CallableTraits

Barrett Adair

根据 Boost 软件许可协议 1.0 版分发。(请参阅随附文件 LICENSE.md 或访问 https://boost.ac.cn/LICENSE_1_0.txt)

概述

动机
关于 Boost.FunctionTypes
兼容性

Boost.CallableTraits 是一个 C++11 的仅头文件库，用于检查、合成和分解可调用类型。Boost.CallableTraits 旨在成为 C++17 提案 p0172 的最后一段中提到的 “用于函数类型的完整类型操作工具”，并消除了对不同函数签名进行模板特化的需求。如果可用，也支持 C++17 noexcept 和事务内存 TS。

#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, "");
}

动机

“不要尝试编写辅助代码来检测 PMF/PMD 并对其进行分派 -- 这绝对是一场噩梦。PMF 类型是核心语言中最糟糕的类型。”

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

考虑一下下面的代码，它定义了 48 个模板特化，这些特化对于在纯 C++17 中为每个有效的函数类型进行特化是必要的

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> {};

对于成员函数指针、函数指针、函数引用、函数对象和 transaction_safe，情况变得更加复杂。

诚然，在日常应用程序代码库中，这种晦涩的特化的用例几乎不存在。即使在库代码中，这些也极其罕见。然而，有一些元编程场景只能通过这种模板“垃圾信息”来解决。编写、测试和维护此类代码既乏味又昂贵。

Boost.CallableTraits 为这个问题提供了一个最终且决定性的库级解决方案，并完全消除了对这些特化的需求（平台特定的调用约定除外）。

关于 `Boost.FunctionTypes`

Boost.CallableTraits 中的功能在很大程度上与 Boost.FunctionTypes 重叠。以下是您可能更喜欢 Boost.CallableTraits 的一些原因

Boost.FunctionTypes 与 Boost.MPL 序列紧密耦合，而 Boost.CallableTraits 除了标准库之外没有其他依赖项。
Boost.CallableTraits 目标是 C++11 及更高版本
1. Boost.CallableTraits 将函数对象/lambda 表达式视为一等公民。
2. Boost.CallableTraits 支持左值/右值引用成员限定符。
3. Boost.CallableTraits 支持 noexcept 和 transaction_safe。
Boost.FunctionTypes 不尝试将所有可调用类型分解为统一的、INVOKE-感知接口。
Boost.FunctionTypes 严重依赖“标签”类型，而 Boost.CallableTraits 遵循 <type_traits> 的风格。在 Boost.FunctionTypes 中支持 C++11 及更高版本将需要大量这些标签的扩散。

例如，以下是如何在 Boost.FunctionTypes 库中从成员函数指针类型中删除成员 const 的方法

#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 使这更容易

#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(){}

Boost.FunctionTypes 库包含一个出色的示例，用于生成类型擦除接口（实现在此）。此示例已使用 Boost.CallableTraits 重新实现，以产生稍微更直观的接口。

Boost.FunctionTypes 是一个优秀的库，但其接口仍有改进空间。

兼容性

Boost.CallableTraits 在 GCC 4.7.4+、Clang 3.5.2+、XCode 6.4+ 和 Visual Studio 2015+ 上受支持。Intel C++ 编译器未正式支持，尽管适用于 Linux 的 2017 版本确实通过了一些测试用例。

表 1. GCC 支持

功能	GCC 7.3.0 及更高版本	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 (无 abominables)	c++11 (无 abominables)
`add_member_cv`	c++11	c++11	c++11	c++11	c++11 (无 abominables)	c++11 (无 abominables)
`add_member_lvalue_reference`	c++11	c++11	c++11	c++11	static_assert 在实例化时失败	static_assert 在实例化时失败
`add_member_rvalue_reference`	c++11	c++11	c++11	c++11	static_assert 在实例化时失败	static_assert 在实例化时失败
`add_member_volatile`	c++11	c++11	c++11	c++11	c++11 (无 abominables)	c++11 (无 abominables)
`add_noexcept`	c++17	c++17	static_assert 在实例化时失败	static_assert 在实例化时失败	static_assert 在实例化时失败	static_assert 在实例化时失败
`add_transaction_safe`	c++17 (需要 -fgnu-tm)	c++17 (需要 -fgnu-tm)	static_assert 在实例化时失败	static_assert 在实例化时失败	static_assert 在实例化时失败	static_assert 在实例化时失败
`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 (无 abominables)	c++11 (无 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 (无 abominables)	c++11 (无 abominables)
`is_cv_member`	c++11	c++11	c++11	c++11	c++11 (无 abominables)	c++11 (无 abominables)
`is_invocable`	c++11	c++11	c++11	c++11	未知	未知
`is_lvalue_reference_member`	c++11	c++11	c++11	c++11	c++11 (始终为 false)	c++11 (始终为 false)
`is_noexcept`	c++17	c++17	c++11 (始终为 false)	c++11 (始终为 false)	c++11 (始终为 false)	c++11 (始终为 false)
`is_reference_member`	c++11	c++11	c++11	c++11	c++11 (始终为 false)	c++11 (始终为 false)
`is_rvalue_reference_member`	c++11	c++11	c++11	c++11	c++11 (始终为 false)	c++11 (始终为 false)
`is_transaction_safe`	c++17 (需要 -fgnu-tm)	c++17 (需要 -fgnu-tm)	c++11 (始终为 false)	c++11 (始终为 false)	c++11 (始终为 false)	c++11 (始终为 false)
`is_volatile_member`	c++11	c++11	c++11	c++11	c++11 (无 abominables)	c++11 (无 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 (无 abominables)	c++11 (无 abominables)
`remove_member_cv`	c++11	c++11	c++11	c++11	c++11 (无 abominables)	c++11 (无 abominables)
`remove_member_reference`	c++11	c++11	c++11	c++11	c++11 (无效果)	c++11 (无效果)
`remove_member_volatile`	c++11	c++11	c++11	c++11	c++11 (无 abominables)	c++11 (无 abominables)
`remove_noexcept`	c++17	c++17	c++11 (无效果)	c++11 (无效果)	c++11 (无效果)	c++11 (无效果)
`remove_transaction_safe`	c++17 (需要 -fgnu-tm)	c++17 (需要 -fgnu-tm)	c++11 (无效果)	c++11 (无效果)	c++11 (无效果)	c++11 (无效果)
`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

表 2. LLVM/Clang 支持

功能	Clang 4.0.0 及更高版本	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 在实例化时失败	static_assert 在实例化时失败	static_assert 在实例化时失败	static_assert 在实例化时失败
`add_transaction_safe`	static_assert 在实例化时失败	static_assert 在实例化时失败	static_assert 在实例化时失败	static_assert 在实例化时失败	static_assert 在实例化时失败
`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 (始终为 false)	c++11 (始终为 false)	c++11 (始终为 false)	c++11 (始终为 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 (始终为 false)	c++11 (始终为 false)	c++11 (始终为 false)	c++11 (始终为 false)	c++11 (始终为 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 (无效果)	c++11 (无效果)	c++11 (无效果)	c++11 (无效果)
`remove_transaction_safe`	c++11 (无效果)	c++11 (无效果)	c++11 (无效果)	c++11 (无效果)	c++11 (无效果)
`remove_varargs`	c++11	c++11	c++11	c++11	c++11
`return_type`	c++11	c++11	c++11	c++11	c++11

表 3. XCode/AppleClang 支持

功能	XCode 8 及更高版本	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`	未知	static_assert 在实例化时失败	static_assert 在实例化时失败	static_assert 在实例化时失败	static_assert 在实例化时失败
`add_transaction_safe`	未知	static_assert 在实例化时失败	static_assert 在实例化时失败	static_assert 在实例化时失败	static_assert 在实例化时失败
`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`	未知	c++11 (始终为 false)	c++11 (始终为 false)	c++11 (始终为 false)	c++11 (始终为 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`	未知	c++11 (始终为 false)	c++11 (始终为 false)	c++11 (始终为 false)	c++11 (始终为 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`	未知	c++11 (无效果)	c++11 (无效果)	c++11 (无效果)	c++11 (无效果)
`remove_transaction_safe`	未知	c++11 (无效果)	c++11 (无效果)	c++11 (无效果)	c++11 (无效果)
`remove_varargs`	c++11	c++11	c++11	c++11	c++11
`return_type`	c++11	c++11	c++11	c++11	c++11

表 4. Visual Studio 支持

功能	MSVC with Visual Studio 2017	MSVC with Visual Studio 2015 (最新更新)
`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 在实例化时失败	static_assert 在实例化时失败
`add_transaction_safe`	static_assert 在实例化时失败	static_assert 在实例化时失败
`add_varargs`	c++11 (定义 BOOST_DISABLE_WIN32 以禁用 PMF varargs 上的 __cdecl)	c++11 (定义 BOOST_DISABLE_WIN32 以禁用 PMF varargs 上的 __cdecl)
`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 (定义 BOOST_DISABLE_WIN32 以禁用 PMF varargs 上的 __cdecl)	c++11 (定义 BOOST_DISABLE_WIN32 以禁用 PMF varargs 上的 __cdecl)
`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 (对于同时具有 ref 和 cv 限定符的函数，始终为 false，原因是编译器错误)
`is_lvalue_reference_member`	c++11	c++11
`is_noexcept`	c++11 (始终为 false)	c++11 (始终为 false)
`is_reference_member`	c++11	c++11
`is_rvalue_reference_member`	c++11	c++11
`is_transaction_safe`	c++11 (始终为 false)	c++11 (始终为 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 (无效果)	c++11 (无效果)
`remove_transaction_safe`	c++11 (无效果)	c++11 (无效果)
`remove_varargs`	c++11 (定义 BOOST_DISABLE_WIN32 以禁用 PMF varargs 上的 __cdecl)	c++11 (定义 BOOST_DISABLE_WIN32 以禁用 PMF varargs 上的 __cdecl)
`return_type`	c++11	c++11

Boost C++ 库

CallableTraits

Barrett Adair

概述

动机

关于 Boost.FunctionTypes

兼容性

关于 `Boost.FunctionTypes`