CallableTraits

Barrett Adair

Distributed under the Boost Software License, Version 1.0. (See accompanying file LICENSE.md or copy at http://www.boost.org/LICENSE_1_0.txt)

Overview

Motivation
Regarding Boost.FunctionTypes
Compatibility

Boost.CallableTraits is a C++11 header-only library for the inspection, synthesis, and decomposition of callable types. Boost.CallableTraits aims to be the "complete type manipulation facility for function types" mentioned in the final paragraph of C++17 proposal p0172, and removes the need for template specializations for different function signatures. C++17 noexcept and the Transactional Memory TS are also supported if available.

#include <type_traits>
#include <tuple>
#include <boost/callable_traits.hpp>

namespace ct = boost::callable_traits;

// This function template helps keep our example code neat
template<typename A, typename B>
void assert_same(){ static_assert(std::is_same<A, B>::value, ""); }

// foo is a function object
struct foo {
    void operator()(int, char, float) const {}
};

int main() {

    // Use args_t to retrieve a parameter list as a std::tuple:
    assert_same<
        ct::args_t<foo>,
        std::tuple<int, char, float>
    >();

    // has_void_return lets us perform a quick check for a void return type
    static_assert(ct::has_void_return<foo>::value, "");

    // Detect C-style variadics (ellipses) in a signature (e.g. printf)
    static_assert(!ct::has_varargs<foo>::value, "");

    // pmf is a pointer-to-member function: void (foo::*)(int, char, float) const
    using pmf = decltype(&foo::operator());

    // remove_member_const_t lets you remove the const member qualifier
    assert_same<
        ct::remove_member_const_t<pmf>,
        void (foo::*)(int, char, float) /*no const!*/
    >();

    // Conversely, add_member_const_t adds a const member qualifier
    assert_same<
        pmf,
        ct::add_member_const_t<void (foo::*)(int, char, float)>
    >();

    // is_const_member_v checks for the presence of member const
    static_assert(ct::is_const_member<pmf>::value, "");
}

Motivation

“Don't try to write helper code to detect PMFs/PMDs and dispatch on them -- it is an absolute nightmare. PMF types are the worst types by far in the core language.”

-- Stephan T. Lavavej, CppCon 2015, "functional: What's New, And Proper Usage"

Consider for a moment the code below, which defines all 48 template specializations necessary to specialize for every valid function type in pure C++17:

template<typename T> struct foo;

template<class Return, class... Args> struct foo<Return(Args...)> {};
template<class Return, class... Args> struct foo<Return(Args...) &> {};
template<class Return, class... Args> struct foo<Return(Args...) &&> {};
template<class Return, class... Args> struct foo<Return(Args...) const> {};
template<class Return, class... Args> struct foo<Return(Args...) const &> {};
template<class Return, class... Args> struct foo<Return(Args...) const &&> {};
template<class Return, class... Args> struct foo<Return(Args...) volatile> {};
template<class Return, class... Args> struct foo<Return(Args...) volatile &> {};
template<class Return, class... Args> struct foo<Return(Args...) volatile &&> {};
template<class Return, class... Args> struct foo<Return(Args...) const volatile> {};
template<class Return, class... Args> struct foo<Return(Args...) const volatile &> {};
template<class Return, class... Args> struct foo<Return(Args...) const volatile &&> {};
template<class Return, class... Args> struct foo<Return(Args..., ...)> {};
template<class Return, class... Args> struct foo<Return(Args..., ...) &> {};
template<class Return, class... Args> struct foo<Return(Args..., ...) &&> {};
template<class Return, class... Args> struct foo<Return(Args..., ...) const> {};
template<class Return, class... Args> struct foo<Return(Args..., ...) const &> {};
template<class Return, class... Args> struct foo<Return(Args..., ...) const &&> {};
template<class Return, class... Args> struct foo<Return(Args..., ...) volatile> {};
template<class Return, class... Args> struct foo<Return(Args..., ...) volatile &> {};
template<class Return, class... Args> struct foo<Return(Args..., ...) volatile &&> {};
template<class Return, class... Args> struct foo<Return(Args..., ...) const volatile> {};
template<class Return, class... Args> struct foo<Return(Args..., ...) const volatile &> {};
template<class Return, class... Args> struct foo<Return(Args..., ...) const volatile &&> {};
template<class Return, class... Args> struct foo<Return(Args...) noexcept> {};
template<class Return, class... Args> struct foo<Return(Args...) & noexcept> {};
template<class Return, class... Args> struct foo<Return(Args...) && noexcept> {};
template<class Return, class... Args> struct foo<Return(Args...) const noexcept> {};
template<class Return, class... Args> struct foo<Return(Args...) const & noexcept> {};
template<class Return, class... Args> struct foo<Return(Args...) const && noexcept> {};
template<class Return, class... Args> struct foo<Return(Args...) volatile noexcept> {};
template<class Return, class... Args> struct foo<Return(Args...) volatile & noexcept> {};
template<class Return, class... Args> struct foo<Return(Args...) volatile && noexcept> {};
template<class Return, class... Args> struct foo<Return(Args...) const volatile noexcept> {};
template<class Return, class... Args> struct foo<Return(Args...) const volatile & noexcept> {};
template<class Return, class... Args> struct foo<Return(Args...) const volatile && noexcept> {};
template<class Return, class... Args> struct foo<Return(Args..., ...) noexcept> {};
template<class Return, class... Args> struct foo<Return(Args..., ...) & noexcept> {};
template<class Return, class... Args> struct foo<Return(Args..., ...) && noexcept> {};
template<class Return, class... Args> struct foo<Return(Args..., ...) const noexcept> {};
template<class Return, class... Args> struct foo<Return(Args..., ...) const & noexcept> {};
template<class Return, class... Args> struct foo<Return(Args..., ...) const && noexcept> {};
template<class Return, class... Args> struct foo<Return(Args..., ...) volatile noexcept> {};
template<class Return, class... Args> struct foo<Return(Args..., ...) volatile & noexcept> {};
template<class Return, class... Args> struct foo<Return(Args..., ...) volatile && noexcept> {};
template<class Return, class... Args> struct foo<Return(Args..., ...) const volatile noexcept> {};
template<class Return, class... Args> struct foo<Return(Args..., ...) const volatile & noexcept> {};
template<class Return, class... Args> struct foo<Return(Args..., ...) const volatile && noexcept> {};

Things get even more complicated with member function pointers, function pointers, function references, function objects, and transaction_safe.

Granted, use cases for such obscure specializations are vitually nonexistent in run-of-the-mill application codebases. Even in library code, these are exceedingly rare. However, there are a handful of metaprogramming scenarios that can only be solved with this kind of template "spam". Writing, testing, and maintaining such code is tedious and costly.

Boost.CallableTraits offers a final and decisive library-level solution to this problem, and removes the need for these specializations entirely (platform-specific calling conventions notwithstanding).

Regarding `Boost.FunctionTypes`

The features in Boost.CallableTraits largely overlap with Boost.FunctionTypes. Here are some reasons why you might prefer Boost.CallableTraits:

Boost.FunctionTypes is tightly coupled to Boost.MPL sequences, while Boost.CallableTraits has no dependencies other than the standard library.
Boost.CallableTraits targets C++11 and later:
1. Boost.CallableTraits treats function objects/lambdas as first-class citizens.
2. Boost.CallableTraits supports lvalue/rvalue reference member qualifiers.
3. Boost.CallableTraits supports noexcept and transaction_safe.
Boost.FunctionTypes does not attempt to factor all callable types into a unified, INVOKE-aware interface.
Boost.FunctionTypes relies heavily on "tag" types, while Boost.CallableTraits follows the style of <type_traits> instead. Supporting C++11 and later in Boost.FunctionTypes would have required significant proliferation of these tags.

For example, here is how to remove member const from a member function pointer type in the Boost.FunctionTypes library:

#include <type_traits>
#include <boost/function_types/components.hpp>
#include <boost/function_types/member_function_pointer.hpp>

struct foo {
    void bar() const {}
};

using const_removed = typename boost::function_types::member_function_pointer<
    typename boost::function_types::components<decltype(&foo::bar)>::types,
    boost::function_types::non_const>::type;

static_assert(std::is_same<const_removed, void(foo::*)()>::value, "");

int main(){}

Boost.CallableTraits makes this easier:

#include <type_traits>
#include <boost/callable_traits/remove_member_const.hpp>

struct foo {
        void bar() const {}
};

using const_removed = boost::callable_traits::remove_member_const_t<decltype(&foo::bar)>;

static_assert(std::is_same<const_removed, void(foo::*)()>::value, "");

int main(){}

The Boost.FunctionTypes library includes an excellent example for generating type-erased interfaces (implementation here). This example was re-implemented using Boost.CallableTraits to yield a slightly more intuitive interface.

Boost.FunctionTypes is a fine library, but its interface left room for improvement.

Compatibility

Boost.CallableTraits supports on GCC 4.7.4+, Clang 3.5.2+, XCode 6.4+, and Visual Studio 2015+. The Intel C++ Compiler is not officially supported, although the 2017 version for Linux does pass a handful of test cases.

Table 1. GCC Support

feature	GCC 7.3.0 and later	GCC 6.3.0	GCC 5.4.0	GCC 4.9.2	GCC 4.8.2	GCC 4.7.4
`add_member_const`	c++11	c++11	c++11	c++11	c++11 (no abominables)	c++11 (no abominables)
`add_member_cv`	c++11	c++11	c++11	c++11	c++11 (no abominables)	c++11 (no abominables)
`add_member_lvalue_reference`	c++11	c++11	c++11	c++11	static_assert fails on instantiation	static_assert fails on instantiation
`add_member_rvalue_reference`	c++11	c++11	c++11	c++11	static_assert fails on instantiation	static_assert fails on instantiation
`add_member_volatile`	c++11	c++11	c++11	c++11	c++11 (no abominables)	c++11 (no abominables)
`add_noexcept`	c++17	c++17	static_assert fails on instantiation	static_assert fails on instantiation	static_assert fails on instantiation	static_assert fails on instantiation
`add_transaction_safe`	c++17 (requires -fgnu-tm)	c++17 (requires -fgnu-tm)	static_assert fails on instantiation	static_assert fails on instantiation	static_assert fails on instantiation	static_assert fails on instantiation
`add_varargs`	c++11	c++11	c++11	c++11	c++11	c++11
`apply_member_pointer`	c++11	c++11	c++11	c++11	c++11	c++11
`apply_return`	c++11	c++11	c++11	c++11	c++11	c++11
`args`	c++11	c++11	c++11	c++11	c++11	c++11
`class_of`	c++11	c++11	c++11	c++11	c++11	c++11
`function_type`	c++11	c++11	c++11	c++11	c++11	c++11
`has_member_qualifiers`	c++11	c++11	c++11	c++11	c++11 (no abominables)	c++11 (no abominables)
`has_varargs`	c++11	c++11	c++11	c++11	c++11	c++11
`has_void_return`	c++11	c++11	c++11	c++11	c++11	c++11
`is_const_member`	c++11	c++11	c++11	c++11	c++11 (no abominables)	c++11 (no abominables)
`is_cv_member`	c++11	c++11	c++11	c++11	c++11 (no abominables)	c++11 (no abominables)
`is_invocable`	c++11	c++11	c++11	c++11	unknown	unknown
`is_lvalue_reference_member`	c++11	c++11	c++11	c++11	c++11 (always false)	c++11 (always false)
`is_noexcept`	c++17	c++17	c++11 (always false)	c++11 (always false)	c++11 (always false)	c++11 (always false)
`is_reference_member`	c++11	c++11	c++11	c++11	c++11 (always false)	c++11 (always false)
`is_rvalue_reference_member`	c++11	c++11	c++11	c++11	c++11 (always false)	c++11 (always false)
`is_transaction_safe`	c++17 (requires -fgnu-tm)	c++17 (requires -fgnu-tm)	c++11 (always false)	c++11 (always false)	c++11 (always false)	c++11 (always false)
`is_volatile_member`	c++11	c++11	c++11	c++11	c++11 (no abominables)	c++11 (no abominables)
`qualified_class_of`	c++11	c++11	c++11	c++11	c++11	c++11
`remove_member_const`	c++11	c++11	c++11	c++11	c++11 (no abominables)	c++11 (no abominables)
`remove_member_cv`	c++11	c++11	c++11	c++11	c++11 (no abominables)	c++11 (no abominables)
`remove_member_reference`	c++11	c++11	c++11	c++11	c++11 (no effect)	c++11 (no effect)
`remove_member_volatile`	c++11	c++11	c++11	c++11	c++11 (no abominables)	c++11 (no abominables)
`remove_noexcept`	c++17	c++17	c++11 (no effect)	c++11 (no effect)	c++11 (no effect)	c++11 (no effect)
`remove_transaction_safe`	c++17 (requires -fgnu-tm)	c++17 (requires -fgnu-tm)	c++11 (no effect)	c++11 (no effect)	c++11 (no effect)	c++11 (no effect)
`remove_varargs`	c++11	c++11	c++11	c++11	c++11	c++11
`return_type`	c++11	c++11	c++11	c++11	c++11	c++11

Table 2. LLVM/Clang Support

feature	Clang 4.0.0 and later	Clang 3.8.0	Clang 3.7.1	Clang 3.6.2	Clang 3.5.2
`add_member_const`	c++11	c++11	c++11	c++11	c++11
`add_member_cv`	c++11	c++11	c++11	c++11	c++11
`add_member_lvalue_reference`	c++11	c++11	c++11	c++11	c++11
`add_member_rvalue_reference`	c++11	c++11	c++11	c++11	c++11
`add_member_volatile`	c++11	c++11	c++11	c++11	c++11
`add_noexcept`	c++17	static_assert fails on instantiation	static_assert fails on instantiation	static_assert fails on instantiation	static_assert fails on instantiation
`add_transaction_safe`	static_assert fails on instantiation	static_assert fails on instantiation	static_assert fails on instantiation	static_assert fails on instantiation	static_assert fails on instantiation
`add_varargs`	c++11	c++11	c++11	c++11	c++11
`apply_member_pointer`	c++11	c++11	c++11	c++11	c++11
`apply_return`	c++11	c++11	c++11	c++11	c++11
`args`	c++11	c++11	c++11	c++11	c++11
`class_of`	c++11	c++11	c++11	c++11	c++11
`function_type`	c++11	c++11	c++11	c++11	c++11
`has_member_qualifiers`	c++11	c++11	c++11	c++11	c++11
`has_varargs`	c++11	c++11	c++11	c++11	c++11
`has_void_return`	c++11	c++11	c++11	c++11	c++11
`is_const_member`	c++11	c++11	c++11	c++11	c++11
`is_cv_member`	c++11	c++11	c++11	c++11	c++11
`is_invocable`	c++11	c++11	c++11	c++11	c++11
`is_lvalue_reference_member`	c++11	c++11	c++11	c++11	c++11
`is_noexcept`	c++17	c++11 (always false)	c++11 (always false)	c++11 (always false)	c++11 (always false)
`is_reference_member`	c++11	c++11	c++11	c++11	c++11
`is_rvalue_reference_member`	c++11	c++11	c++11	c++11	c++11
`is_transaction_safe`	c++11 (always false)	c++11 (always false)	c++11 (always false)	c++11 (always false)	c++11 (always false)
`is_volatile_member`	c++11	c++11	c++11	c++11	c++11
`qualified_class_of`	c++11	c++11	c++11	c++11	c++11
`remove_member_const`	c++11	c++11	c++11	c++11	c++11
`remove_member_cv`	c++11	c++11	c++11	c++11	c++11
`remove_member_reference`	c++11	c++11	c++11	c++11	c++11
`remove_member_volatile`	c++11	c++11	c++11	c++11	c++11
`remove_noexcept`	c++17	c++11 (no effect)	c++11 (no effect)	c++11 (no effect)	c++11 (no effect)
`remove_transaction_safe`	c++11 (no effect)	c++11 (no effect)	c++11 (no effect)	c++11 (no effect)	c++11 (no effect)
`remove_varargs`	c++11	c++11	c++11	c++11	c++11
`return_type`	c++11	c++11	c++11	c++11	c++11

Table 3. XCode/AppleClang Support

feature	XCode 8 and later	XCode 7.3	XCode 7.2	XCode 7.1	XCode 6.4
`add_member_const`	c++11	c++11	c++11	c++11	c++11
`add_member_cv`	c++11	c++11	c++11	c++11	c++11
`add_member_lvalue_reference`	c++11	c++11	c++11	c++11	c++11
`add_member_rvalue_reference`	c++11	c++11	c++11	c++11	c++11
`add_member_volatile`	c++11	c++11	c++11	c++11	c++11
`add_noexcept`	unknown	static_assert fails on instantiation	static_assert fails on instantiation	static_assert fails on instantiation	static_assert fails on instantiation
`add_transaction_safe`	unknown	static_assert fails on instantiation	static_assert fails on instantiation	static_assert fails on instantiation	static_assert fails on instantiation
`add_varargs`	c++11	c++11	c++11	c++11	c++11
`apply_member_pointer`	c++11	c++11	c++11	c++11	c++11
`apply_return`	c++11	c++11	c++11	c++11	c++11
`args`	c++11	c++11	c++11	c++11	c++11
`class_of`	c++11	c++11	c++11	c++11	c++11
`function_type`	c++11	c++11	c++11	c++11	c++11
`has_member_qualifiers`	c++11	c++11	c++11	c++11	c++11
`has_varargs`	c++11	c++11	c++11	c++11	c++11
`has_void_return`	c++11	c++11	c++11	c++11	c++11
`is_const_member`	c++11	c++11	c++11	c++11	c++11
`is_cv_member`	c++11	c++11	c++11	c++11	c++11
`is_invocable`	c++11	c++11	c++11	c++11	c++11
`is_lvalue_reference_member`	c++11	c++11	c++11	c++11	c++11
`is_noexcept`	unknown	c++11 (always false)	c++11 (always false)	c++11 (always false)	c++11 (always false)
`is_reference_member`	c++11	c++11	c++11	c++11	c++11
`is_rvalue_reference_member`	c++11	c++11	c++11	c++11	c++11
`is_transaction_safe`	unknown	c++11 (always false)	c++11 (always false)	c++11 (always false)	c++11 (always false)
`is_volatile_member`	c++11	c++11	c++11	c++11	c++11
`qualified_class_of`	c++11	c++11	c++11	c++11	c++11
`remove_member_const`	c++11	c++11	c++11	c++11	c++11
`remove_member_cv`	c++11	c++11	c++11	c++11	c++11
`remove_member_reference`	c++11	c++11	c++11	c++11	c++11
`remove_member_volatile`	c++11	c++11	c++11	c++11	c++11
`remove_noexcept`	unknown	c++11 (no effect)	c++11 (no effect)	c++11 (no effect)	c++11 (no effect)
`remove_transaction_safe`	unknown	c++11 (no effect)	c++11 (no effect)	c++11 (no effect)	c++11 (no effect)
`remove_varargs`	c++11	c++11	c++11	c++11	c++11
`return_type`	c++11	c++11	c++11	c++11	c++11

Table 4. Visual Studio Support

feature	MSVC with Visual Studio 2017	MSVC with Visual Studio 2015 (latest update)
`add_member_const`	c++11	c++11
`add_member_cv`	c++11	c++11
`add_member_lvalue_reference`	c++11	c++11
`add_member_rvalue_reference`	c++11	c++11
`add_member_volatile`	c++11	c++11
`add_noexcept`	static_assert fails on instantiation	static_assert fails on instantiation
`add_transaction_safe`	static_assert fails on instantiation	static_assert fails on instantiation
`add_varargs`	c++11 (define BOOST_DISABLE_WIN32 to disable __cdecl on PMF varargs)	c++11 (define BOOST_DISABLE_WIN32 to disable __cdecl on PMF varargs)
`apply_member_pointer`	c++11	c++11
`apply_return`	c++11	c++11
`args`	c++11	c++11
`class_of`	c++11	c++11
`function_type`	c++11	c++11
`has_member_qualifiers`	c++11	c++11
`has_varargs`	c++11 (define BOOST_DISABLE_WIN32 to disable __cdecl on PMF varargs)	c++11 (define BOOST_DISABLE_WIN32 to disable __cdecl on PMF varargs)
`has_void_return`	c++11	c++11
`is_const_member`	c++11	c++11
`is_cv_member`	c++11	c++11
`is_invocable`	c++11	c++11 (always false for functions that are simultaneously ref and cv-qualified due to compiler bug)
`is_lvalue_reference_member`	c++11	c++11
`is_noexcept`	c++11 (always false)	c++11 (always false)
`is_reference_member`	c++11	c++11
`is_rvalue_reference_member`	c++11	c++11
`is_transaction_safe`	c++11 (always false)	c++11 (always false)
`is_volatile_member`	c++11	c++11
`qualified_class_of`	c++11	c++11
`remove_member_const`	c++11	c++11
`remove_member_cv`	c++11	c++11
`remove_member_reference`	c++11	c++11
`remove_member_volatile`	c++11	c++11
`remove_noexcept`	c++11 (no effect)	c++11 (no effect)
`remove_transaction_safe`	c++11 (no effect)	c++11 (no effect)
`remove_varargs`	c++11 (define BOOST_DISABLE_WIN32 to disable __cdecl on PMF varargs)	c++11 (define BOOST_DISABLE_WIN32 to disable __cdecl on PMF varargs)
`return_type`	c++11	c++11

Boost C++ Libraries

CallableTraits

Barrett Adair

Overview

Motivation

Regarding Boost.FunctionTypes

Compatibility

Regarding `Boost.FunctionTypes`