...one of the most highly
regarded and expertly designed C++ library projects in the
world.
— Herb Sutter and Andrei
Alexandrescu, C++
Coding Standards
Author: | David Abrahams, Jeremy Siek, Thomas Witt |
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Contact: | dave@boost-consulting.com, jsiek@osl.iu.edu, witt@ive.uni-hannover.de |
Organization: | Boost Consulting, Indiana University Open Systems Lab, University of Hanover Institute for Transport Railway Operation and Construction |
Date: | 2006-09-11 |
Copyright: | Copyright David Abrahams, Jeremy Siek, and Thomas Witt 2003. |
abstract: | indirect_iterator adapts an iterator by applying an extra dereference inside of operator*(). For example, this iterator adaptor makes it possible to view a container of pointers (e.g. list<foo*>) as if it were a container of the pointed-to type (e.g. list<foo>). indirect_iterator depends on two auxiliary traits, pointee and indirect_reference, to provide support for underlying iterators whose value_type is not an iterator. |
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Table of Contents
template < class Iterator , class Value = use_default , class CategoryOrTraversal = use_default , class Reference = use_default , class Difference = use_default > class indirect_iterator { public: typedef /* see below */ value_type; typedef /* see below */ reference; typedef /* see below */ pointer; typedef /* see below */ difference_type; typedef /* see below */ iterator_category; indirect_iterator(); indirect_iterator(Iterator x); template < class Iterator2, class Value2, class Category2 , class Reference2, class Difference2 > indirect_iterator( indirect_iterator< Iterator2, Value2, Category2, Reference2, Difference2 > const& y , typename enable_if_convertible<Iterator2, Iterator>::type* = 0 // exposition ); Iterator const& base() const; reference operator*() const; indirect_iterator& operator++(); indirect_iterator& operator--(); private: Iterator m_iterator; // exposition };
The member types of indirect_iterator are defined according to the following pseudo-code, where V is iterator_traits<Iterator>::value_type
if (Value is use_default) then typedef remove_const<pointee<V>::type>::type value_type; else typedef remove_const<Value>::type value_type; if (Reference is use_default) then if (Value is use_default) then typedef indirect_reference<V>::type reference; else typedef Value& reference; else typedef Reference reference; if (Value is use_default) then typedef pointee<V>::type* pointer; else typedef Value* pointer; if (Difference is use_default) typedef iterator_traits<Iterator>::difference_type difference_type; else typedef Difference difference_type; if (CategoryOrTraversal is use_default) typedef iterator-category ( iterator_traversal<Iterator>::type,``reference``,``value_type`` ) iterator_category; else typedef iterator-category ( CategoryOrTraversal,``reference``,``value_type`` ) iterator_category;
The expression *v, where v is an object of iterator_traits<Iterator>::value_type, shall be valid expression and convertible to reference. Iterator shall model the traversal concept indicated by iterator_category. Value, Reference, and Difference shall be chosen so that value_type, reference, and difference_type meet the requirements indicated by iterator_category.
[Note: there are further requirements on the iterator_traits<Iterator>::value_type if the Value parameter is not use_default, as implied by the algorithm for deducing the default for the value_type member.]
In addition to the concepts indicated by iterator_category and by iterator_traversal<indirect_iterator>::type, a specialization of indirect_iterator models the following concepts, Where v is an object of iterator_traits<Iterator>::value_type:
- Readable Iterator if reference(*v) is convertible to value_type.
- Writable Iterator if reference(*v) = t is a valid expression (where t is an object of type indirect_iterator::value_type)
- Lvalue Iterator if reference is a reference type.
indirect_iterator<X,V1,C1,R1,D1> is interoperable with indirect_iterator<Y,V2,C2,R2,D2> if and only if X is interoperable with Y.
In addition to the operations required by the concepts described above, specializations of indirect_iterator provide the following operations.
indirect_iterator();
Requires: | Iterator must be Default Constructible. |
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Effects: | Constructs an instance of indirect_iterator with a default-constructed m_iterator. |
indirect_iterator(Iterator x);
Effects: | Constructs an instance of indirect_iterator with m_iterator copy constructed from x. |
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template < class Iterator2, class Value2, unsigned Access, class Traversal , class Reference2, class Difference2 > indirect_iterator( indirect_iterator< Iterator2, Value2, Access, Traversal, Reference2, Difference2 > const& y , typename enable_if_convertible<Iterator2, Iterator>::type* = 0 // exposition );
Requires: | Iterator2 is implicitly convertible to Iterator. |
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Effects: | Constructs an instance of indirect_iterator whose m_iterator subobject is constructed from y.base(). |
Iterator const& base() const;
Returns: | m_iterator |
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reference operator*() const;
Returns: | **m_iterator |
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indirect_iterator& operator++();
Effects: | ++m_iterator |
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Returns: | *this |
indirect_iterator& operator--();
Effects: | --m_iterator |
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Returns: | *this |
This example prints an array of characters, using indirect_iterator to access the array of characters through an array of pointers. Next indirect_iterator is used with the transform algorithm to copy the characters (incremented by one) to another array. A constant indirect iterator is used for the source and a mutable indirect iterator is used for the destination. The last part of the example prints the original array of characters, but this time using the make_indirect_iterator helper function.
char characters[] = "abcdefg"; const int N = sizeof(characters)/sizeof(char) - 1; // -1 since characters has a null char char* pointers_to_chars[N]; // at the end. for (int i = 0; i < N; ++i) pointers_to_chars[i] = &characters[i]; // Example of using indirect_iterator boost::indirect_iterator<char**, char> indirect_first(pointers_to_chars), indirect_last(pointers_to_chars + N); std::copy(indirect_first, indirect_last, std::ostream_iterator<char>(std::cout, ",")); std::cout << std::endl; // Example of making mutable and constant indirect iterators char mutable_characters[N]; char* pointers_to_mutable_chars[N]; for (int j = 0; j < N; ++j) pointers_to_mutable_chars[j] = &mutable_characters[j]; boost::indirect_iterator<char* const*> mutable_indirect_first(pointers_to_mutable_chars), mutable_indirect_last(pointers_to_mutable_chars + N); boost::indirect_iterator<char* const*, char const> const_indirect_first(pointers_to_chars), const_indirect_last(pointers_to_chars + N); std::transform(const_indirect_first, const_indirect_last, mutable_indirect_first, std::bind1st(std::plus<char>(), 1)); std::copy(mutable_indirect_first, mutable_indirect_last, std::ostream_iterator<char>(std::cout, ",")); std::cout << std::endl; // Example of using make_indirect_iterator() std::copy(boost::make_indirect_iterator(pointers_to_chars), boost::make_indirect_iterator(pointers_to_chars + N), std::ostream_iterator<char>(std::cout, ",")); std::cout << std::endl;
The output is:
a,b,c,d,e,f,g, b,c,d,e,f,g,h, a,b,c,d,e,f,g,
The source code for this example can be found here.