Filesystem Library Version 4

Reference Documentation

Table of Contents

Introduction

This reference documentation describes components that C++ programs may use to perform operations involving file systems, including paths, regular files, and directories.

Conformance [fs.conformance]

ISO/IEC 9945 conformance [fs.conform.9945]

Some behavior in this reference documentation is specified by reference to ISO/IEC 9945. How such behavior is actually implemented is unspecified.

[Note: This constitutes an "as if" rule for implementation of operating system dependent behavior. In practice implementations will usually call native operating system API's. —end note]

Implementations are encouraged to provide such behavior as it is defined by ISO/IEC 9945. Implementations shall document any behavior that differs from the behavior defined by ISO/IEC 9945. Implementations that do not support exact ISO/IEC 9945 behavior are encouraged to provide behavior as close to ISO/IEC 9945 behavior as is reasonable given the limitations of actual operating systems and file systems. If an implementation cannot provide any reasonable behavior, the implementation shall report an error in an implementation-defined manner.

[Note: Such errors might be reported by an #error directive, a static_assert, a filesystem_error exception, a special return value, or some other manner. —end note]

Implementations are not required to provide behavior that is not supported by a particular file system.

[Example: The FAT file system used by some memory cards, camera memory, and floppy discs does not support hard links, symlinks, and many other features of more capable file systems. Implementations are only required to support the FAT features supported by the host operating system. —end example]

The behavior of functions described in this reference may differ from their specification in the presence of file system races. No diagnostic is required.

If the possibility of a file system race would make it unreliable for a program to test for a precondition before calling a function described in this reference documentation, Requires is not specified for the condition. Instead, the condition is specified as a Throws condition.

[Note: As a design practice, preconditions are not specified when it is unreasonable for a program to detect them prior to calling the function. —end note]

Operating system dependent conformance [fs.conform.os]

Some behavior is specified in this reference documentation as being operating system dependent ([fs.def.osdep]). The operation system an implementation is dependent upon is implementation defined.

It is permissible for an implementation to be dependent upon an operating system emulator rather than the actual operating system.

[Example: An implementation uses Cygwin, a Linux® API emulator for some Windows® operating system versions. The implementation would define Cygwin as its operating system. Users could refer to the Cygwin documentation to find details of the operating system dependent behavior.  —end example]

It is user and conformance test detectable that such an implementation is running on Cygwin. Users would be misled and conformance tests would fail if the implementation defined Linux or Windows rather than Cygwin as the operating system, since real behavior is a blend of the two.

Definitions [fs.definitions]

The following definitions shall apply throughout this reference documentation:

operating system dependent behavior [fs.def.osdep]

Behavior that is dependent upon the behavior and characteristics of an operating system. See [fs.conform.os].

file [fs.def.file]

An object that can be written to, or read from, or both. A file has certain attributes, including type. File types include regular files and directories. Other types of files, such as symbolic links, may be supported by the implementation.

file system [fs.def.filesystem]

A collection of files and certain of their attributes.

filename [fs.def.filename]

The name of a file. Filenames "."  and ".."  have special meaning. The follow characteristics of filenames are operating system dependent:

• The permitted characters. See [fs.os.examples].

• Specific filenames that are not permitted.

• Additional filenames that have special meaning.

• Case awareness and sensitivity during path resolution.

• Special rules that may apply to file types other than regular files, such as directories.

path [fs.def.path]

A sequence of elements that identify the location of a file within a filesystem. The elements are the root-name_opt, root-directory_opt, and an optional sequence of filenames. [Note: A pathname is the concrete representation of a path. —end note]

absolute path [fs.def.absolute-path]

A path that unambiguously identifies the location of a file without reference to an additional starting location. The elements of a path that determine if it is absolute are operating system dependent.

relative path [fs.def.relative-path]

A path that is not absolute, and so only unambiguously identifies the location of a file when resolved relative to an implied starting location. The elements of a path that determine if it is relative are operating system dependent.  [Note: Paths "." and ".." are relative paths. —end note]

canonical path [fs.def.canonical-path]

An absolute path that has no elements that are symbolic links, and no "." or ".." elements.

pathname [fs.def.pathname]

A character string that represents the name of a path. Pathnames are formatted according to the generic pathname grammar or an operating system dependent native pathname format.

native pathname format [fs.def.native]

The operating system dependent pathname format accepted by the host operating system.

normal form path [fs.def.normal]

A path with no redundant directory separators, current directory (dot) or parent directory (dot-dot) elements. The normal form for an empty path is an empty path. [v3: The normal form for a path ending in a directory-separator that is not the root directory is the same path with a current directory (dot) element appended.]

link [fs.def.link]

A directory entry object that associates a filename with a file. On some file systems, several directory entries can associate names with the same file.

hard link [fs.def.hardlink]

A link to an existing file. Some file systems support multiple hard links to a file. If the last hard link to a file is removed, the file itself is removed.

[Note: A hard link can be thought of as a shared-ownership smart pointer to a file. —end note]

symbolic link [fs.def.symlink]

A type of file with the property that when the file is encountered during pathname resolution, a string stored by the file is used to modify the pathname resolution.

[Note: A symbolic link can be thought of as a raw pointer to a file. If the file pointed to does not exist, the symbolic link is said to be a "dangling" symbolic link. —end note]

file system race [fs.def.race]

The condition that occurs when multiple threads, processes, or computers interleave access and modification of the same object within a file system.

Generic pathname format [path.generic]

pathname:
            root-name_opt root-directory_opt relative-path_opt

root-name:
            An operating system dependent name that identifies the starting location for absolute paths.

[Note: Many operating systems define a name beginning with two directory-separator characters as a root-name that identifies network or other resource locations. Some operating systems define a single letter followed by a colon as a drive specifier - a root-name identifying a specific device such as a disc drive. —end note]

root-directory:
            directory-separator

relative-path:
            filename
            relative-path directory-separator
            relative-path directory-separator filename

filename:
            name
            "."
            ".."

preferred-separator:
            An operating system dependent directory separator character. May be a synonym for "/".

directory-separator:
            "/"
      "/" directory-separator
            preferred-separator
            preferred-separator directory-separator

Multiple successive directory-separator characters are considered to be the same as one directory-separator character.

The filename "." is considered to be a reference to the current directory. The filename ".." is considered to be a reference to the parent directory. Specific filenames may have special meanings for a particular operating system.

Operating system dependent examples (Informative) [fs.os.examples]

Certain features are specified in this reference documentation as being operating system dependent. The following table shows the application of those specifications for operating systems that use the ISO/IEC 9945 or Windows® application program interfaces (APIs).^[footnote1]

^[footnote1] OS X® and Windows® are examples of commercially available operating systems. This information is given for the convenience of users of this document and does not constitute an endorsement by ISO or IEC of these products.

^{[footnote 2]} Rationale: ISO C specifies std::locale("") as "the locale-specific native environment", while ISO/IEC 9945 says it "Specifies an implementation-defined native environment."

^{[footnote 3]} Rationale: This is the current behavior of C and C++ standard library functions that perform file operations using narrow character strings to identify paths. Changing this behavior would be surprising and at variance with existing code, particularly where user input is involved.

^{[footnote 4]} Rationale: Vendor's documentation states "All BSD system functions expect their string parameters to be in UTF-8 encoding and nothing else."

Header <boost/filesystem.hpp> synopsis [filesystem.synopsis]

namespace boost
{
  namespace filesystem
  {
    class path;

    bool lexicographical_compare(path::iterator first1, path::iterator last1,
      path::iterator first2, path::iterator last2);
    void swap(path& lhs, path& rhs) noexcept;
    std::size_t hash_value(const path& p);

    bool operator==(const path& lhs, const path& rhs);
    bool operator!=(const path& lhs, const path& rhs);
    bool operator< (const path& lhs, const path& rhs);
    bool operator<=(const path& lhs, const path& rhs);
    bool operator> (const path& lhs, const path& rhs);
    bool operator>=(const path& lhs, const path& rhs);

    path operator/ (const path& lhs, const path& rhs);

    std::ostream&  operator<<( std::ostream& os, const path& p );
    std::wostream& operator<<( std::wostream& os, const path& p );
    std::istream&  operator>>( std::istream& is, path& p );
    std::wistream& operator>>( std::wistream& is, path& p )

    class filesystem_error;
    class directory_entry;

    class directory_iterator;

    // enable c++11 range-based for statements
    const directory_iterator& begin(const directory_iterator& iter);
    directory_iterator end(const directory_iterator&);


    // enable BOOST_FOREACH
    directory_iterator& range_begin(directory_iterator& iter);
    directory_iterator range_begin(const directory_iterator& iter);
    directory_iterator range_end(const directory_iterator&);

    class recursive_directory_iterator;

    // enable c++11 range-based for statements
    const recursive_directory_iterator&
      begin(const recursive_directory_iterator& iter);
    recursive_directory_iterator
      end(const recursive_directory_iterator&);


    // enable BOOST_FOREACH
    recursive_directory_iterator&
      range_begin(recursive_directory_iterator& iter);
    recursive_directory_iterator
      range_begin(const recursive_directory_iterator& iter);
    recursive_directory_iterator
      range_end(const recursive_directory_iterator&);

    enum file_type
    {
      status_error, file_not_found, regular_file, directory_file,
      symlink_file, block_file, character_file, fifo_file, socket_file,
      reparse_file, type_unknown
    };

    enum perms
    {
      no_perms,
      owner_read, owner_write, owner_exe, owner_all,
      group_read, group_write, group_exe, group_all,
      others_read, others_write, others_exe, others_all, all_all,
      set_uid_on_exe, set_gid_on_exe, sticky_bit,
      perms_mask, perms_not_known,
      add_perms, remove_perms, symlink_perms
    };

    class file_status;

    struct space_info  // returned by space function
    {
      uintmax_t capacity;
      uintmax_t free;
      uintmax_t available; // free space available to non-privileged process
    };

    enum class copy_options
    {
      none = 0u,
      // copy_file options
      skip_existing,
      overwrite_existing,
      update_existing,
      synchronize_data,
      synchronize,
      ignore_attribute_errors,
      // copy options
      recursive,
      copy_symlinks,
      skip_symlinks,
      directories_only,
      create_symlinks,
      create_hard_links
    };

    enum class directory_options
    {
      none = 0u,
      skip_permission_denied,
      follow_directory_symlink,
      pop_on_error
    };

    // operational functions

    path         absolute(const path& p, const path& base=current_path());
    path         absolute(const path& p, system::error_code& ec);
    path         absolute(const path& p, const path& base,
                   system::error_code& ec);

    path         canonical(const path& p, const path& base = current_path());
    path         canonical(const path& p, system::error_code& ec);
    path         canonical(const path& p, const path& base,
                   system::error_code& ec);

    void         copy(const path& from, const path& to);
    void         copy(const path& from, const path& to,
                   system::error_code& ec);
    void         copy(const path& from, const path& to,
                   copy_options options);
    void         copy(const path& from, const path& to,
                   copy_options options, system::error_code& ec);

    bool         copy_file(const path& from, const path& to);
    bool         copy_file(const path& from, const path& to,
                   system::error_code& ec);
    bool         copy_file(const path& from, const path& to,
                   copy_options options);
    bool         copy_file(const path& from, const path& to,
                   copy_options options, system::error_code& ec);

    void         copy_symlink(const path& existing_symlink,
                   const path& new_symlink);
    void         copy_symlink(const path& existing_symlink,
                   const path& new_symlink, system::error_code& ec);

    bool         create_directories(const path& p);
    bool         create_directories(const path& p,
                   system::error_code& ec);

    bool         create_directory(const path& p);
    bool         create_directory(const path& p, system::error_code& ec);
    bool         create_directory(const path& p, const path& existing);
    bool         create_directory(const path& p, const path& existing, system::error_code& ec);

    void         create_directory_symlink(const path& to,
                   const path& new_symlink);
    void         create_directory_symlink(const path& to,
                   const path& new_symlink, system::error_code& ec);

    void         create_hard_link(const path& to, const path& new_hard_link);
    void         create_hard_link(const path& to, const path& new_hard_link,
                                  system::error_code& ec);

    void         create_symlink(const path& to, const path& new_symlink);
    void         create_symlink(const path& to, const path& new_symlink,
                                system::error_code& ec);

    std::time_t  creation_time(const path& p);
    std::time_t  creation_time(const path& p, system::error_code& ec);

    path         current_path();
    path         current_path(system::error_code& ec);
    void         current_path(const path& p);
    void         current_path(const path& p, system::error_code& ec);

    bool         exists(file_status s) noexcept;
    bool         exists(const path& p);
    bool         exists(const path& p, system::error_code& ec) noexcept;

    bool         equivalent(const path& p1, const path& p2);
    bool         equivalent(const path& p1, const path& p2,
                   system::error_code& ec);

    uintmax_t    file_size(const path& p);
    uintmax_t    file_size(const path& p, system::error_code& ec);

    uintmax_t    hard_link_count(const path& p);
    uintmax_t    hard_link_count(const path& p, system::error_code& ec);

    const path&  initial_path();
    const path&  initial_path(system::error_code& ec);

    bool         is_block_file(file_status s) noexcept;
    bool         is_block_file(const path& p);
    bool         is_block_file(const path& p,
                   system::error_code& ec) noexcept;

    bool         is_character_file(file_status s) noexcept;
    bool         is_character_file(const path& p);
    bool         is_character_file(const path& p,
                   system::error_code& ec) noexcept;

    bool         is_directory(file_status s) noexcept;
    bool         is_directory(const path& p);
    bool         is_directory(const path& p,
                   system::error_code& ec) noexcept;

    bool         is_empty(const path& p);
    bool         is_empty(const path& p, system::error_code& ec);

    bool         is_fifo(file_status s) noexcept;
    bool         is_fifo(const path& p);
    bool         is_fifo(const path& p,
                   system::error_code& ec) noexcept;

    bool         is_other(file_status s) noexcept;
    bool         is_other(const path& p,);
    bool         is_other(const path& p, system::error_code& ec) noexcept;

    bool         is_regular_file(file_status s) noexcept;
    bool         is_regular_file(const path& p);
    bool         is_regular_file(const path& p,
                   system::error_code& ec) noexcept;

    bool         is_reparse_file(file_status s) noexcept;
    bool         is_reparse_file(const path& p);
    bool         is_reparse_file(const path& p,
                   system::error_code& ec) noexcept;

    bool         is_socket(file_status s) noexcept;
    bool         is_socket(const path& p);
    bool         is_socket(const path& p,
                   system::error_code& ec) noexcept;

    bool         is_symlink(file_status s noexcept);
    bool         is_symlink(const path& p);
    bool         is_symlink(const path& p, system::error_code& ec) noexcept;

    std::time_t  last_write_time(const path& p);
    std::time_t  last_write_time(const path& p, system::error_code& ec);
    void         last_write_time(const path& p, const std::time_t new_time);
    void         last_write_time(const path& p, const std::time_t new_time,
                                 system::error_code& ec);

    path         read_symlink(const path& p);
    path         read_symlink(const path& p, system::error_code& ec);

    path         relative(const path& p, system::error_code& ec);
    path         relative(const path& p, const path& base=current_path());
    path         relative(const path& p,
                     const path& base, system::error_code& ec);

    bool         remove(const path& p);
    bool         remove(const path& p, system::error_code& ec);

    uintmax_t    remove_all(const path& p);
    uintmax_t    remove_all(const path& p, system::error_code& ec);

    void         rename(const path& from, const path& to);
    void         rename(const path& from, const path& to,
                   system::error_code& ec);

    void         resize_file(const path& p, uintmax_t size);
    void         resize_file(const path& p, uintmax_t size,
                   system::error_code& ec);

    space_info   space(const path& p);
    space_info   space(const path& p, system::error_code& ec);

    file_status  status(const path& p);
    file_status  status(const path& p, system::error_code& ec) noexcept;

    bool         status_known(file_status s) noexcept;

    file_status  symlink_status(const path& p);
    file_status  symlink_status(const path& p,
                   system::error_code& ec) noexcept;

    path         system_complete(const path& p);
    path         system_complete(const path& p, system::error_code& ec);

    path         temp_directory_path();
    path         temp_directory_path(system::error_code& ec);

    path         unique_path(const path& model="%%%%-%%%%-%%%%-%%%%");
    path         unique_path(system::error_code& ec);
    path         unique_path(const path& model, system::error_code& ec);

    path         weakly_canonical(const path& p, const path& base=current_path());
    path         weakly_canonical(const path& p, system::error_code& ec);
    path         weakly_canonical(const path& p, const path& base,
                   system::error_code& ec);

  }  // namespace filesystem
}  // namespace boost

Error reporting [fs.err.report]

Filesystem library functions often provide two overloads, one that throws an exception to report file system errors, and another that sets an error_code.

[Note: This supports two common use cases:

• Uses where file system errors are truly exceptional and indicate a serious failure. Throwing an exception is the most appropriate response. This is the preferred default for most everyday programming.

• Uses where file system errors are routine and do not necessarily represent failure. Returning an error code is the most appropriate response. This allows application specific error handling, including simply ignoring the error.

—end note]

Functions not having an argument of type system::error_code& report errors as follows, unless otherwise specified:

• When a call by the implementation to an operating system or other underlying API results in an error that prevents the function from meeting its specifications, an exception of type filesystem_error is thrown.

• Failure to allocate storage is reported by throwing an exception as described in the C++ standard, 17.6.4.10 [res.on.exception.handling].

• Destructors throw nothing.

Functions having an argument of type system::error_code& report errors as follows, unless otherwise specified:

• If a call by the implementation to an operating system or other underlying API results in an error that prevents the function from meeting its specifications, the system::error_code& argument is set as appropriate for the specific error. Otherwise, clear() is called on the system::error_code& argument.

• Failure to allocate storage is reported by throwing an exception as described in the C++ standard, 17.6.4.10 [res.on.exception.handling].

Class path [class.path]

An object of class path represents a path, and contains a pathname Such an object is concerned only with the lexical and syntactic aspects of a path. The path does not necessarily exist in external storage, and the pathname is not necessarily valid for the current operating system or for a particular file system.

namespace boost
{
  namespace filesystem
  {
      class path
      {
      public:
        typedef see below                                    value_type;
        typedef std::basic_string<value_type>                string_type;
        typedef std::codecvt<wchar_t, char, std::mbstate_t>  codecvt_type;
        constexpr value_type                                 dot;
        constexpr value_type                                 separator;
        constexpr value_type                                 preferred_separator;

        // constructors and destructor
        path();
        path(const path& p);
        path(path&& p) noexcept;

        template <class Source>
          path(Source const& source, const codecvt_type& cvt=codecvt());

        template <class InputIterator>
          path(InputIterator begin, InputIterator end,
            const codecvt_type& cvt=codecvt());

       ~path();

        // assignments
        path& operator=(const path& p);
        path& operator=(path&& p) noexcept;

        template <class Source>
          path& operator=(Source const& source);

        path& assign(const path& p);
        path& assign(path&& p) noexcept;
        template <class Source>
          path& assign(Source const& source,
            const codecvt_type& cvt=codecvt())

        template <class InputIterator>
          path& assign(InputIterator begin, InputIterator end,
            const codecvt_type& cvt=codecvt());

        // appends
        path& operator/=(const path& p);

        template <class Source>
          path& operator/=(Source const& source);

        path& append(const path& x);
        template <class Source>
          path& append(Source const& source,
            const codecvt_type& cvt=codecvt());

        template <class InputIterator>
          path& append(InputIterator begin, InputIterator end,
            const codecvt_type& cvt=codecvt());

        // concatenation
        path& operator+=(const path& x);
        template <class Source>
          path& operator+=(Source const& source);
        path& operator+=(value_type x);
        template <class CharT>
          path& operator+=(CharT x);

        path& concat(const path& x);
        template <class Source>
          path& concat(Source const& x,
            const codecvt_type& cvt=codecvt());
        template <class InputIterator>
          path& concat(InputIterator begin, InputIterator end,
            const codecvt_type& cvt=codecvt());

        // modifiers
        void  clear();
        path& make_preferred();
        path& remove_filename();
        path& remove_filename_and_trailing_separators();
        path& replace_filename(const path& replacement);
        path& replace_extension(const path& new_extension = path());
        void  swap(path& rhs) noexcept;

        // lexical operations
        path lexically_normal() const;
        path lexically_relative(const path& base) const;
        path lexically_proximate(const path& base) const;

        // native format observers
        const string_type&  native() const noexcept;  // native format, encoding
        const value_type*   c_str() const noexcept;   // native().c_str()
        string_type::size_type size() const noexcept; // native().size()

        template <class String>
          String  string(const codecvt_type& cvt=codecvt()) const;
        string    string(const codecvt_type& cvt=codecvt()) const;
        wstring   wstring(const codecvt_type& cvt=codecvt()) const;

        // generic format observers
        template <class String>
          String  generic_string() const;

        string    generic_string(const codecvt_type& cvt=codecvt()) const;
        wstring   generic_wstring(const codecvt_type& cvt=codecvt()) const;

        // compare
        int   compare(const path& p) const noexcept;
        int   compare(const std::string& s) const;
        int   compare(const value_type* s) const;

        // decomposition
        path  root_name() const;
        path  root_directory() const;
        path  root_path() const;
        path  relative_path() const;
        path  parent_path() const;
        path  filename() const;
        path  stem() const;
        path  extension() const;

        // query
        bool empty() const;
        bool filename_is_dot() const;
        bool filename_is_dot_dot() const;
        bool has_root_name() const;
        bool has_root_directory() const;
        bool has_root_path() const;
        bool has_relative_path() const;
        bool has_parent_path() const;
        bool has_filename() const;
        bool has_stem() const;
        bool has_extension() const;
        bool is_absolute() const;
        bool is_relative() const;

        // iterators
        class iterator;
        typedef iterator const_iterator;
        class reverse_iterator;
        typedef reverse_iterator const_reverse_iterator;

        iterator begin() const;
        iterator end() const;
        reverse_iterator rbegin() const;
        reverse_iterator rend() const;

        // imbued locale
        static std::locale imbue(const std::locale& loc);
        static const codecvt_type & codecvt();

      private:
        string_type pathname;  // exposition only
      };

  }  // namespace filesystem
}  // namespace boost

value_type is a typedef for the character type used by the operating system to represent pathnames.

path Usage concerns [path.usage]

Multithreading concerns

Filesystem library functions are not protected against data races. [Modifying an object of a Filesystem library type that is shared between threads risks undefined behavior unless objects of that type are explicitly specified as being sharable without data races or the user supplies a locking mechanism. —end note] [Note: Thus the Filesystem library behaves as if it were part of the standard library, and C++ standard 17.6.4.10 Shared objects and the library [res.on.objects] would thus apply.  —end note]

Windows concerns

Visual C++ at least through version 2012 does not employ C++11-style static initialization locks, so the initialization of path::codecvt() can race, either with itself or path::imbue() if they are called from a different thread. A workaround is to call:

path::codecvt();  // ensure VC++ does not race during initialization.

in the main thread before launching any additional threads. [Note: The obvious fix of the Filesystem implementation doing the locking doesn't work because of unrelated problems with the Microsoft compiler; for static linking the runtime tries to do the initialization before main() starts, but doesn't permit operating system lock calls at that time.  —end note]

POSIX concerns

Filesystem library initialization may throw an exception on POSIX systems (e.g. Linux, but not Mac OS X) that use environmental variables to determine the encoding of paths. This happens when std::locale("") throws because an environmental variable such as LANG is set to an invalid value, so it can affect any use of  std::locale(""), not just the Filesystem library. Filesystem uses lazy initialization so the exception is only thrown if a valid std::locale("") is actually needed, and also so that the exception is thrown after main() starts.

Rather than waiting until a call to some Filesystem library function unexpectedly triggers the exception when it calls path::codecvt(), a program that needs be highly robust against environmental variable problems may want to preemptively call std::locale("") within a try block, catch the exception, and diagnose or repair the invalid environmental variable.

path Conversions [path.cvt]

path argument conversions [path.arg.cvt]

path argument format conversions [path.arg.fmt.cvt]

Member function arguments that take character sequences representing paths may use the generic pathname format or the native pathname format. Iff such arguments are in the generic format and the generic format is not acceptable to the operating system as a native path, conversion to native format shall be performed during the processing of the argument. See [fs.os.examples].

[Note: Depending on the operating system, there may be no unambiguous way for an implementation to always be able to distinguish between native format and generic format arguments. This is by design as it simplifies use for operating systems that do not require disambiguation. Should an implementation encounter an operating system where disambiguation is required, an implementation can defined an extension to distinguish between the formats. —end note]

If the native format requires paths for regular files to be formatted differently from paths for directories, the path shall be treated as a directory path if last element is a separator, otherwise it shall be treated as a regular file path.

path argument encoding conversions [path.arg.encoding.cvt]

For member function arguments that take character sequences representing paths, if the value type of the argument is not value_type and one of the value types is char and the other is wchar_t, conversion to value_type shall be performed by the path::codecvt() facet. ([path.imbued.locale]).

path Conversions to generic format [fs.cvt.to.generic]

Generic format observer functions shall return strings formatted according to the generic pathname format using preferred-separator. See [fs.os.examples].

path Requirements [path.req]

Template parameters named InputIterator are required to meet the requirements for a C++ standard library InputIterator compliant iterator. The iterator's value type is required to be one of: char, wchar_t. Collectively, these types are referred to as supported path character types.

Template parameters named Source are required to be one of:

• A std::basic_string, std::basic_string_view, boost::container::basic_string or boost::basic_string_view specialization with a value type of one of the supported path character types.

• v3, deprecated: A container with a value type of one of the supported path character types.

• A pointer into a null terminated string. The value type is required to be a supported path character type.

• A C-array of supported path character type containing a null terminated string.

• A boost::filesystem::directory_entry.

path constructors [path.construct]

template <class Source>
path(Source const& source, const codecvt_type& cvt=codecvt());

template <class InputIterator>
path(InputIterator begin, InputIterator end, const codecvt_type& cvt=codecvt());

Effects: Stores the contents [begin,end) of source in pathname, converting format and encoding if required ([path.arg.convert]).

path assignments [path.assign]

path& operator=(const path& p);
path& operator=(path&& p) noexcept;
template <class Source>
path& operator=(Source const& source);
path& assign(const path& p);
path& assign(path&& p) noexcept;
template <class Source>
path& assign(Source const& source, const codecvt_type& cvt=codecvt());
template <class InputIterator>
path& assign(InputIterator begin, InputIterator end, const codecvt_type& cvt=codecvt());

Effects: Stores the contents [begin,end) or source or p in pathname, converting format and encoding if required ([path.arg.convert]).

Returns: *this

path appends [path.append]

The append operations use operator/= to denote their semantic effect of appending preferred-separator when needed.

path& operator/=(const path& p);
path& append(const path& p);

Effects:

v3: Appends path::preferred_separator to pathname, converting format and encoding if required ([path.arg.convert]), unless:

• an added separator would be redundant, or

• would change a relative path to an absolute path, or

• p.empty(), or

• *p.native().cbegin() is a directory separator.

Then appends p.native() to pathname.

v4: If p.is_absolute() || (p.has_root_name() && p.root_name() != root_name()), assigns p to *this. Otherwise, modifies *this as if by these steps:

• If p.has_root_directory(), removes root directory and relative path, if any.
• Let x be a path with contents of p without a root name. If has_filename() is true and x does not start with a directory separator, appends path::preferred_separator.
• Appends x.native().

[Note: Whether the path is absolute or not depends on the target OS conventions. Because of this, the result of append operation may be different for different operating systems for some paths. For example, path("//net/foo") / "/bar" will result in "/bar" on POSIX systems and "//net/foo/bar" on Windows because "/bar" is an absolute path on POSIX systems but not on Windows. For portable behavior avoid appending paths with non-empty root path. —end note]

Returns: *this

template <class Source>
path& operator/=(Source const & source);
template <class Source>
path& append(Source const & source, const codecvt_type& cvt=codecvt());
template <class InputIterator>
path& append(InputIterator begin, InputIterator end, const codecvt_type& cvt=codecvt());

Effects:

As if append(path(args)), where args is the list of arguments passed to the operation.

Returns: *this

path concatenation [path.concat]

path& operator+=(const path& p);
path& operator+=(value_type x);
template <class Source>
path& operator+=(Source const& source);
template <class CharT>
path& operator+=(CharT x);
path& concat(const path& p);
template <class Source>
path& concat(Source const& source, const codecvt_type& cvt=codecvt());
template <class InputIterator>
path& concat(InputIterator begin, InputIterator end, const codecvt_type& cvt=codecvt());

Postcondition: native() == prior_native + effective-argument, where prior_native is native() prior to the call to operator+=, and effective-argument is:

• p.native() if p is present and is const path&, otherwise

• s, where s is std::basic_string<typename std::iterator_traits<InputIterator>::value_type>
s(begin, end), if begin and end arguments are present, otherwise

• x.

If the value type of effective-argument would not be path::value_type, the actual argument or argument range is first converted so that effective-argument has value type path::value_type.

Returns: *this

path modifiers [path.modifiers]

void clear();

Postcondition: this->empty() is true.

path& make_preferred();

Effects: directory-separators are converted to preferred-separators. See [fs.os.examples].

Returns: *this

path& remove_filename();

Effects: v3: As if, *this = parent_path();

[Note: This function is needed to efficiently implement directory_iterator. It is exposed to allow additional uses. The actual implementation may be much more efficient than *this = parent_path()  —end note]

v4: Removes the filename() path element.

[Note: Unlike v3, the trailing directory separator(s) are not removed.—end note]

Returns: *this.

path& remove_filename_and_trailing_separators();

Effects: As if, *this = parent_path();

[Note: This function is similar to path::remove_filename from v3, but is also usable in v4.—end note]

Returns: *this.

path& replace_filename(const path& replacement);

Effects: As if, remove_filename().append(replacement);

Returns: *this.

path& replace_extension(const path& new_extension = path());

Effects:

• Any existing extension() is removed from the stored path, then

• iff new_extension is not empty and does not begin with a dot character, a dot character is appended to the stored path, then

• new_extension is appended to the stored path.

Returns: *this

void swap(path& rhs) noexcept;

Effects: Swaps the contents of the two paths.

Complexity: constant time.

path lexical operations [path.lex.ops]

path lexically_normal() const;

Overview: Returns *this with redundant current directory (dot), parent directory (dot-dot), and directory-separator elements removed.

Returns: *this in normal form.

Remarks: Uses operator/= to compose the returned path.

[Example:

std::cout << path("foo/./bar/..").lexically_normal() << std::endl;    // outputs "foo"
std::cout << path("foo/.///bar/../").lexically_normal() << std::endl; // v3: outputs "foo/."
                                                                      // v4: outputs "foo/"

On Windows, the returned path's directory-separator characters will be backslashes rather than slashes, but that does not affect path equality. —end example]

path lexically_relative(const path& base) const;

Overview: Returns *this made relative to base. Treats empty or identical paths as corner cases, not errors. Does not resolve symlinks. Does not first normalize *this or base.

Remarks: Uses std::mismatch(begin(), end(), base.begin(), base.end()), to determine the first mismatched element of *this and base. Uses operator== to determine if elements match.

Returns:

• path() if the first mismatched element of *this is equal to begin() or the first mismatched element of base is equal to base.begin(), or
   
• path(".") if the first mismatched element of *this is equal to end() and the first mismatched element of base is equal to base.end(), or
   
• An object of class path composed via application of operator/= path("..") for each element in the half-open range [first mismatched element of base, base.end()), and then application of operator/= for each element in the half-open range [first mismatched element of *this, end()).

[Example:

assert(path("/a/d").lexically_relative("/a/b/c") == "../../d");
assert(path("/a/b/c").lexically_relative("/a/d") == "../b/c");
assert(path("a/b/c").lexically_relative("a") == "b/c");
assert(path("a/b/c").lexically_relative("a/b/c/x/y") == "../..");
assert(path("a/b/c").lexically_relative("a/b/c") == ".");
assert(path("a/b").lexically_relative("c/d") == "");

The above assertions will succeed. On Windows, the returned path's directory-separators will be backslashes rather than forward slashes, but that does not affect path equality. —end example]

[Note: If symlink following semantics are desired, use the operational function relative  —end note]

[Note: If normalization is needed to ensure consistent matching of elements, apply lexically_normal() to *this, base, or both. —end note]

path lexically_proximate(const path& base) const;

Returns: If lexically_relative(base) returns a non-empty path, returns that path. Otherwise returns *this.

[Note: If symlink following semantics are desired, use the operational function relative  —end note]

[Note: If normalization is needed to ensure consistent matching of elements, apply lexically_normal() to *this, base, or both. —end note]

path native format observers [path.native.obs]

The string returned by all native format observers is in the native pathname format.

const string_type& native() const noexcept;

Returns: pathname.

const value_type* c_str() const noexcept;

Returns: pathname.c_str().

string_type::size_type size() const noexcept;

Returns: pathname.size().

template <class String>
String string(const codecvt_type& cvt=codecvt()) const;

Returns: pathname.

Remarks: If string_type is a different type than String, conversion is performed by cvt.

string string(const codecvt_type& cvt=codecvt()) const;
wstring wstring(const codecvt_type& cvt=codecvt()) const;

Returns: pathname.

Remarks: If string_type is a different type than function's return type, conversion is performed by cvt.

path generic format observers [path.generic.obs]

The string returned by all generic format observers is in the generic pathname format. The returned strings use a single forward slash ('/') as directory separators.

template <class String>
String generic_string(const codecvt_type& cvt=codecvt()) const;

Returns: pathname.

Remarks: If string_type is a different type than String, conversion is performed by cvt.

string generic_string(const codecvt_type& cvt=codecvt()) const;
wstring generic_wstring(const codecvt_type& cvt=codecvt()) const;

Returns: pathname.

Remarks: If string_type is a different type than function's return type, conversion is performed by cvt.

path compare [path.compare]

int compare(const path& p) const noexcept;

Returns: A value less than 0 if the elements of *this are lexicographically less than the elements of p, otherwise a value greater than 0 if the elements of *this are lexicographically greater than the elements of p, otherwise 0.

Remark: The elements are determined as if by iteration over the half-open range [begin(), end()) for *this and p.

int compare(const std::string& s) const

Returns: compare(path(s)).

int compare(const value_type* s) const

Returns: compare(path(s)).

path decomposition [path.decompose]

See the Path decomposition table for examples for values returned by decomposition functions. The Tutorial may also be helpful.

path root_name() const;

Returns: root-name, if pathname includes root-name, otherwise path().

path root_directory() const;

Returns: root-directory, if pathname includes root-directory, otherwise path().

If root-directory is composed of slash name, slash is excluded from the returned string.

path root_path() const;

Returns: root_name() / root_directory()

path relative_path() const;

Returns: A path composed from pathname, if !empty(), beginning with the first filename after root-path. Otherwise, path().

path parent_path() const;

Returns: (empty() || begin() == --end()) ? path() : pp, where pp is constructed as if by starting with an empty path and successively applying operator/= for each element in the range begin(), --end().

[Example:

std::cout << path("/foo/bar.txt").parent_path(); // outputs "/foo"
std::cout << path("/foo/bar").parent_path();     // outputs "/foo"
std::cout << path("/foo/bar/").parent_path();    // outputs "/foo/bar"
std::cout << path("/").parent_path();            // outputs ""
std::cout << path(".").parent_path();            // outputs ""
std::cout << path("..").parent_path();           // outputs ""

See the last bullet item in the forward traversal order list for why the "/foo/bar/" example doesn't output "/foo".

—end example]

path filename() const;

Returns: v3: empty() ? path() : *--end()
v4: *this == root_path() ? path() : *--end()

[Example:

std::cout << path("/foo/bar.txt").filename(); // outputs "bar.txt"
std::cout << path("/foo/bar").filename();     // outputs "bar"
std::cout << path("/foo/bar/").filename();    // v3 outputs "."
                                              // v4 outputs ""
std::cout << path("/").filename();            // v3 outputs "/"
                                              // v4 outputs ""
std::cout << path(".").filename();            // outputs "."
std::cout << path("..").filename();           // outputs ".."

See the last bullet item in the forward traversal order list for why the "/foo/bar/" example doesn't output "bar".

—end example]

path stem() const;

Returns: If p.filename() does not contain dots, consist solely of one or to two dots, [Since v4: or contains exactly one dot as the initial character,] returns p.filename(). Otherwise returns the substring of p.filename() starting at its beginning and ending at the last dot (the dot is not included).

[Example:

std::cout << path("/foo/bar.txt").stem() << '\n'; // outputs "bar"
std::cout << path(".hidden").stem() << '\n';      // v3 outputs ""
                                                  // v4 outputs ".hidden"
path p = "foo.bar.baz.tar";
for (; !p.extension().empty(); p = p.stem())
  std::cout << p.extension() << '\n';
  // outputs: .tar
  //          .baz
  //          .bar

—end example]

path extension() const;

Returns: The substring of p.filename() that is not included in p.stem().

Remarks: Implementations are permitted but not required to define additional behavior for file systems which append additional elements to extensions, such as alternate data streams or partitioned dataset names.

[Example:

std::cout << path("/foo/bar.txt").extension(); // outputs ".txt"

—end example]

[Note: The dot is included in the return value so that it is possible to distinguish between no extension and an empty extension. See https://lists.boost.org/Archives/boost/2010/02/162028.php for more extensive rationale.  —end note]

path query [path.query]

bool empty() const;

Returns: m_pathname.empty().

bool filename_is_dot() const;

Returns: filename() == path(".")

[Example:

std::cout << path(".").filename_is_dot();     // outputs 1
std::cout << path("/.").filename_is_dot();    // outputs 1
std::cout << path("foo/.").filename_is_dot(); // outputs 1
std::cout << path("foo/").filename_is_dot();  // v3 outputs 1, v4 outputs 0
std::cout << path("/").filename_is_dot();     // outputs 0
std::cout << path("/foo").filename_is_dot();  // outputs 0
std::cout << path("/foo.").filename_is_dot(); // outputs 0
std::cout << path("..").filename_is_dot();    // outputs 0

See the last bullet item in the forward traversal order list for why path("foo/").filename() is a dot filename in v3.

—end example]

bool filename_is_dot_dot() const;

Returns: filename() == path("..")

bool has_root_path() const;

Returns: !root_path().empty()

bool has_root_name() const;

Returns: !root_name().empty()

bool has_root_directory() const;

Returns: !root_directory().empty()

bool has_relative_path() const;

Returns: !relative_path().empty()

bool has_parent_path() const;

Returns: !parent_path().empty()

bool has_filename() const;

Returns: !filename().empty()

bool has_stem() const;

Returns: !stem().empty()

bool has_extension() const;

Returns: !extension().empty()

bool is_absolute() const;

Returns: true if the elements of root_path() uniquely identify a directory, else false.

[Note: Path is considered absolute on POSIX systems if it has a root directory, and on Windows - if it has both root name and root directory. —end note]

bool is_relative() const;

Returns: !is_absolute().

path iterators [path.itr]

Path iterators iterator, const_iterator, reverse_iterator, and const_reverse_iterator iterate over the elements of the stored pathname.

Path iterators are constant iterators satisfying the requirements of a bidirectional iterator (C++ Std, 24.1.4 Bidirectional iterators [lib.bidirectional.iterators]). The value_type  of an iterator is path.

[Note: Path iterators store their value objects internally and when dereferenced return references to those internal objects. They cannot be used with iterator adaptors such as std::reverse_iterator that assume references obtained by dereferencing an iterator point to objects that out-live the iterator itself. —end note]

Calling any non-const member function of a path object invalidates all iterators referring to elements of that object.

The forward traversal order is as follows:

• The root-name element, if present.
• The root-directory element, if present, in the generic format. [Note: the generic format is required to ensure lexicographical comparison works correctly. —end note]
• Each successive filename element, if present.
• [v3: Dot] [v4: Empty path], if one or more trailing non-root directory separators are present.

[Note: Treating the last element during iteration as [v3: dot] [v4: an empty path] when there is a trailing directory separator enables lexical (i.e. syntactic) distinction between paths to directories versus paths to regular files. Such a distinction is usually irrelevant on POSIX and Windows based operating systems, but may be a requirement on other operating systems. —end note]

The backward traversal order is the reverse of forward traversal.

iterator begin() const;

Returns: An iterator for the first element in forward traversal order. If no elements are present, the end iterator.

iterator end() const;

Returns: The end iterator.

reverse_iterator rbegin() const;

Returns: An iterator for the first element in backward traversal order. If no elements are present, the end iterator.

reverse_iterator rend() const;

Returns: The end iterator.

path imbued locale [path.imbued.locale]

path operations sometimes require encoding conversions between pathname and some other string object where one of the value types is char and the other is wchar_t. Such conversions shall be performed by the path::codecvt() facet.

[Example: ... —end example]

static std::locale imbue(const std::locale& loc);

Effects: Stores a copy of loc as the imbued path locale.

Returns: The previous imbued path locale.

Remarks: The initial value of the imbued path locale is operating system dependent. It shall be a locale with a codecvt facet for a char string encoding appropriate for the operating system. See ([fs.os.examples]). 

static const codecvt_type& codecvt();

Returns: The codecvt facet for the imbued path locale .

path non-member functions [path.non-member]

bool lexicographical_compare(path::iterator first1, path::iterator last1,
                             path::iterator first2, path::iterator last2);

Returns: true if the sequence of native() strings for the elements defined by the half-open range [first1, last1) is lexicographically less than the sequence of native() strings for the elements defined by the half-open range [first2, last2). Returns false otherwise.

Remarks: If two sequences have the same number of elements and their corresponding elements are equivalent, then neither sequence is lexicographically less than the other. If one sequence is a prefix of the other, then the shorter sequence is lexicographically less than the longer sequence. Otherwise, the lexicographical comparison of the sequences yields the same result as the comparison of the first corresponding pair of elements that are not equivalent.

[Note: A path aware lexicographical_compare algorithm is provided for historical reasons. —end note]

path lexically_normal(const path& p);

Overview: Returns p with redundant current directory (dot), parent directory (dot-dot), and directory-separator elements removed.

Returns: p in normal form.

Remarks: Uses operator/= to compose the returned path.

[Example:

assert(lexically_normal("foo/./bar/..") == "foo");
assert(lexically_normal("foo/.///bar/../") == "foo/.");

All of the above assertions will succeed. On Windows, the returned path's directory-separator characters will be backslashes rather than slashes, but that does not affect path equality. —end example]

path lexically_relative(const path& p, const path& base);

Overview: Returns p made relative to base. Treats empty or identical paths as corner cases, not errors. Does not resolve symlinks. Does not first normalize p or base.

Remarks: Uses std::mismatch(p.begin(), p.end(), base.begin(), base.end()), to determine the first mismatched element of p and base. Uses operator== to determine if elements match.

Returns:

• path() if the first mismatched element of p is equal to p.begin() or the first mismatched element of base is equal to base.begin(), or
   

• path(".") if the first mismatched element of p is equal to p.end() and the first mismatched element of base is equal to base.end(), or
   

• An object of class path composed via application of operator/= path("..") for each element in the half-open range [first mismatched element of base, base.end()), and then application of operator/= for each element in the half-open range [first mismatched element of p, p.end()).

[Example:

assert(lexically_relative("/a/d", "/a/b/c") == "../../d");
assert(lexically_relative("/a/b/c", "/a/d") == "../b/c");
assert(lexically_relative("a/b/c", "a") == "b/c");
assert(lexically_relative("a/b/c", "a/b/c/x/y") == "../..");
assert(lexically_relative("a/b/c", "a/b/c") == ".");
assert(lexically_relative("a/b", "c/d") == "");

All of the above assertions will succeed. On Windows, the returned path's directory-separators will be backslashes rather than forward slashes, but that does not affect path equality. —end example]

[Note: If symlink following semantics are desired, use the operational function relative  —end note]

[Note: If normalization is needed to ensure consistent matching of elements, wrap p, base, or both in calls lexically_normal(). —end note]

void swap(path& lhs, path& rhs) noexcept;

Effects: lhs.swap(rhs).

std::size_t hash_value(const path& p);

Returns: A hash value for the path p. If for two paths, p1 == p2 then hash_value(p1) == hash_value(p2).

This allows paths to be used with Boost.Hash.

bool operator< (const path& lhs, const path& rhs);

Returns: return lhs.compare(rhs.begin) < 0.

bool operator<=(const path& lhs, const path& rhs);

Returns: !(rhs < lhs).

bool operator> (const path& lhs, const path& rhs);

Returns: rhs < lhs.

bool operator>=(const path& lhs, const path& rhs);

Returns: !(lhs < rhs).

bool operator==(const path& lhs, const path& rhs);

Returns: !(lhs < rhs) && !(rhs < lhs).

[Note: Path equality and path equivalence have different semantics.

Equality is determined by the path non-member operator==, which considers the two path's lexical representations only. Thus path("foo") == "bar" is never true.

Equivalence is determined by the equivalent() non-member function, which determines if two paths resolve to the same file system entity. Thus equivalent("foo", "bar") will be true when both paths resolve to the same file.

Programmers wishing to determine if two paths are "the same" must decide if "the same" means "the same representation" or "resolve to the same actual file", and choose the appropriate function accordingly. —end note]

bool operator!=(const path& lhs, const path& rhs);

Returns: !(lhs == rhs).

path operator/ (const path& lhs, const path& rhs);

Returns: path(lhs) /= rhs.

path inserter and extractor [path.io]

The inserter and extractor delimit the string with double-quotes (") so that paths with embedded spaces will round trip correctly. Ampersand (&) is as an escape character, so the path can itself contain double quotes.

template <class Char, class Traits>
std::basic_ostream<Char, Traits>& operator<<(std::basic_ostream<Char, Traits>& os,
                                             const path& p);

Effects: Insert characters into os:

• A double-quote.

• Each character in p.string<std::basic_string<Char>>(). If the character to be inserted is equal to the escape character or a double-quote, as determined by operator==, first insert the escape character.

• A double-quote.

[Note: Effects are similar to:

std::basic_string<Char> str(p.string<std::basic_string<Char>>());
os << boost::io::quoted(str, static_cast<Char>('&'));

—end note]

Returns: os

template <class Char, class Traits>
std::basic_istream<Char, Traits>& operator>>(std::basic_istream<Char, Traits>& is,
                                             path& p);

Effects:  Extract characters from is:

• If the first character that would be extracted is equal to double-quote, as determined by operator==, then:
  • Discard the initial double-quote.
  • Save the value and then turn off the skipws flag.
  • p.clear()
  • Until an unescaped double-quote character is reached or is.not_good(), extract characters from is and append them to p, except that if an escape character is reached, ignore it and append the next character to p.
  • Discard the final double-quote character.
  • Restore the skipws flag to its original value.
• Otherwise, is >> p.

[Note: Effects are similar to:

std::basic_string<Char> str;
is >> boost::io::quoted(str, static_cast<Char>('&'));
p = str;

—end note]

Returns: is

Class filesystem_error [class.filesystem_error]

namespace boost
{
  namespace filesystem
  {
      class filesystem_error : public system_error
      {
      public:
        filesystem_error();
        filesystem_error(const filesystem_error&);
        filesystem_error(const std::string& what_arg,
          system::error_code ec);
        filesystem_error(const std::string& what_arg,
          const path& p1, system::error_code ec);
        filesystem_error(const std::string& what_arg,
          const path& p1, const path& p2, system::error_code ec);

        filesystem_error& filesystem_error(const filesystem_error&);
       ~filesystem_error();

        filesystem_error& operator=(const filesystem_error&);

        const path& path1() const;
        const path& path2() const;

        const char * what() const;
      };
  }  // namespace filesystem
}  // namespace boost

The class template filesystem_error defines the type of objects thrown as exceptions to report file system errors from functions described in this reference documentation.

filesystem_error members [filesystem_error.members]

filesystem_error(const std::string& what_arg, error_code ec);

Postcondition:

Expression	Value
runtime_error::what()	what_arg.c_str()
code()	ec
path1().empty()	true
path2().empty()	true

filesystem_error(const std::string& what_arg, const path& p1, error_code ec);

Postcondition:

Expression	Value
runtime_error::what()	what_arg.c_str()
code()	ec
path1()	Reference to stored copy of p1
path2().empty()	true

filesystem_error(const std::string& what_arg, const path& p1, const path& p2, error_code ec);

Postcondition:

Expression	Value
runtime_error::what()	what_arg.c_str()
code()	ec
path1()	Reference to stored copy of p1
path2()	Reference to stored copy of p2

const path& path1() const;

Returns: Reference to copy of p1 stored by the constructor, or, if none, an empty path.

const path& path2() const;

Returns: Reference to copy of p2 stored by the constructor, or, if none, an empty path.

const char* what() const;

Returns: A string containing runtime_error::what(). The exact format is unspecified. Implementations are encouraged but not required to include path1.native_string()if not empty, path2.native_string()if not empty, and system_error::what() strings in the returned string.

Enum file_type [enum.file_type]

This enum specifies constants uses to identify file types.

Enum perms [enum.perms]

This enum specifies bitmask constants uses to identify file permissions. ISO/IEC 9945 (POSIX) specifies actual values, and those values have been adopted here because they are very familiar and ingrained for many POSIX users.

Windows: All permissions except write are currently ignored. There is only a single write permission; setting write permission for owner, group, or others sets write permission for all, and removing write permission for owner, group, or others removes write permission for all.

Class file_status [class.file_status]

namespace boost
{
  namespace filesystem
  {
      class file_status
      {
      public:

        // constructors
        file_status() noexcept;
        explicit file_status(file_type ft, perms prms = perms_not_known) noexcept;

        // compiler generated
        file_status(const file_status&) noexcept;
        file_status& operator=(const file_status&) noexcept;
       ~file_status() noexcept;

        // observers
        file_type  type() const noexcept;
        perms      permissions() const noexcept;

        // modifiers
        void       type(file_type ft) noexcept;
        void       permissions(perms prms) noexcept;
      };
  }  // namespace filesystem
}  // namespace boost

An object of type file_status stores information about the type and permissions of a file.

file_status constructors [file_status.cons]

explicit file_status() noexcept;

Postconditions: type() == status_error, permissions() == perms_not_known.

explicit file_status(file_type ft, perms prms = perms_not_known) noexcept;

Postconditions: type() == ft, permissions() == prms.

file_status observers [file_status.obs]

file_type type() const noexcept;

Returns: The value of type() specified by the postconditions of the most recent call to a constructor, operator=, or type(file_type) function.

perms permissions() const noexcept;

Returns: The value of permissions() specified by the postconditions of the most recent call to a constructor, operator=, or permissions(perms) function.

file_status modifiers [file_status.mods]

void type(file_type ft) noexcept;

Postconditions: type() == ft.

void permissions(perms prms) noexcept;

Postconditions: permissions() == prms.

Class directory_entry [class.directory_entry]

namespace boost
{
  namespace filesystem
  {
      class directory_entry
      {
      public:

        // constructors and destructor
        directory_entry();
        directory_entry(const directory_entry&);
        explicit directory_entry(const path& p);
        directory_entry(const path& p, system::error_code& ec);       // v4-only
        directory_entry(const path& p, file_status st,                // v3-only
          file_status symlink_st=file_status());
       ~directory_entry();

        // modifiers
        directory_entry& operator=(const directory_entry&);
        void assign(const path& p);
        void assign(const path& p, system::error_code& ec);           // v4-only
        void assign(const path& p, file_status st,                    // v3-only
          file_status symlink_st=file_status());
        void replace_filename(const path& p);
        void replace_filename(const path& p, system::error_code& ec); // v4-only
        void replace_filename(const path& p, file_status st,          // v3-only
          file_status symlink_st=file_status());

        void refresh();
        void refresh(system::error_code& ec);

        // observers
        const path&  path() const;

        file_status  status() const;
        file_status  status(system::error_code& ec) const;
        file_status  symlink_status() const;
        file_status  symlink_status(system::error_code& ec) const;
        file_type file_type() const;
        file_type file_type(system::error_code& ec) const;
        file_type symlink_file_type() const;
        file_type symlink_file_type(system::error_code& ec) const;

        bool exists() const;
        bool exists(system::error_code& ec) const;
        bool is_regular_file() const;
        bool is_regular_file(system::error_code& ec) const;
        bool is_directory() const;
        bool is_directory(system::error_code& ec) const;
        bool is_symlink() const;
        bool is_symlink(system::error_code& ec) const;
        bool is_block_file() const;
        bool is_block_file(system::error_code& ec) const;
        bool is_character_file() const;
        bool is_character_file(system::error_code& ec) const;
        bool is_fifo() const;
        bool is_fifo(system::error_code& ec) const;
        bool is_socket() const;
        bool is_socket(system::error_code& ec) const;
        bool is_reparse_file() const;
        bool is_reparse_file(system::error_code& ec) const;
        bool is_other() const;
        bool is_other(system::error_code& ec) const;

        bool operator< (const directory_entry& rhs);
        bool operator==(const directory_entry& rhs);
        bool operator!=(const directory_entry& rhs);
        bool operator< (const directory_entry& rhs);
        bool operator<=(const directory_entry& rhs);
        bool operator> (const directory_entry& rhs);
        bool operator>=(const directory_entry& rhs);
      private:
        path                 m_path;           // for exposition only
        mutable file_status  m_status;         // for exposition only; stat()-like
        mutable file_status  m_symlink_status; // for exposition only; lstat()-like
      };

  }  // namespace filesystem
}  // namespace boost

A directory_entry object stores a path object, as well as some amount of cached information about the file identified by the path. Currently, the cached information includes a file_status object for non-symbolic link status and a file_status object for symbolic link status.

[Note: Because status() on a pathname may be a relatively expensive operation, some operating systems provide status information as a byproduct of directory iteration. Caching such status information can result is significant time savings. Cached and non-cached results may differ in the presence of file system races. —end note]

Actual cold-boot timing of iteration over a directory with 15,047 entries was six seconds for non-cached status queries versus one second for cached status queries. Windows XP, 3.0 GHz processor, with a moderately fast hard-drive. Similar speedups are expected on Linux and BSD-derived systems that provide status as a by-product of directory iteration.

[Note: The exact set of cached information may vary from one Boost.Filesystem version to another, and also between different operating systems and underlying file systems. Users' code must not rely on whether a certain piece of information is cached or not. This means that calling most observers and modifiers of directory_entry may or may not result in a filesystem query that may potentially fail. Information caching is exclusively a performance feature aimed at reducing the amount of such queries. —end note]

directory_entry constructors [directory_entry.cons]

directory_entry();

Postcondition:

Expression	Value
path().empty()	true
status()	file_status()
symlink_status()	file_status()

explicit directory_entry(const path& p);
directory_entry(const path& p, system::error_code& ec); // v4-only

Effects:

v3: Initializes m_path from p and default-constructs m_status and m_symlink_status.

[Note: The cached file statuses will be updated when queried by the caller or by an explicit call to refresh. —end note]

v4: Initializes m_path from p and calls refresh() or refresh(ec), respectively.

Postcondition: path() == p if no error occurs, otherwise path().empty() == true.

directory_entry(const path& p, file_status st, file_status symlink_st=file_status()); // v3-only

v3: Postcondition:

Expression	Value
path()	p
status()	st
symlink_status()	symlink_st

directory_entry modifiers [directory_entry.mods]

void assign(const path& p);
void assign(const path& p, system::error_code& ec); // v4-only

Effects:

v3: Assigns p to m_path and file_status() to m_status and m_symlink_status.

[Note: The cached file statuses will be updated when queried by the caller or by an explicit call to refresh. —end note]

v4: Assigns p to m_path and calls refresh() or refresh(ec), respectively. If an error occurs, the value of the cached data is unspecified.

void assign(const path& p, file_status st, file_status symlink_st=file_status()); // v3-only

v3: Postcondition:

Expression	Value
path()	p
status()	st
symlink_status()	symlink_st

void replace_filename(const path& p);
void replace_filename(const path& p, system::error_code& ec); // v4-only

Effects:

v3: Calls m_path.replace_filename(p) and assigns file_status() to m_status and m_symlink_status.

[Note: The cached file statuses will be updated when queried by the caller or by an explicit call to refresh. —end note]

v4: Calls m_path.replace_filename(p) and then refresh() or refresh(ec), respectively. If an error occurs, the value of the cached data is unspecified.

void replace_filename(const path& p, file_status st, file_status symlink_st=file_status()); // v3-only

Effects: v3: Calls m_path.replace_filename(p) and assigns st to m_status and symlink_st to m_symlink_status.

void refresh();
void refresh(system::error_code& ec);

Effects: Updates any cached data by querying the filesystem about the file identified by m_path. If an error occurs, the value of the cached data is unspecified.

directory_entry observers [directory_entry.obs]

const path& path() const;

Returns: m_path

file_status status() const;
file_status status(system::error_code& ec) const;

Effects: If !status_known(m_status), calls refresh() or refresh(ec), respectively.

Returns: m_status

Throws: As specified in Error reporting.

file_status  symlink_status() const;
file_status  symlink_status(system::error_code& ec) const;

Effects: If !status_known(m_symlink_status), calls refresh() or refresh(ec), respectively.

Returns: m_symlink_status

Throws: As specified in Error reporting.

file_type file_type() const;
file_type file_type(system::error_code& ec) const;

Effects: Equivalent to status().type() or status(ec).type(), respectively.

[Note: The implementation may be more efficient than calling status, if the information about the file type is cached, but permissions are not. —end note]

file_type symlink_file_type() const;
file_type symlink_file_type(system::error_code& ec) const;

Effects: Equivalent to symlink_status().type() or symlink_status(ec).type(), respectively.

[Note: The implementation may be more efficient than calling symlink_status, if the information about the file type is cached, but permissions are not. —end note]

bool exists() const;
bool exists(system::error_code& ec) const;

Effects: Equivalent to exists(status()) or exists(status(ec)), respectively.

bool is_regular_file() const;
bool is_regular_file(system::error_code& ec) const;

Effects: Equivalent to is_regular_file(status()) or is_regular_file(status(ec)), respectively.

bool is_directory() const;
bool is_directory(system::error_code& ec) const;

Effects: Equivalent to is_directory(status()) or is_directory(status(ec)), respectively.

bool is_symlink() const;
bool is_symlink(system::error_code& ec) const;

Effects: Equivalent to is_symlink(symlink_status()) or is_symlink(symlink_status(ec)), respectively.

bool is_block_file() const;
bool is_block_file(system::error_code& ec) const;

Effects: Equivalent to is_block_file(status()) or is_block_file(status(ec)), respectively.

bool is_character_file() const;
bool is_character_file(system::error_code& ec) const;

Effects: Equivalent to is_character_file(status()) or is_character_file(status(ec)), respectively.

bool is_fifo() const;
bool is_fifo(system::error_code& ec) const;

Effects: Equivalent to is_fifo(status()) or is_fifo(status(ec)), respectively.

bool is_socket() const;
bool is_socket(system::error_code& ec) const;

Effects: Equivalent to is_socket(status()) or is_socket(status(ec)), respectively.

bool is_reparse_file() const;
bool is_reparse_file(system::error_code& ec) const;

Effects: Equivalent to is_reparse_file(symlink_status()) or is_reparse_file(symlink_status(ec)), respectively.

bool is_other() const;
bool is_other(system::error_code& ec) const;

Effects: Equivalent to is_other(status()) or is_other(status(ec)), respectively.

bool operator==(const directory_entry& rhs);

Returns: m_path == rhs.m_path.

bool operator!=(const directory_entry& rhs);

Returns: m_path != rhs.m_path.

bool operator< (const directory_entry& rhs);

Returns: m_path < rhs.m_path.

bool operator<=(const directory_entry& rhs);

Returns: m_path <= rhs.m_path.

bool operator> (const directory_entry& rhs);

Returns: m_path > rhs.m_path.

bool operator>=(const directory_entry& rhs);

Returns: m_path >= rhs.m_path.

Class directory_iterator [class.directory_iterator]

Objects of type directory_iterator provide standard library compliant iteration over the contents of a directory. Also see class recursive_directory_iterator.

namespace boost
{
  namespace filesystem
  {
      class directory_iterator
      {
      public:
        // member functions

        directory_iterator() noexcept;  // creates the "end" iterator
        directory_iterator(const directory_iterator&);
        explicit directory_iterator(const path& p, directory_options opts = directory_options::none);
        directory_iterator(const path& p, system::error_code& ec);
        directory_iterator(const path& p, directory_options opts, system::error_code& ec);
       ~directory_iterator();

        directory_iterator& operator=(const directory_iterator&);

        directory_iterator& operator++();
        directory_iterator& increment(system::error_code& ec);

        // other members as required by
        //  C++ Std, 24.1.1 Input iterators [input.iterators]
      };

  }  // namespace filesystem
}  // namespace boost

directory_iterator satisfies the requirements of an input iterator (C++ Std, 24.2.1, Input iterators [input.iterators]).

A directory_iterator reads successive elements from the directory for which it was constructed, as if by calling ISO/IEC 9945 readdir() or readdir_r(). After a directory_iterator is constructed, and every time operator++ is called, it reads a directory element and stores information about it in an object of type directory_entry. operator++ is not equality preserving; that is, i == j does not imply that ++i == ++j.

[Note: The practical consequence of not preserving equality is that directory iterators can only be used for single-pass algorithms. —end note]

If the end of the directory elements is reached, the iterator shall become equal to the end iterator value. The constructor directory_iterator() with no arguments always constructs an end iterator object, which shall be the only valid iterator for the end condition. The result of operator* on an end iterator is not defined. For any other iterator value a const directory_entry& is returned. The result of operator-> on an end iterator is undefined behavior. For any other iterator value a const directory_entry* is returned.

Two end iterators are always equal. An end iterator shall not be equal to a non-end iterator.

The above wording is based on the Standard Library's istream_iterator wording.

The result of calling the path() member of the directory_entry object obtained by dereferencing a directory_iterator is a reference to a path object composed of the directory argument from which the iterator was constructed with filename of the directory entry appended as if by operator/=.

Directory iteration shall not yield directory entries for the current (dot) and parent (dot dot) directories.

The order of directory entries obtained by dereferencing successive increments of a directory_iterator is unspecified.

[Note: Programs performing directory iteration may wish to test if the path obtained by dereferencing a directory iterator actually exists. It could be a symbolic link to a non-existent file. Programs recursively walking directory trees for purposes of removing and renaming entries may wish to avoid following symbolic links.

If a file is removed from or added to a directory after the construction of a directory_iterator for the directory, it is unspecified whether or not subsequent incrementing of the iterator will ever result in an iterator whose value is the removed or added directory entry. See ISO/IEC 9945 readdir_r(). —end note]

directory_iterator members [directory_iterator.members]

directory_iterator() noexcept;

Effects: Constructs the end iterator.

explicit directory_iterator(const path& p, directory_options opts = directory_options::none);
directory_iterator(const path& p, system::error_code& ec);
directory_iterator(const path& p, directory_options opts, system::error_code& ec);

Effects: Constructs an iterator representing the first entry in the directory p resolves to, if any; otherwise, the end iterator. If opening the directory fails with a permission_denied error and (opts & directory_options::skip_permission_denied) != 0, constructs the end iterator and ignores the error (the operation completes successfully). If opts is not specified, it is assumed to be directory_options::none.

Throws: As specified in Error reporting.

[Note: To iterate over the current directory, use directory_iterator(".") rather than directory_iterator(""). —end note]

directory_iterator& operator++();
directory_iterator& increment(system::error_code& ec);

Effects: As specified by the C++ Standard, 24.1.1 Input iterators [input.iterators]. In case of error the iterator is left in the end state.

Returns: *this.

Throws: As specified in Error reporting.

directory_iterator non-member functions

const directory_iterator& begin(const directory_iterator& iter);

Returns: iter.

directory_iterator end(const directory_iterator&);

Returns: directory_iterator().

Class recursive_directory_iterator [class.rec.dir.itr]

Objects of type recursive_directory_iterator provide standard library compliant iteration over the contents of a directory, including recursion into its sub-directories.

namespace boost
{
  namespace filesystem
  {
      class recursive_directory_iterator :
        public iterator<input_iterator_tag, directory_entry>
      {
      public:

        // constructors and destructor
        recursive_directory_iterator() noexcept;
        recursive_directory_iterator(const recursive_directory_iterator&);
        explicit recursive_directory_iterator(const path& p,
          directory_options opts = directory_options::none);
        recursive_directory_iterator(const path& p,
          directory_options opts, system::error_code& ec);
        recursive_directory_iterator(const path& p, system::error_code& ec);
       ~recursive_directory_iterator();

        // observers
        int depth() const noexcept;
        bool recursion_pending() const noexcept;

        // modifiers
        recursive_directory_iterator& operator=(const recursive_directory_iterator&);

        recursive_directory_iterator& operator++();
        recursive_directory_iterator& increment(system::error_code& ec);

        void pop();
        void pop(system::error_code& ec);
        void disable_recursion_pending(bool value = true) noexcept;

        // other members as required by
        //  C++ Std, Input iterators [input.iterators]

      private:
        // actual data members will probably be stored in a shared object,
        // or some similar mechanism, to achieve the required input iterator
        // copy semantics
        int            m_depth;              // for exposition only
        bool           m_recursion_pending;  // for exposition only
        directory_options m_options;         // for exposition only
      };

  }  // namespace filesystem
}  // namespace boost

The behavior of a recursive_directory_iterator is the same as a directory_iterator unless otherwise specified.

• Incrementing a recursive_directory_iterator pointing to a directory causes that directory itself to be iterated over, as specified by the operator++ and increment functions.
   
• When a recursive_directory_iterator reaches the end of the directory currently being iterated over, or when pop() is called, m_depth is decremented, and iteration of the parent directory continues.

recursive_directory_iterator() noexcept;

Effects: Constructs the end iterator.

explicit recursive_directory_iterator(const path& p, directory_options opts = directory_options::none);
recursive_directory_iterator(const path& p, directory_options opts, system::error_code& ec);
recursive_directory_iterator(const path& p, system::error_code& ec);

Effects:  Constructs an iterator representing the first entry in the directory p resolves to, if any; otherwise, the end iterator.

Postcondition: Unless the end iterator was constructed, depth() == 0 && recursion_pending() == true && m_options == opts. For the signature without the opts argument, opts is assumed to be directory_options::none.

Throws: As specified in Error reporting.

[Note: To iterate over the current directory, use recursive_directory_iterator(".") rather than recursive_directory_iterator(""). —end note]

[Note: By default, recursive_directory_iterator does not follow directory symlinks. To follow directory symlinks, specify directory_options::follow_directory_symlink in opts. —end note]

int depth() const noexcept;

Requires: *this != recursive_directory_iterator().

Returns: m_depth.

bool recursion_pending() const noexcept;

Requires: *this != recursive_directory_iterator().

Returns: m_recursion_pending.

recursive_directory_iterator& operator++();
recursive_directory_iterator& increment(system::error_code& ec);

Effects: As specified by the C++ Standard, 24.1.1 Input iterators [input.iterators], except:

• if recursion_pending() && is_directory(this->status()) && (!is_symlink(this->symlink_status()) || (m_options & directory_options::follow_directory_symlink) != 0) then:

  • directory (*this)->path() is opened and recursively iterated into and m_depth is incremented;

  • if opening the directory fails with a permission_denied error and (m_options & directory_options::skip_permission_denied) != 0, increment the iterator on the current level and ignore the error (the operation completes successfully).

• if there are no more directory entries at this level then m_depth is decremented and iteration of the parent directory resumes.

If the operation completes with an error, then

• if (m_options & directory_options::pop_on_error) != 0, the iterator is left in a state as if after repeatedly calling pop() until it succeeds or the iterator becomes equal to an end iterator; any pop() failures are not reported to the caller;
• otherwise, the iterator is left equal to an end iterator.

Postcondition: recursion_pending() == true.

Returns: *this.

Throws: As specified in Error reporting.

void pop();
void pop(system::error_code& ec);

Requires: *this != recursive_directory_iterator().

Effects: If depth() == 0, set *this to recursive_directory_iterator(). Otherwise, --m_depth, cease iteration of the directory currently being iterated over, and continue iteration over the parent directory.

If the operation completes with an error, then

• if (m_options & directory_options::pop_on_error) != 0, the iterator is left in a state as if after repeatedly calling pop() until it succeeds or the iterator becomes equal to an end iterator; any pop() failures are not reported to the caller;
• otherwise, the iterator is left equal to an end iterator.

void disable_recursion_pending(bool value = true) noexcept;

Requires: *this != recursive_directory_iterator().

Postcondition: recursion_pending() == !value.

[Note: These functions are used to prevent unwanted recursion into a directory. —end note]

recursive_directory_iterator non-member functions

const recursive_directory_iterator& begin(const recursive_directory_iterator& iter);

Returns: iter.

recursive_directory_iterator end(const recursive_directory_iterator&);

Returns: recursive_directory_iterator().

Operational functions [fs.op.funcs]

Operational functions query or modify files, including directories, in external storage.

Operational functions access a file by resolving an object of class path to a particular file in a file hierarchy. The path is resolved as if by the ISO/IEC 9945 Pathname Resolution mechanism.

[Note: Because hardware failures, network failures, file system races, and many other kinds of errors occur frequently in file system operations, users should be aware that any filesystem operational function, no matter how apparently innocuous, may encounter an error. See Error reporting. —end note]

path absolute(const path& p, const path& base=current_path());
path absolute(const path& p, system::error_code& ec);
path absolute(const path& p, const path& base, system::error_code& ec);

Returns: If p.is_absolute() is true then p, otherwise an absolute path composed according to the following table:

	p. has_root_directory()	!p.has_root_directory()
p.has_root_name()	return p	return  p.root_name()  / absolute(base)     .root_directory()  / absolute(base)     .relative_path()  / p.relative_path()
!p.has_root_name()	return  absolute(base)   .root_name()  / p	return absolute(base)  / p

For the overload without a base argument, base is current_path().

[Note: For the returned path, rp, rp.is_absolute() is true. —end note]

Throws: As specified in Error reporting.

path canonical(const path& p, const path& base = current_path());
path canonical(const path& p, system::error_code& ec);
path canonical(const path& p, const path& base, system::error_code& ec);

Overview: Converts p to an absolute path that has no symbolic link, dot, or dot-dot elements. Testing path elements for symbolic links is done as if by calling is_symlink and requires absolute(p, base) to exist.

Returns: A canonical path that refers to the same file system object as absolute(p, base). For the overload without a base argument, base is current_path().

Throws:  As specified in Error reporting.

Remarks: !exists(absolute(p, base)) is an error.

[Note: Canonical pathnames allow security checking of a path (e.g. does this path live in /home/goodguy or /home/badguy?) —end note]

void copy(const path& from, const path& to);
void copy(const path& from, const path& to, system::error_code& ec);

Effects: copy(from, to, copy_options::none[, ec]).

Throws: As specified in Error reporting.

void copy(const path& from, const path& to, copy_options options);
void copy(const path& from, const path& to, copy_options options, system::error_code& ec);

Precondition: options must contain at most one option from each of the following groups:

• copy_options::skip_existing, copy_options::overwrite_existing or copy_options::update_existing;
• copy_options::synchronize_data or copy_options::synchronize;
• copy_options::ignore_attribute_errors;
• copy_options::recursive;
• copy_options::copy_symlinks or copy_options::skip_symlinks;
• copy_options::directories_only, copy_options::create_symlinks or copy_options::create_hard_links.

Effects: Let f and t be file_status objects obtained the following way:

• If (options & (copy_options::create_symlinks | copy_options::skip_symlinks)) != copy_options::none then f = symlink_status(from) and t = symlink_status(to);
• Otherwise, if (options & copy_options::copy_symlinks) != copy_options::none then f = symlink_status(from) and t = status(to);
• Otherwise, f = status(from) and t = status(to).

Then, report an error if:

• !exists(f), or
• equivalent(from, to), or
• is_other(f) || is_other(t), or
• is_directory(f) && is_regular_file(t).

Otherwise, if is_symlink(f), then:

• If (options & copy_options::skip_symlinks) != copy_options::none then return;
• Otherwise if !exists(t) && (options & copy_options::copy_symlinks) != copy_options::none then copy_symlink(from, to);
• Otherwise report error.

Otherwise, if is_regular_file(f), then:

• If (options & copy_options::directories_only) != copy_options::none then return;
• Otherwise if (options & copy_options::create_symlinks) != copy_options::none then create_symlink(link, to), where link is determined as follows:
  • If from.is_absolute() then link equals to from;
  • Otherwise, link is equivalent to relative(absolute(to.parent_path()), absolute(from.parent_path())) / from.filename().
• Otherwise if (options & copy_options::create_hard_links) != copy_options::none then create_hard_link(from, to);
• Otherwise if is_directory(t) then copy_file(from, to / from.filename(), options);
• Otherwise copy_file(from, to, options).

Otherwise, if is_directory(f), then:

• If (options & copy_options::create_symlinks) != copy_options::none then report error with error code equal to make_error_code(system::errc::is_a_directory);
• Otherwise if
  • (options & copy_options::recursive) != copy_options::none, or
  • options == copy_options::none and this call to copy is not a recursive call from copy
  then:
  • If !exists(t), then create_directory(to, from).
  • Then, iterate over files in from and for each directory_entry x obtained during iteration invoke copy(x.path(), to / x.path().filename(), options).
• Otherwise, return.

Otherwise, for all unsupported file types of f report error.

Throws: As specified in Error reporting.

bool copy_file(const path& from, const path& to);
bool copy_file(const path& from, const path& to, system::error_code& ec);

Effects: return copy_file(from, to, copy_options::none[, ec]).

Throws: As specified in Error reporting.

bool copy_file(const path& from, const path& to, copy_options options);
bool copy_file(const path& from, const path& to, copy_options options, system::error_code& ec);

Precondition: options must contain at most one option from each of the following groups:

• copy_options::skip_existing, copy_options::overwrite_existing or copy_options::update_existing;
• copy_options::synchronize_data or copy_options::synchronize;
• copy_options::ignore_attribute_errors.

Effects: Report an error if:

• !is_regular_file(from), or
• exists(to) && !is_regular_file(to), or
• exists(to) && equivalent(from, to), or
• exists(to) && (options & (copy_options::skip_existing | copy_options::overwrite_existing)) == copy_options::none.

Otherwise, return successfully with no effect if:

• exists(to) && (options & copy_options::skip_existing) != copy_options::none, or
• exists(to) && (options & copy_options::update_existing) != copy_options::none and last write time of to is more recent than that of from.

Otherwise:

• The contents and attributes of the file from resolves to are copied to the file to resolves to. If copying file attributes (but not contents) fails with an error and (options & copy_options::ignore_attribute_errors) != copy_options::none then that error is ignored. After that,
• If (options & copy_options::synchronize) != copy_options::none, the written data and attributes are synchronized with the permanent storage; otherwise
• If (options & copy_options::synchronize_data) != copy_options::none, the written data is synchronized with the permanent storage.

Returns: true if the file was copied without error, otherwise false.

Throws: As specified in Error reporting.

[Note: When copy_options::update_existing is specified, checking the write times of from and to may not be atomic with the copy operation. Another process may create or modify the file identified by to after the file modification times have been checked but before copying starts. In this case the target file will be overwritten.]

[Note: The copy_options::synchronize_data and copy_options::synchronize options may have a significant performance impact. The copy_options::synchronize_data option may be less expensive than copy_options::synchronize. However, without these options, upon returning from copy_file it is not guaranteed that the copied file is completely written and preserved in case of a system failure. Any delayed write operations may fail after the function returns, at the point of physically writing the data to the underlying media, and this error will not be reported to the caller.]

[Note: The copy_options::ignore_attribute_errors option can be used when the caller does not require file attributes to be copied. The implementation is permitted to make an attempt to copy the file attributes, but still succeed the file copying operation if that attempt fails. This option may be useful with file systems that do not fully support operations of file attributes.]

void copy_symlink(const path& existing_symlink, const path& new_symlink);
void copy_symlink(const path& existing_symlink, const path& new_symlink, system::error_code& ec);

Effects: create_symlink(read_symlink(existing_symlink[, ec]), new_symlink[, ec]).

Throws: As specified in Error reporting.

bool create_directories(const path& p);
bool create_directories(const path& p, system::error_code& ec);

Effects: Establishes the postcondition by calling create_directory() for any element of p that does not exist.

Postcondition: is_directory(p)

Returns: true if a new directory was created, otherwise false.

Throws: As specified in Error reporting.

Complexity: O(n+1) where n is the number of elements of p that do not exist.

bool create_directory(const path& p);
bool create_directory(const path& p, system::error_code& ec);
bool create_directory(const path& p, const path& existing);
bool create_directory(const path& p, const path& existing, system::error_code& ec);

Effects: Establishes the postcondition by attempting to create the directory p resolves to, as if by ISO/IEC 9945 mkdir(). For overloads without existing argument, the new directory is created with S_IRWXU|S_IRWXG|S_IRWXO mode. Overloads with existing argument obtain mode from existing, which must be a path to an existing directory. On Windows, CreateDirectoryW(p.c_str(), NULL) is used when existing is not specified and CreateDirectoryExW(existing.c_str(), p.c_str(), NULL) otherwise. Creation failure because p resolves to an existing directory shall not be treated as an error.

Postcondition: is_directory(p)

Returns: true if a new directory was created, otherwise false.

Throws: As specified in Error reporting.

void create_directory_symlink(const path& to, const path& new_symlink);
void create_directory_symlink(const path& to, const path& new_symlink, system::error_code& ec);

Effects: Establishes the postcondition, as if by ISO/IEC 9945 symlink().

Postcondition: new_symlink resolves to a symbolic link file that contains an unspecified representation of to.

Throws: As specified in Error reporting.

[Note: Some operating systems, such as Windows, require symlink creation to identify that the link is to a directory. Portable code should use create_directory_symlink() to create directory symlinks rather than create_symlink() —end note]

[Note: Some operating systems do not support symbolic links at all or support them only for regular files. Some file systems do not support symbolic links regardless of the operating system - the FAT file system used on memory cards and flash drives, for example. —end note]

void create_hard_link(const path& to, const path& new_hard_link);
void create_hard_link(const path& to, const path& new_hard_link, system::error_code& ec);

Effects: Establishes the postcondition, as if by ISO/IEC 9945 link().

Postcondition:

•  exists(to) && exists(new_hard_link) && equivalent(to, new_hard_link)
• The contents of the file or directory to resolves to are unchanged.

Throws: As specified in Error reporting.

[Note: Some operating systems do not support hard links at all or support them only for regular files. Some file systems do not support hard links regardless of the operating system - the FAT file system used on memory cards and flash drives, for example. Some file systems limit the number of links per file. —end note]

void create_symlink(const path& to, const path& new_symlink);
void create_symlink(const path& to, const path& new_symlink, system::error_code& ec);

Effects: Establishes the postcondition, as if by ISO/IEC 9945 symlink().

Postcondition: new_symlink resolves to a symbolic link file that contains an unspecified representation of to.

Throws: As specified in Error reporting.

[Note: Some operating systems do not support symbolic links at all or support them only for regular files. Some file systems do not support symbolic links regardless of the operating system - the FAT system used on memory cards and flash drives, for example. —end note]

path current_path();
path current_path(system::error_code& ec);

Returns: The current working directory path, as if by ISO/IEC 9945 getcwd(). is_absolute() is true for the returned path.

Throws: As specified in Error reporting.

[Note: The current_path() name was chosen to emphasize that the return is a path, not just a single directory name.

The current path as returned by many operating systems is a dangerous global variable. It may be changed unexpectedly by a third-party or system library functions, or by another thread.  —end note]

void current_path(const path& p);
void current_path(const path& p, system::error_code& ec);

Effects: Establishes the postcondition, as if by ISO/IEC 9945 chdir().

Postcondition: equivalent(p, current_path()).

Throws: As specified in Error reporting.

[Note: The current path for many operating systems is a dangerous global state. It may be changed unexpectedly by a third-party or system library functions, or by another thread.  —end note]

bool exists(file_status s) noexcept;

Returns: status_known(s) && s.type() != file_not_found

bool exists(const path& p);
bool exists(const path& p, system::error_code& ec) noexcept;

Returns: exists(status(p)) or exists(status(p, ec)), respectively.

Throws: As specified in Error reporting.

bool equivalent(const path& p1, const path& p2);
bool equivalent(const path& p1, const path& p2, system::error_code& ec);

Returns: true, if p1 and p2 resolve to the same file system entity, else false.

Two paths are considered to resolve to the same file system entity if two candidate entities reside on the same device at the same location. This is determined as if by the values of the ISO/IEC 9945 stat structure, obtained as if by stat() for the two paths, having equal st_dev values and equal st_ino values.

[Note: ISO/IEC 9945 requires that "st_dev must be unique within a Local Area Network". Conservative ISO/IEC 9945 implementations may also wish to check for equal st_size and st_mtime values. Windows implementations may use GetFileInformationByHandle() as a surrogate for stat(), and consider "same" to be equal values for dwVolumeSerialNumber, nFileIndexHigh, nFileIndexLow, nFileSizeHigh, nFileSizeLow, ftLastWriteTime.dwLowDateTime, and ftLastWriteTime.dwHighDateTime. —end note]

Throws: As specified in Error reporting. [v3: !exists(p1) && !exists(p2) is an error. If only one of the paths doesn't exist, false is returned without error.] [v4: !exists(p1) || !exists(p2) is an error.]

uintmax_t file_size(const path& p);
uintmax_t file_size(const path& p, system::error_code& ec);

Returns: If exists(p) && is_regular_file(p), the size in bytes of the file p resolves to, determined as if by the value of the ISO/IEC 9945 stat structure member st_size obtained as if by ISO/IEC 9945 stat(). Otherwise, static_cast<uintmax_t>(-1).

Throws: As specified in Error reporting.

uintmax_t hard_link_count(const path& p);
uintmax_t hard_link_count(const path& p, system::error_code& ec);

Returns: The number of hard links for p.

Throws: As specified in Error reporting.

const path& initial_path();
const path& initial_path(system::error_code& ec);

Returns: current_path() as of the first call to initial_path().

[Note: initial_path() is not thread safe, and may return an undesirable result if called subsequent to a change to the current directory. These problems can be avoided by calling initial_path() immediately on entry to main().  —end note]

Throws: For the first call, as specified in Error reporting. Subsequent calls throw nothing.

bool is_block_file(file_status s) noexcept;

Returns: s.type() == block_file

bool is_block_file(const path& p);
bool is_block_file(const path& p, system::error_code& ec) noexcept;

Returns: is_block_file(status(p)) or is_block_file(status(p, ec)), respectively. The overload with argument ec returns false if an error occurs.

Throws: filesystem_error; the overload with argument ec throws nothing.

bool is_character_file(file_status s) noexcept;

Returns: s.type() == character_file

bool is_character_file(const path& p);
bool is_character_file(const path& p, system::error_code& ec) noexcept;

Returns: is_character_file(status(p)) or is_character_file(status(p, ec)), respectively. The overload with argument ec returns false if an error occurs.

Throws: filesystem_error; the overload with argument ec throws nothing.

bool is_directory(file_status s) noexcept;

Returns: s.type() == directory_file

bool is_directory(const path& p);
bool is_directory(const path& p, system::error_code& ec) noexcept;

Returns: is_directory(status(p)) or is_directory(status(p, ec)), respectively. The overload with argument ec returns false if an error occurs.

Throws: filesystem_error; the overload with argument ec throws nothing.

bool is_empty(const path& p);
bool is_empty(const path& p, system::error_code& ec);

Effects: Determines file_status s, as if by status(p, ec).

Returns: is_directory(s)
         ? directory_iterator(p) == directory_iterator()
         : file_size(p) == 0;

bool is_fifo(file_status s) noexcept;

Returns: s.type() == fifo_file

bool is_fifo(const path& p);
bool is_fifo(const path& p, system::error_code& ec) noexcept;

Returns: is_fifo(status(p)) or is_fifo(status(p, ec)), respectively. The overload with argument ec returns false if an error occurs.

Throws: filesystem_error; the overload with argument ec throws nothing.

bool is_other(file_status s) noexcept;

Returns: return exists(s) && !is_regular_file(s) && !is_directory(s) && !is_symlink(s)

bool is_other(const path& p);
bool is_other(const path& p, system::error_code& ec) noexcept;

Returns: is_other(status(p)) or is_other(status(p, ec)), respectively. The overload with argument ec returns false if an error occurs.

Throws: filesystem_error; the overload with argument ec throws nothing.

bool is_regular_file(file_status s) noexcept;

Returns: s.type() == regular_file

bool is_regular_file(const path& p);
bool is_regular_file(const path& p, system::error_code& ec) noexcept;

Returns: is_regular_file(status(p)) or is_regular_file(status(p, ec)), respectively. The overload with argument ec returns false if an error occurs.

Throws: filesystem_error; the overload with argument ec throws nothing.

bool is_reparse_file(file_status s) noexcept;

Returns: s.type() == reparse_file

bool is_reparse_file(const path& p);
bool is_reparse_file(const path& p, system::error_code& ec) noexcept;

Returns: is_reparse_file(symlink_status(p)) or is_reparse_file(symlink_status(p, ec)), respectively. The overload with argument ec returns false if an error occurs.

Throws: filesystem_error; the overload with argument ec throws nothing.

bool is_socket(file_status s) noexcept;

Returns: s.type() == socket_file

bool is_socket(const path& p);
bool is_socket(const path& p, system::error_code& ec) noexcept;

Returns: is_socket(status(p)) or is_socket(status(p, ec)), respectively. The overload with argument ec returns false if an error occurs.

Throws: filesystem_error; the overload with argument ec throws nothing.

bool is_symlink(file_status s) noexcept;

Returns: s.type() == symlink_file

bool is_symlink(const path& p);
bool is_symlink(const path& p, system::error_code& ec) noexcept;

Returns: is_symlink(symlink_status(p)) or is_symlink(symlink_status(p, ec)), respectively. The overload with argument ec returns false if an error occurs.

Throws: filesystem_error; the overload with argument ec throws nothing.

std::time_t creation_time(const path& p);
std::time_t creation_time(const path& p, system::error_code& ec);

Returns: The time of creation of the file to which p resolves.

Throws: As specified in Error reporting.

[Note: Not all platforms support querying file creation time. Where not supported, the operation will fail with errc::function_not_supported error code. —end note]

std::time_t last_write_time(const path& p);
std::time_t last_write_time(const path& p, system::error_code& ec);

Returns: The time of last data modification of p, determined as if by the value of the ISO/IEC 9945 stat structure member st_mtime  obtained as if by ISO/IEC 9945 stat().

Throws: As specified in Error reporting.

void last_write_time(const path& p, const std::time_t new_time);
void last_write_time(const path& p, const std::time_t new_time, system::error_code& ec);

Effects: Sets the time of last data modification of the file resolved to by p to new_time, as if by ISO/IEC 9945 stat() followed by ISO/IEC 9945 utime().

Throws: As specified in Error reporting.

[Note: A postcondition of last_write_time(p) == new_time is not specified since it might not hold for file systems with coarse time granularity. —end note]

void permissions(const path& p, perms prms);
void permissions(const path& p, perms prms, system::error_code& ec);

Requires: !((prms & add_perms) && (prms & remove_perms)).

Effects: Applies the effective permissions bits from prms to the file p resolves to, as if by ISO/IEC 9945 fchmodat(). The effective permission bits are determined as specified by the following table.

bits present in prms	Effective bits applied
Neither add_perms nor remove_perms	prms & perms_mask
add_perms	status(p).permissions() | (prms & perms_mask)
remove_perms	status(p).permissions() & ~(prms & perms_mask)

[Note: Conceptually permissions are viewed as bits, but the actual implementation may use some other mechanism. —end note]

Throws: As specified in Error reporting.

path read_symlink(const path& p);
path read_symlink(const path& p, system::error_code& ec);

Returns:  If p resolves to a symbolic link, a path object containing the contents of that symbolic link. Otherwise an empty path object.

Throws: As specified in Error reporting. [Note: It is an error if p does not resolve to a symbolic link. —end note]

path relative(const path& p, system::error_code& ec);

Returns: relative(p, current_path(), ec).

Throws:  As specified in Error reporting.

path relative(const path& p, const path& base=current_path());
path relative(const path& p, const path& base, system::error_code& ec);

Overview: Returns p made relative to base. Treats empty or identical paths as corner cases, not errors. Resolves symlinks and normalizes both p and base before other processing.

Returns: weakly_canonical(p).lexically_relative(weakly_canonical(base)). The second form returns path() if an error occurs.

Throws: As specified in Error reporting.

bool remove(const path& p);
bool remove(const path& p, system::error_code& ec);

Effects:  If exists(symlink_status(p,ec)), it is removed as if by ISO/IEC 9945 remove().

[Note: A symbolic link is itself removed, rather than the file it resolves to being removed. —end note]

Postcondition: !exists(symlink_status(p)).

Returns:  false if p did not exist, otherwise true.

Throws: As specified in Error reporting.

uintmax_t remove_all(const path& p);
uintmax_t remove_all(const path& p, system::error_code& ec);

Effects:  Recursively deletes the contents of p if it exists, then deletes file p itself, as if by ISO/IEC 9945 remove().

[Note: A symbolic link is itself removed, rather than the file it resolves to being removed. —end note]

Postcondition: !exists(p)

Returns: The number of files removed.

Throws: As specified in Error reporting.

void rename(const path& old_p, const path& new_p);
void rename(const path& old_p, const path& new_p, system::error_code& ec);

Effects: Renames old_p to new_p, as if by ISO/IEC 9945 rename().

[Note: If old_p and new_p resolve to the same existing file, no action is taken. Otherwise, if new_p resolves to an existing non-directory file, it is removed, while if new_p resolves to an existing directory, it is removed if empty on ISO/IEC 9945 but is an error on Windows. A symbolic link is itself renamed, rather than the file it resolves to being renamed. —end note]

Throws: As specified in Error reporting.

void resize_file(const path& p, uintmax_t new_size);
void resize_file(const path& p, uintmax_t new_size, system::error_code& ec);

Postcondition: file_size() == new_size.

Throws: As specified in Error reporting.

Remarks: Achieves its postconditions as if by ISO/IEC 9945 truncate().

space_info space(const path& p);
space_info space(const path& p, system::error_code& ec);

Returns: An object of type space_info. The value of the space_info object is determined as if by using ISO/IEC 9945 statvfs() to obtain an ISO/IEC 9945 struct statvfs, and then multiplying its f_blocks, f_bfree, and f_bavail members by its f_frsize member, and assigning the results to the capacity, free, and available members respectively. Any members for which the value cannot be determined shall be set to -1.

Throws: As specified in Error reporting.

file_status status(const path& p);

Effects: As if:

system::error_code ec;
file_status result = status(p, ec);
if (result == status_error)
  throw filesystem_error(implementation-supplied-message, p, ec);
return result;

Returns: See above.

Throws: filesystem_error. [Note: result values of file_status(file_not_found)and file_status(type_unknown) are not considered failures and do not cause an exception to be thrown. —end note]

file_status status(const path& p, system::error_code& ec) noexcept;

Effects:

If possible, determines the attributes of the file p resolves to, as if by ISO/IEC 9945 stat().

If, during attribute determination, the underlying file system API reports an error, sets ec to indicate the specific error reported. Otherwise, ec.clear().

[Note: This allows users to inspect the specifics of underlying API errors even when the value returned by status() is not file_status(status_error).  —end note]

Returns:

If ec != error_code():

• If the specific error indicates that p cannot be resolved because some element of the path does not exist, return file_status(file_not_found). [Note: ISO/IEC 9945 errors that indicate this are ENOENT or ENOTDIR. Windows equivalents include ERROR_FILE_NOT_FOUND, ERROR_PATH_NOT_FOUND, ERROR_INVALID_NAME, ERROR_INVALID_PARAMETER, ERROR_BAD_PATHNAME, and ERROR_BAD_NETPATH. -- end note]
   
• Otherwise, if the specific error indicates that p can be resolved but the attributes cannot be determined, return file_status(type_unknown). [Note: For example, Windows ERROR_SHARING_VIOLATION errors. For ISO/IEC 9945, the case never arises. —end note]
   
• Otherwise, return file_status(status_error).

[Note: These semantics distinguish between p being known not to exist, p existing but not being able to determine its attributes, and there being an error that prevents even knowing if p exists. These distinctions are important to some use cases. —end note]

Otherwise,

• If the attributes indicate a regular file, as if by ISO/IEC 9945 S_ISREG(), return file_status(regular_file). [Note: regular_file implies appropriate <fstream> operations would succeed, assuming no hardware, permission, access, or file system race errors. Lack of regular_file does not necessarily imply <fstream> operations would fail on a directory. —end note]
   
• Otherwise, if the attributes indicate a directory, as if by ISO/IEC 9945 S_ISDIR(), return file_status(directory_file). [Note: directory_file implies directory_iterator(p)would succeed. —end note]
   
• Otherwise, if the attributes indicate a block special file, as if by ISO/IEC 9945 S_ISBLK(), return file_status(block_file).
   
• Otherwise, if the attributes indicate a character special file, as if by ISO/IEC 9945 S_ISCHR(), return file_status(character_file).
   
• Otherwise, if the attributes indicate a fifo or pipe file, as if by ISO/IEC 9945 S_ISFIFO(), return file_status(fifo_file).
   
• Otherwise, if the attributes indicate a socket, as if by ISO/IEC 9945 S_ISSOCK(), return file_status(socket_file).
   
• Otherwise, return file_status(type_unknown).

Remarks: If a symbolic link is encountered during pathname resolution, pathname resolution continues using the contents of the symbolic link.

bool status_known(file_status s) noexcept;

Returns: s.type() != status_error

file_status symlink_status(const path& p);
file_status symlink_status(const path& p, system::error_code& ec) noexcept;

Effects:  Same as status(), above, except that the attributes of p are determined as if by ISO/IEC 9945 lstat().

Returns: Same as status(), above, except that if the attributes indicate a symbolic link, as if by ISO/IEC 9945 S_ISLNK(), return file_status(symlink_file).

Remarks: Pathname resolution terminates if p names a symbolic link.

Throws: filesystem_error; overload with error_code& throws nothing.

path system_complete(const path& p);
path system_complete(const path& p, system::error_code& ec);

Effects: Composes an absolute path from p, using the same rules used by the operating system to resolve a path passed as the filename argument to standard library open functions.

Returns: The composed path.

Postcondition: For the returned path, rp, rp.is_absolute() is true.

Throws: As specified in Error reporting.

[Note: For ISO/IEC 9945, system_complete(p) has the same semantics as absolute(p, current_path()).

For Windows, system_complete(p) has the same semantics as absolute(p, current_path()) if p.is_absolute() || !p.has_root_name() or p and base have the same root_name(). Otherwise it acts like absolute(p, kinky), where kinky is the current directory for the p.root_name() drive. This will be the current directory of that drive the last time it was set, and thus may be residue left over from a prior program run by the command processor! Although these semantics are often useful, they are also very error-prone.

path temp_directory_path();
path temp_directory_path(system::error_code& ec);

Returns: A directory path suitable for temporary files under the conventions of the operating system. The specifics of how this path is determined are implementation defined. An error shall be reported if !exists(p) || !is_directory(p), where p is the path to be returned.

ISO/IEC 9945: The path supplied by the first environment variable found in the list TMPDIR, TMP, TEMP, TEMPDIR. If none of these are found, "/tmp", or, if macro __ANDROID__ is defined, "/data/local/tmp".

Windows: The path reported by the Windows GetTempPath API function.

Throws: As specified in Error reporting.

[Note: The temp_directory_path() name was chosen to emphasize that the return is a path, not just a single directory name.  —end note]

path unique_path(const path& model="%%%%-%%%%-%%%%-%%%%");
path unique_path(system::error_code& ec);
path unique_path(const path& model, system::error_code& ec);

The unique_path function generates a path name suitable for creating temporary files, including directories. The name is based on a model that uses the percent sign character to specify replacement by a random hexadecimal digit. Where model is not specified, a default model equivalent to "%%%%-%%%%-%%%%-%%%%" is used. [Note: The more bits of randomness in the generated path name, the less likelihood of prior existence or being guessed. Each replacement hexadecimal digit in the model adds four bits of randomness. The default model thus provides 64 bits of randomness. This is sufficient for most applications. —end note]

Returns: A path identical to model, except that each occurrence of a percent sign character is replaced by a random hexadecimal digit character in the range 0-9, a-f.

Throws: As specified in Error reporting.

Remarks: Implementations are encouraged to obtain the required randomness via a cryptographically secure pseudo-random number generator, such as one provided by the operating system. [Note: Such generators may block until sufficient entropy develops. —end note]

path weakly_canonical(const path& p, const path& base=current_path());
path weakly_canonical(const path& p, system::error_code& ec);
path weakly_canonical(const path& p, const path& base, system::error_code& ec);

Overview: Returns p with symlinks resolved and the result normalized.

Effects: Let head be the path composed of the leading elements of p that exist and tail — from the rest of p. Where not specified, base is obtained from current_path() or current_path(ec) Calls canonical(head, base) or canonical(head, base, ec), appends tail to the returned path as if by using operator/. The result is then normalized and returned.

Postcondition: The returned path is in normal form.

Remarks: Uses operator/= to compose the returned path. Uses the status function to determine existence.

Remarks: Implementations are encouraged to avoid unnecessary normalization such as when canonical has already been called on the entirety of p.

Throws:  As specified in Error reporting.

File streams

<boost/filesystem/fstream.hpp> header

Replacements are provided for the file stream classes from the C++ standard library's <fstream> header. These replacement classes publicly inherit from the standard library classes. In the Boost.Filesystem version, constructors and open functions take const path& arguments instead of const char* arguments. There are no other differences in syntax or semantics.

namespace boost
{
  namespace filesystem
  {
    template < class charT, class traits = std::char_traits<charT> >
    class basic_filebuf : public std::basic_filebuf<charT,traits>
    {
    public:
      basic_filebuf<charT,traits>*
        open(const path& p, std::ios_base::openmode mode);
    };

    template < class charT, class traits = std::char_traits<charT> >
    class basic_ifstream : public std::basic_ifstream<charT,traits>
    {
    public:
      explicit basic_ifstream(const path& p, std::ios_base::openmode mode=std::ios_base::in)
      void open(const path& p, std::ios_base::openmode mode=std::ios_base::in);
    };

    template < class charT, class traits = std::char_traits<charT> >
    class basic_ofstream : public std::basic_ofstream<charT,traits>
    {
    public:
      explicit basic_ofstream(const path& p, std::ios_base::openmode mode=std::ios_base::out);
      void open(const path& p, std::ios_base::openmode mode=std::ios_base::out);
    };

    template < class charT, class traits = std::char_traits<charT> >
    class basic_fstream : public std::basic_fstream<charT,traits>
    {
    public:
      explicit basic_fstream(const path& p,
        std::ios_base::openmode mode=std::ios_base::in | std::ios_base::out);
      void open(const path& p,
        std::ios_base::openmode mode=std::ios_base::in | std::ios_base::out);
    };

    typedef basic_filebuf<char> filebuf;
    typedef basic_ifstream<char> ifstream;
    typedef basic_ofstream<char> ofstream;
    typedef basic_fstream<char> fstream;

    typedef basic_filebuf<wchar_t> wfilebuf;
    typedef basic_ifstream<wchar_t> wifstream;
    typedef basic_fstream<wchar_t> wfstream;
    typedef basic_ofstream<wchar_t> wofstream;

  }  // namespace filesystem
}  // namespace boost

<boost/filesystem/cstdio.hpp> header

This header provides a std::fopen overload from <cstdio> C++ standard library header. The overload accepts const path& as its first argument instead of const char* and has no other differences from its standard counterpart.

namespace boost
{
  namespace filesystem
  {
    std::FILE* fopen(const path& p, const char* mode);

  }  // namespace filesystem
}  // namespace boost

Path decomposition table

The table is generated by a program compiled with the Boost implementation.

Shaded entries indicate cases where ISO/IEC 9945 (POSIX) and Windows implementations yield different results. The top value is the ISO/IEC 9945 result and the bottom value is the Windows result.

Long paths on Windows and the \\?\ namespace prefix

The Microsoft Windows "Maximum Path Length Limitation" specifies:

In the Windows API (with some exceptions ...), the maximum length for a path is MAX_PATH, which is defined as 260 characters.

The Windows API has many functions that also have Unicode versions to permit an extended-length path for a maximum total path length of 32,767 characters. ... To specify an extended-length path, use the "\\?\" prefix. For example, "\\?\D:\very long path". [C++ string literals require backslashes be doubled, of course.]

Because most Boost.Filesystem operational functions just pass the contents of a class path object to the Windows API, they do work with the extended-length prefixes. But some won't work, because to the limitations imposed by Windows.

Effectively, the "\\?\" prefix informs the underlying Windows API that the path identifies an object in the Win32 filesystem namespace and disables most of the internal path processing, including normalization, by the Windows API. While it lifts the length limitation, this imposes a number of other restrictions on the acceptable paths that are listed in the section below. There are a number of other prefixes that have different special meanings, such as "\\.\" (a local device namespace prefix) and "\??\" (NT object namespace prefix). Note that UNC paths can be extended with the prefixes by referring to a special "UNC" device. For example, "\\server\share" can be extended as "\\?\UNC\server\share".

In terms of Boost.Filesystem, the namespace prefixes are considered as parts of the path's root name. For example, the root name of "\\?\c:\foo" is "\\?\c:". This interpretation is chosen to allow root paths to be iterable, that is you can list the contents of the "\\?\c:\" directory, provided that drive C: exists and you have the necessary permissions. [Note: You can't list the contents of "\\?\" as such path does not exist.]

Cautions for paths with namespace prefixes

• Individual components of a path are still are limited to whatever is supported for the particular filesystem, commonly 255 characters.
• Only backslashes are acceptable as directory separators. Slashes are not treated as separators.
• All paths must be absolute and must not contain "." or ".." components.
• Once an absolute path grows beyond 260 characters, it is essentially poisoned and all operations must use extended-length prefixes. So even a simple operation like create_directory("a") will fail if the absolute path of the resulting directory would exceed 260 characters.
• Certain Boost.Filesystem functions that decompose their argument path and then work on individual relative directories or files will not work properly with namespace prefix paths.

Acknowledgements

This Filesystem Library is dedicated to my wife, Sonda, who provided the support necessary to see both a trial implementation and the proposal itself through to completion. She gave me the strength to continue after a difficult year of cancer treatment in the middle of it all.

Many people contributed technical comments, ideas, and suggestions to the Boost Filesystem Library. See Version history.

Dietmar Kühl contributed the original Boost Filesystem Library directory_iterator design. Peter Dimov, Walter Landry, Rob Stewart, and Thomas Witt were particularly helpful in refining the library.

The create_directories, extension, basename, and replace_extension functions were developed by Vladimir Prus. The temp_directory_path function was contributed by Jeff Flinn. David Svoboda suggested the canonical function and provided pseudo-code.

Howard Hinnant and John Maddock reviewed a draft of the version 2 proposal, and identified a number of mistakes or weaknesses, resulting in a more polished final document.

Peter Dimov suggested a single class path, with member templates to adapt to multiple string types. His idea became the basis for the version 3 path design.

References

© Copyright Beman Dawes, 2002, 2006, 2007, 2009, 2010, 2011

© Copyright Andrey Semashev, 2019-2024

Distributed under the Boost Software License, Version 1.0. See www.boost.org/LICENSE_1_0.txt