C++11 元编程学习

本文主要是介绍C++11 元编程学习，对大家解决编程问题具有一定的参考价值，需要的程序猿们随着小编来一起学习吧！

主要对C++11中的<type_traist>中的代码进行简单的注释说明

// C++11 <type_traits> -*- C++ -*-

// Copyright (C) 2007-2015 Free Software Foundation, Inc.
//
// This file is part of the GNU ISO C++ Library.  This library is free
// software; you can redistribute it and/or modify it under the
// terms of the GNU General Public License as published by the
// Free Software Foundation; either version 3, or (at your option)
// any later version.

// This library is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
// GNU General Public License for more details.

// Under Section 7 of GPL version 3, you are granted additional
// permissions described in the GCC Runtime Library Exception, version
// 3.1, as published by the Free Software Foundation.

// You should have received a copy of the GNU General Public License and
// a copy of the GCC Runtime Library Exception along with this program;
// see the files COPYING3 and COPYING.RUNTIME respectively.  If not, see
// <http://www.gnu.org/licenses/>.

/** @file include/type_traits
 *  This is a Standard C++ Library header.
 */

#ifndef _GLIBCXX_TYPE_TRAITS
#define _GLIBCXX_TYPE_TRAITS 1

#pragma GCC system_header

#if __cplusplus < 201103L
# include <bits/c++0x_warning.h>
#else

#include <bits/c++config.h>

#ifdef _GLIBCXX_USE_C99_STDINT_TR1
# if defined (__UINT_LEAST16_TYPE__) && defined(__UINT_LEAST32_TYPE__)
namespace std
{
typedef __UINT_LEAST16_TYPE__ uint_least16_t;
typedef __UINT_LEAST32_TYPE__ uint_least32_t;
}
# else
#  include <cstdint>
# endif
#endif

namespace std _GLIBCXX_VISIBILITY(default)
{
_GLIBCXX_BEGIN_NAMESPACE_VERSION

/**
   * @defgroup metaprogramming Metaprogramming
   * @ingroup utilities
   *
   * Template utilities for compile-time introspection and modification,
   * including type classification traits, type property inspection traits
   * and type transformation traits.
   *
   * @{
   */

/// integral_constant: 常整数
template<typename _Tp, _Tp __v>
struct integral_constant
{
    static constexpr _Tp                  value = __v;
    typedef _Tp                           value_type;
    typedef integral_constant<_Tp, __v>   type;
    constexpr operator value_type() const { return value; }
#if __cplusplus > 201103L

#define __cpp_lib_integral_constant_callable 201304

    constexpr value_type operator()() const { return value; }
#endif
};

// 定义静态变量
template<typename _Tp, _Tp __v>
constexpr _Tp integral_constant<_Tp, __v>::value;

// 编译时布尔类型取值为true的类型
/// The type used as a compile-time boolean with true value.
typedef integral_constant<bool, true>     true_type;

// 编译时布尔类型取值为false的类型
/// The type used as a compile-time boolean with false value.
typedef integral_constant<bool, false>    false_type;

// 布尔类型常量
template<bool __v>
using __bool_constant = integral_constant<bool, __v>;

// Meta programming helper types.
// 第一个参数为true时 ，返回第二个的数值
// 第二个参数为false时，返回第三个的数值
template<bool, typename, typename>
struct conditional;

template<typename...>
struct __or_;

// __or_的无类型特化版本, __or_<>::value = false_type::value
template<>
struct __or_<>
        : public false_type
{ };

// __or_特化版本, __or_<_B1>::value = _B1::value
template<typename _B1>
struct __or_<_B1>
        : public _B1
{ };

// __or_特化版本: __or_<_B1, _B2>::value = 如果_B1::value = true时，返回_B1::value; 如果_B2::value = true时, 返回_B2::value
template<typename _B1, typename _B2>
struct __or_<_B1, _B2>
        : public conditional<_B1::value, _B1, _B2>::type
{ };

// __or_三个类型及以上类型的特化版本:
template<typename _B1, typename _B2, typename _B3, typename... _Bn>
struct __or_<_B1, _B2, _B3, _Bn...>
        : public conditional<_B1::value, _B1, __or_<_B2, _B3, _Bn...>>::type
{ };

template<typename...>
struct __and_;

// __and_的无类型特化版本: __and_<>::value = true_type::value
template<>
struct __and_<>
        : public true_type
{ };

// 单类型特化版本: __and_<_B1>::value = _B1:value
template<typename _B1>
struct __and_<_B1>
        : public _B1
{ };

// 两个类型特化版本: __and_<_B1, _B2>::value = _B1::value = true, 则返回 _B2::value; 如果_B1::value = false, 则返回 _B1::value
// 即一个取值为false， 则不在进行判断
template<typename _B1, typename _B2>
struct __and_<_B1, _B2>
        : public conditional<_B1::value, _B2, _B1>::type
{ };

template<typename _B1, typename _B2, typename _B3, typename... _Bn>
struct __and_<_B1, _B2, _B3, _Bn...>
        : public conditional<_B1::value, __and_<_B2, _B3, _Bn...>, _B1>::type
{ };

// 非
template<typename _Pp>
struct __not_
        : public integral_constant<bool, !_Pp::value>
{ };

// For several sfinae-friendly trait implementations we transport both the
// result information (as the member type) and the failure information (no
// member type). This is very similar to std::enable_if, but we cannot use
// them, because we need to derive from them as an implementation detail.
// 获取类型
template<typename _Tp>
struct __success_type
{ typedef _Tp type; };

struct __failure_type
{ };

// Primary type categories.
// 移除const和volatile的类型
template<typename>
struct remove_cv;

template<typename>
struct __is_void_helper
        : public false_type { };

// 特化为void类型
template<>
struct __is_void_helper<void>
        : public true_type { };

// 判断类型Tp是否为空类型
/// is_void
template<typename _Tp>
struct is_void
        : public __is_void_helper<typename remove_cv<_Tp>::type>::type
{ };

// 判断为整型: bool/char/signed char/unsigned char/wchar_t/char16_t/char32_t/
//           short/unsigned short/int/unsigned int/long/unsigned long/long long/unsigned long long
//           __GLIBCXX_TYPE_INT_N_0/unsigned __GLIBCXX_TYPE_INT_N_0
//           __GLIBCXX_TYPE_INT_N_1/unsigned __GLIBCXX_TYPE_INT_N_1
//           __GLIBCXX_TYPE_INT_N_2/unsigned __GLIBCXX_TYPE_INT_N_2
//           __GLIBCXX_TYPE_INT_N_3/unsigned __GLIBCXX_TYPE_INT_N_3
template<typename>
struct __is_integral_helper
        : public false_type { };

template<>
struct __is_integral_helper<bool>
        : public true_type { };

template<>
struct __is_integral_helper<char>
        : public true_type { };

template<>
struct __is_integral_helper<signed char>
        : public true_type { };

template<>
struct __is_integral_helper<unsigned char>
        : public true_type { };

#ifdef _GLIBCXX_USE_WCHAR_T
template<>
struct __is_integral_helper<wchar_t>
        : public true_type { };
#endif

template<>
struct __is_integral_helper<char16_t>
        : public true_type { };

template<>
struct __is_integral_helper<char32_t>
        : public true_type { };

template<>
struct __is_integral_helper<short>
        : public true_type { };

template<>
struct __is_integral_helper<unsigned short>
        : public true_type { };

template<>
struct __is_integral_helper<int>
        : public true_type { };

template<>
struct __is_integral_helper<unsigned int>
        : public true_type { };

template<>
struct __is_integral_helper<long>
        : public true_type { };

template<>
struct __is_integral_helper<unsigned long>
        : public true_type { };

template<>
struct __is_integral_helper<long long>
        : public true_type { };

template<>
struct __is_integral_helper<unsigned long long>
        : public true_type { };

// Conditionalizing on __STRICT_ANSI__ here will break any port that
// uses one of these types for size_t.
#if defined(__GLIBCXX_TYPE_INT_N_0)
template<>
struct __is_integral_helper<__GLIBCXX_TYPE_INT_N_0>
        : public true_type { };

template<>
struct __is_integral_helper<unsigned __GLIBCXX_TYPE_INT_N_0>
        : public true_type { };
#endif
#if defined(__GLIBCXX_TYPE_INT_N_1)
template<>
struct __is_integral_helper<__GLIBCXX_TYPE_INT_N_1>
        : public true_type { };

template<>
struct __is_integral_helper<unsigned __GLIBCXX_TYPE_INT_N_1>
        : public true_type { };
#endif
#if defined(__GLIBCXX_TYPE_INT_N_2)
template<>
struct __is_integral_helper<__GLIBCXX_TYPE_INT_N_2>
        : public true_type { };

template<>
struct __is_integral_helper<unsigned __GLIBCXX_TYPE_INT_N_2>
        : public true_type { };
#endif
#if defined(__GLIBCXX_TYPE_INT_N_3)
template<>
struct __is_integral_helper<__GLIBCXX_TYPE_INT_N_3>
        : public true_type { };

template<>
struct __is_integral_helper<unsigned __GLIBCXX_TYPE_INT_N_3>
        : public true_type { };
#endif

// 判断_Tp是否为整型
/// is_integral
template<typename _Tp>
struct is_integral
        : public __is_integral_helper<typename remove_cv<_Tp>::type>::type
{ };

// 判断是否为浮点数类型: float/double/ long double / _GLIBCXX_USE_FLOAT128
template<typename>
struct __is_floating_point_helper
        : public false_type { };

template<>
struct __is_floating_point_helper<float>
        : public true_type { };

template<>
struct __is_floating_point_helper<double>
        : public true_type { };

template<>
struct __is_floating_point_helper<long double>
        : public true_type { };

#if !defined(__STRICT_ANSI__) && defined(_GLIBCXX_USE_FLOAT128)
template<>
struct __is_floating_point_helper<__float128>
        : public true_type { };
#endif

/// is_floating_point
template<typename _Tp>
struct is_floating_point
        : public __is_floating_point_helper<typename remove_cv<_Tp>::type>::type
{ };

// 判断是否为数组
/// is_array
template<typename>
struct is_array
        : public false_type { };

template<typename _Tp, std::size_t _Size>
struct is_array<_Tp[_Size]>
        : public true_type { };

template<typename _Tp>
struct is_array<_Tp[]>
        : public true_type { };

// 判断是否为指针
template<typename>
struct __is_pointer_helper
        : public false_type { };

template<typename _Tp>
struct __is_pointer_helper<_Tp*>
        : public true_type { };

/// is_pointer
template<typename _Tp>
struct is_pointer
        : public __is_pointer_helper<typename remove_cv<_Tp>::type>::type
{ };

// 判断是否为左值引用
/// is_lvalue_reference
template<typename>
struct is_lvalue_reference
        : public false_type { };

template<typename _Tp>
struct is_lvalue_reference<_Tp&>
        : public true_type { };

// 判断是否为右值引用
/// is_rvalue_reference
template<typename>
struct is_rvalue_reference
        : public false_type { };

template<typename _Tp>
struct is_rvalue_reference<_Tp&&>
        : public true_type { };

// 判断是否为函数
template<typename>
struct is_function;

// 判断是否为类的数据成员变量的指针
template<typename>
struct __is_member_object_pointer_helper
        : public false_type { };

template<typename _Tp, typename _Cp>
struct __is_member_object_pointer_helper<_Tp _Cp::*>
        : public integral_constant<bool, !is_function<_Tp>::value> { };

/// is_member_object_pointer
template<typename _Tp>
struct is_member_object_pointer
        : public __is_member_object_pointer_helper<
        typename remove_cv<_Tp>::type>::type
{ };

// 判断是否为类的成员函数的指针
template<typename>
struct __is_member_function_pointer_helper
        : public false_type { };

template<typename _Tp, typename _Cp>
struct __is_member_function_pointer_helper<_Tp _Cp::*>
        : public integral_constant<bool, is_function<_Tp>::value> { };

/// is_member_function_pointer
template<typename _Tp>
struct is_member_function_pointer
        : public __is_member_function_pointer_helper<
        typename remove_cv<_Tp>::type>::type
{ };

// 是否为枚举类型
/// is_enum
template<typename _Tp>
struct is_enum
        : public integral_constant<bool, __is_enum(_Tp)>
{ };

// 是否为union类型
/// is_union
template<typename _Tp>
struct is_union
        : public integral_constant<bool, __is_union(_Tp)>
{ };

// 是否为类
/// is_class
template<typename _Tp>
struct is_class
        : public integral_constant<bool, __is_class(_Tp)>
{ };

/// is_function
// 默认返回 false
template<typename>
struct is_function
        : public false_type { };
// 判断函数
template<typename _Res, typename... _ArgTypes>
struct is_function<_Res(_ArgTypes...)>
    : public true_type { };
// 返回值为引用的函数
template<typename _Res, typename... _ArgTypes>
struct is_function<_Res(_ArgTypes...) &>
    : public true_type { };
// 返回值为右值引用的函数
template<typename _Res, typename... _ArgTypes>
struct is_function<_Res(_ArgTypes...) &&>
    : public true_type { };
// 变参数函数
template<typename _Res, typename... _ArgTypes>
struct is_function<_Res(_ArgTypes......)>
    : public true_type { };
// 返回值为引用的变参数函数
template<typename _Res, typename... _ArgTypes>
struct is_function<_Res(_ArgTypes......) &>
    : public true_type { };
// 返回值为右值引用的变参数函数
template<typename _Res, typename... _ArgTypes>
struct is_function<_Res(_ArgTypes......) &&>
    : public true_type { };
// const类型函数
template<typename _Res, typename... _ArgTypes>
struct is_function<_Res(_ArgTypes...) const>
    : public true_type { };

template<typename _Res, typename... _ArgTypes>
struct is_function<_Res(_ArgTypes...) const &>
    : public true_type { };

template<typename _Res, typename... _ArgTypes>
struct is_function<_Res(_ArgTypes...) const &&>
    : public true_type { };

template<typename _Res, typename... _ArgTypes>
struct is_function<_Res(_ArgTypes......) const>
    : public true_type { };

template<typename _Res, typename... _ArgTypes>
struct is_function<_Res(_ArgTypes......) const &>
    : public true_type { };

template<typename _Res, typename... _ArgTypes>
struct is_function<_Res(_ArgTypes......) const &&>
    : public true_type { };

template<typename _Res, typename... _ArgTypes>
struct is_function<_Res(_ArgTypes...) volatile>
    : public true_type { };

template<typename _Res, typename... _ArgTypes>
struct is_function<_Res(_ArgTypes...) volatile &>
    : public true_type { };

template<typename _Res, typename... _ArgTypes>
struct is_function<_Res(_ArgTypes...) volatile &&>
    : public true_type { };

template<typename _Res, typename... _ArgTypes>
struct is_function<_Res(_ArgTypes......) volatile>
    : public true_type { };

template<typename _Res, typename... _ArgTypes>
struct is_function<_Res(_ArgTypes......) volatile &>
    : public true_type { };

template<typename _Res, typename... _ArgTypes>
struct is_function<_Res(_ArgTypes......) volatile &&>
    : public true_type { };

template<typename _Res, typename... _ArgTypes>
struct is_function<_Res(_ArgTypes...) const volatile>
    : public true_type { };

template<typename _Res, typename... _ArgTypes>
struct is_function<_Res(_ArgTypes...) const volatile &>
    : public true_type { };

template<typename _Res, typename... _ArgTypes>
struct is_function<_Res(_ArgTypes...) const volatile &&>
    : public true_type { };

template<typename _Res, typename... _ArgTypes>
struct is_function<_Res(_ArgTypes......) const volatile>
    : public true_type { };

template<typename _Res, typename... _ArgTypes>
struct is_function<_Res(_ArgTypes......) const volatile &>
    : public true_type { };

template<typename _Res, typename... _ArgTypes>
struct is_function<_Res(_ArgTypes......) const volatile &&>
    : public true_type { };

#define __cpp_lib_is_null_pointer 201309
// 判断是否为nullptr
template<typename>
struct __is_null_pointer_helper
        : public false_type { };

template<>
struct __is_null_pointer_helper<std::nullptr_t>
        : public true_type { };

/// is_null_pointer (LWG 2247).
template<typename _Tp>
struct is_null_pointer
        : public __is_null_pointer_helper<typename remove_cv<_Tp>::type>::type
{ };

/// __is_nullptr_t (extension).
template<typename _Tp>
struct __is_nullptr_t
        : public is_null_pointer<_Tp>
{ };

// Composite type categories.

// 判断是否为引用类型: 包含左值引用/右值引用
/// is_reference
template<typename _Tp>
struct is_reference
        : public __or_<is_lvalue_reference<_Tp>,
        is_rvalue_reference<_Tp>>::type
{ };

// 判断是否为算数的类型
/// is_arithmetic
template<typename _Tp>
struct is_arithmetic
        : public __or_<is_integral<_Tp>, is_floating_point<_Tp>>::type
{ };

// 是否为基础类型: 算数类型/void/nullptr
/// is_fundamental
template<typename _Tp>
struct is_fundamental
        : public __or_<is_arithmetic<_Tp>, is_void<_Tp>,
        is_null_pointer<_Tp>>::type
{ };

// 是否为对象类型: 非(函数/引用/void)类型
/// is_object
template<typename _Tp>
struct is_object
        : public __not_<__or_<is_function<_Tp>, is_reference<_Tp>,
        is_void<_Tp>>>::type
{ };

// 判断是否为成员指针: 成员变量指针/成员函数指针
template<typename>
struct is_member_pointer;

// 判断是否为标量: 算数类型 | 枚举类型 | 指针类型 | 成员指针类型 | 空指针类型
/// is_scalar
template<typename _Tp>
struct is_scalar
        : public __or_<is_arithmetic<_Tp>, is_enum<_Tp>, is_pointer<_Tp>,
        is_member_pointer<_Tp>, is_null_pointer<_Tp>>::type
{ };

// 判断是否为复合类型: 即非基础类型
/// is_compound
template<typename _Tp>
struct is_compound
        : public integral_constant<bool, !is_fundamental<_Tp>::value> { };

template<typename _Tp>
struct __is_member_pointer_helper
        : public false_type { };

template<typename _Tp, typename _Cp>
struct __is_member_pointer_helper<_Tp _Cp::*>
        : public true_type { };

/// is_member_pointer
template<typename _Tp>
struct is_member_pointer
        : public __is_member_pointer_helper<typename remove_cv<_Tp>::type>::type
{ };

// Utility to detect referenceable types ([defns.referenceable]).
// 判断是否可引用: 对象类型 | 引用类型
template<typename _Tp>
struct __is_referenceable
        : public __or_<is_object<_Tp>, is_reference<_Tp>>::type
{ };

// 函数类型特化
template<typename _Res, typename... _Args>
struct __is_referenceable<_Res(_Args...)>
    : public true_type
{ };

// 变参数函数类型特化
template<typename _Res, typename... _Args>
struct __is_referenceable<_Res(_Args......)>
    : public true_type
{ };

// Type properties.
// 判断是否为常量
/// is_const
template<typename>
struct is_const
        : public false_type { };

template<typename _Tp>
struct is_const<_Tp const>
        : public true_type { };

// 判断是否为volatile
/// is_volatile
template<typename>
struct is_volatile
        : public false_type { };

template<typename _Tp>
struct is_volatile<_Tp volatile>
        : public true_type { };

// https://www.cnblogs.com/shuaihanhungry/p/5764019.html
// 判断是否为"平凡的"
/// is_trivial
template<typename _Tp>
struct is_trivial
        : public integral_constant<bool, __is_trivial(_Tp)>
{ };

// 是否为"平凡可拷贝的"
// is_trivially_copyable
template<typename _Tp>
struct is_trivially_copyable
        : public integral_constant<bool, __is_trivially_copyable(_Tp)>
{ };

// 是否为standard 内存排布
/// is_standard_layout
template<typename _Tp>
struct is_standard_layout
        : public integral_constant<bool, __is_standard_layout(_Tp)>
{ };

// 是否为standard 内部排布 + 平凡类型(Plain Old Data, 即和C语言的struct一致，可以直接通过内存拷贝即可保持全部的成员变量保持不变)
/// is_pod
// Could use is_standard_layout && is_trivial instead of the builtin.
template<typename _Tp>
struct is_pod
        : public integral_constant<bool, __is_pod(_Tp)>
{ };

// 编译期计算量类型: 标量都符合
/// is_literal_type
template<typename _Tp>
struct is_literal_type
        : public integral_constant<bool, __is_literal_type(_Tp)>
{ };

// 是否为empty
/// is_empty
template<typename _Tp>
struct is_empty
        : public integral_constant<bool, __is_empty(_Tp)>
{ };

// 是否为多态
/// is_polymorphic
template<typename _Tp>
struct is_polymorphic
        : public integral_constant<bool, __is_polymorphic(_Tp)>
{ };

// 是否为final: c++14及之后版本
#if __cplusplus >= 201402L
#define __cpp_lib_is_final 201402L
/// is_final
template<typename _Tp>
struct is_final
        : public integral_constant<bool, __is_final(_Tp)>
{ };
#endif

// 是否为抽象类
/// is_abstract
template<typename _Tp>
struct is_abstract
        : public integral_constant<bool, __is_abstract(_Tp)>
{ };

// 是否为有符号类型: 首先必须是算数类型
template<typename _Tp,
         bool = is_arithmetic<_Tp>::value>
struct __is_signed_helper
        : public false_type { };

template<typename _Tp>
struct __is_signed_helper<_Tp, true>
        : public integral_constant<bool, _Tp(-1) < _Tp(0)>
{ };

/// is_signed
template<typename _Tp>
struct is_signed
        : public __is_signed_helper<_Tp>::type
{ };

// 判断是否为有符号类型
/// is_unsigned
template<typename _Tp>
struct is_unsigned
        : public __and_<is_arithmetic<_Tp>, __not_<is_signed<_Tp>>>::type
{ };


// Destructible and constructible type properties.
// (1) 默认为 add_rvalue_reference<Tp>::value = Tp
// (2) 如果为可引用的类型，则add_rvalue_reference<Tp>::value = Tp&&
template<typename>
struct add_rvalue_reference;

/**
   *  @brief  Utility to simplify expressions used in unevaluated operands
   *  @ingroup utilities
   */
template<typename _Tp>
typename add_rvalue_reference<_Tp>::type declval() noexcept;

template<typename, unsigned = 0>
struct extent;

template<typename>
struct remove_all_extents;

template<typename _Tp>
struct __is_array_known_bounds
        : public integral_constant<bool, (extent<_Tp>::value > 0)>
{ };

template<typename _Tp>
struct __is_array_unknown_bounds
        : public __and_<is_array<_Tp>, __not_<extent<_Tp>>>::type
{ };

// In N3290 is_destructible does not say anything about function
// types and abstract types, see LWG 2049. This implementation
// describes function types as non-destructible and all complete
// object types as destructible, iff the explicit destructor
// call expression is wellformed(合乎语法的).
struct __do_is_destructible_impl
{
    template<typename _Tp, typename = decltype(declval<_Tp&>().~_Tp())>
    static true_type __test(int);

    template<typename>
    static false_type __test(...);
};

template<typename _Tp>
struct __is_destructible_impl
        : public __do_is_destructible_impl
{
    typedef decltype(__test<_Tp>(0)) type;
};

template<typename _Tp,
         bool = __or_<is_void<_Tp>,
                      __is_array_unknown_bounds<_Tp>,
                      is_function<_Tp>>::value,
         bool = __or_<is_reference<_Tp>, is_scalar<_Tp>>::value>
struct __is_destructible_safe;

template<typename _Tp>
struct __is_destructible_safe<_Tp, false, false>
        : public __is_destructible_impl<typename
        remove_all_extents<_Tp>::type>::type
{ };

template<typename _Tp>
struct __is_destructible_safe<_Tp, true, false>
        : public false_type { };

template<typename _Tp>
struct __is_destructible_safe<_Tp, false, true>
        : public true_type { };

// 可析构的: 不是void类型/数组/函数/引用/标量
/// is_destructible
template<typename _Tp>
struct is_destructible
        : public __is_destructible_safe<_Tp>::type
{ };

// is_nothrow_destructible requires that is_destructible is
// satisfied as well.  We realize that by mimicing the
// implementation of is_destructible but refer to noexcept(expr)
// instead of decltype(expr).
struct __do_is_nt_destructible_impl
{
    // 这里增加了析构函数为noexcept的判断
    template<typename _Tp>
    static integral_constant<bool, noexcept(declval<_Tp&>().~_Tp())>
    __test(int);

    template<typename>
    static false_type __test(...);
};

template<typename _Tp>
struct __is_nt_destructible_impl
        : public __do_is_nt_destructible_impl
{
    typedef decltype(__test<_Tp>(0)) type;
};

template<typename _Tp,
         bool = __or_<is_void<_Tp>,
                      __is_array_unknown_bounds<_Tp>,
                      is_function<_Tp>>::value,
         bool = __or_<is_reference<_Tp>, is_scalar<_Tp>>::value>
struct __is_nt_destructible_safe;

template<typename _Tp>
struct __is_nt_destructible_safe<_Tp, false, false>
        : public __is_nt_destructible_impl<typename
        remove_all_extents<_Tp>::type>::type
{ };

template<typename _Tp>
struct __is_nt_destructible_safe<_Tp, true, false>
        : public false_type { };

template<typename _Tp>
struct __is_nt_destructible_safe<_Tp, false, true>
        : public true_type { };

// 不抛出异常的析构，需要满足是可析构的
/// is_nothrow_destructible
template<typename _Tp>
struct is_nothrow_destructible
        : public __is_nt_destructible_safe<_Tp>::type
{ };

// 判断是否有默认构造函数
struct __do_is_default_constructible_impl
{
    template<typename _Tp, typename = decltype(_Tp())>
    static true_type __test(int);

    template<typename>
    static false_type __test(...);
};

template<typename _Tp>
struct __is_default_constructible_impl
        : public __do_is_default_constructible_impl
{
    typedef decltype(__test<_Tp>(0)) type;
};

// 非void类型 & 包含默认构造函数
template<typename _Tp>
struct __is_default_constructible_atom
        : public __and_<__not_<is_void<_Tp>>,
        __is_default_constructible_impl<_Tp>>::type
{ };

// 特化数组
template<typename _Tp, bool = is_array<_Tp>::value>
struct __is_default_constructible_safe;

// The following technique is a workaround for a current core language
// restriction, which does not allow for array types to occur in
// functional casts of the form T().  Complete arrays can be default-
// constructed, if the element type is default-constructible, but
// arrays with unknown bounds are not.
// 特化已知size的数组
template<typename _Tp>
struct __is_default_constructible_safe<_Tp, true>
        : public __and_<__is_array_known_bounds<_Tp>,
        __is_default_constructible_atom<typename
        remove_all_extents<_Tp>::type>>::type
{ };

template<typename _Tp>
struct __is_default_constructible_safe<_Tp, false>
        : public __is_default_constructible_atom<_Tp>::type
{ };

// 判断是否有默认构造函数
/// is_default_constructible
template<typename _Tp>
struct is_default_constructible
        : public __is_default_constructible_safe<_Tp>::type
{ };


// Implementation of is_constructible.

// The hardest part of this trait is the binary direct-initialization
// case, because we hit into a functional cast of the form T(arg).
// This implementation uses different strategies depending on the
// target type to reduce the test overhead as much as possible:
//
// a) For a reference target type, we use a static_cast expression
//    modulo its extra cases.
//
// b) For a non-reference target type we use a ::new expression.
struct __do_is_static_castable_impl
{
    template<typename _From, typename _To, typename
             = decltype(static_cast<_To>(declval<_From>()))>
    static true_type __test(int);

    template<typename, typename>
    static false_type __test(...);
};

template<typename _From, typename _To>
struct __is_static_castable_impl
        : public __do_is_static_castable_impl
{
    typedef decltype(__test<_From, _To>(0)) type;
};

template<typename _From, typename _To>
struct __is_static_castable_safe
        : public __is_static_castable_impl<_From, _To>::type
{ };

// 判断是否可以进行static_cast操作
// __is_static_castable
template<typename _From, typename _To>
struct __is_static_castable
        : public integral_constant<bool, (__is_static_castable_safe<
                                          _From, _To>::value)>
{ };

// Implementation for non-reference types. To meet the proper
// variable definition semantics, we also need to test for
// is_destructible in this case.
// This form should be simplified by a single expression:
// ::delete ::new _Tp(declval<_Arg>()), see c++/51222.
struct __do_is_direct_constructible_impl
{
    template<typename _Tp, typename _Arg, typename
             = decltype(::new _Tp(declval<_Arg>()))>
    static true_type __test(int);

    template<typename, typename>
    static false_type __test(...);
};

template<typename _Tp, typename _Arg>
struct __is_direct_constructible_impl
        : public __do_is_direct_constructible_impl
{
    typedef decltype(__test<_Tp, _Arg>(0)) type;
};

// 判断可以进行直接的new
template<typename _Tp, typename _Arg>
struct __is_direct_constructible_new_safe
        : public __and_<is_destructible<_Tp>,
        __is_direct_constructible_impl<_Tp, _Arg>>::type
{ };

// 判断是否为相同类型
template<typename, typename>
struct is_same;

// 判断是否为基类
template<typename, typename>
struct is_base_of;

// 移除引用
template<typename>
struct remove_reference;

// 非void 或 函数类型
template<typename _From, typename _To, bool
         = __not_<__or_<is_void<_From>,
                        is_function<_From>>>::value>
struct __is_base_to_derived_ref;

// Detect whether we have a downcast situation during
// reference binding.
// 判断基类引用到子类引用: from和to的类型不相同 && from必须为to的基类
template<typename _From, typename _To>
struct __is_base_to_derived_ref<_From, _To, true>
{
    typedef typename remove_cv<typename remove_reference<_From
    >::type>::type __src_t;
    typedef typename remove_cv<typename remove_reference<_To
    >::type>::type __dst_t;
    typedef __and_<__not_<is_same<__src_t, __dst_t>>,
    is_base_of<__src_t, __dst_t>> type;
    static constexpr bool value = type::value;
};

template<typename _From, typename _To>
struct __is_base_to_derived_ref<_From, _To, false>
        : public false_type
{ };

// 判断左值引用 => 右值引用
template<typename _From, typename _To, bool
         = __and_<is_lvalue_reference<_From>,
                  is_rvalue_reference<_To>>::value>
struct __is_lvalue_to_rvalue_ref;

// Detect whether we have an lvalue of non-function type
// bound to a reference-compatible rvalue-reference.
template<typename _From, typename _To>
struct __is_lvalue_to_rvalue_ref<_From, _To, true>
{
    typedef typename remove_cv<typename remove_reference<
    _From>::type>::type __src_t;
    typedef typename remove_cv<typename remove_reference<
    _To>::type>::type __dst_t;
    typedef __and_<__not_<is_function<__src_t>>,
    __or_<is_same<__src_t, __dst_t>,
    is_base_of<__dst_t, __src_t>>> type;
    static constexpr bool value = type::value;
};

template<typename _From, typename _To>
struct __is_lvalue_to_rvalue_ref<_From, _To, false>
        : public false_type
{ };

// Here we handle direct-initialization to a reference type as
// equivalent to a static_cast modulo overshooting conversions.
// These are restricted to the following conversions:
//    a) A base class value to a derived class reference
//    b) An lvalue to an rvalue-reference of reference-compatible
//       types that are not functions
// 判断Arg是否可以转为Tp的引用类型
template<typename _Tp, typename _Arg>
struct __is_direct_constructible_ref_cast
        : public __and_<__is_static_castable<_Arg, _Tp>,
        __not_<__or_<__is_base_to_derived_ref<_Arg, _Tp>,
        __is_lvalue_to_rvalue_ref<_Arg, _Tp>
        >>>::type
{ };

// 判断是否可以对new使用构造函数
template<typename _Tp, typename _Arg>
struct __is_direct_constructible_new
        : public conditional<is_reference<_Tp>::value,
        __is_direct_constructible_ref_cast<_Tp, _Arg>,
        __is_direct_constructible_new_safe<_Tp, _Arg>
        >::type
{ };

// 判断是否可以直接调用构造函数
template<typename _Tp, typename _Arg>
struct __is_direct_constructible
        : public __is_direct_constructible_new<_Tp, _Arg>::type
{ };

// Since default-construction and binary direct-initialization have
// been handled separately, the implementation of the remaining
// n-ary construction cases is rather straightforward. We can use
// here a functional cast, because array types are excluded anyway
// and this form is never interpreted as a C cast.
// 判断是否可以调用非默认构造函数: Tp(args)
struct __do_is_nary_constructible_impl
{
    template<typename _Tp, typename... _Args, typename
             = decltype(_Tp(declval<_Args>()...))>
    static true_type __test(int);

    template<typename, typename...>
    static false_type __test(...);
};

template<typename _Tp, typename... _Args>
struct __is_nary_constructible_impl
        : public __do_is_nary_constructible_impl
{
    typedef decltype(__test<_Tp, _Args...>(0)) type;
};

template<typename _Tp, typename... _Args>
struct __is_nary_constructible
        : public __is_nary_constructible_impl<_Tp, _Args...>::type
{
    static_assert(sizeof...(_Args) > 1,
                  "Only useful for > 1 arguments");
};

template<typename _Tp, typename... _Args>
struct __is_constructible_impl
        : public __is_nary_constructible<_Tp, _Args...>
{ };

template<typename _Tp, typename _Arg>
struct __is_constructible_impl<_Tp, _Arg>
        : public __is_direct_constructible<_Tp, _Arg>
{ };

template<typename _Tp>
struct __is_constructible_impl<_Tp>
        : public is_default_constructible<_Tp>
{ };

// 判断构造函数
/// is_constructible
template<typename _Tp, typename... _Args>
struct is_constructible
        : public __is_constructible_impl<_Tp, _Args...>::type
{ };

template<typename _Tp, bool = __is_referenceable<_Tp>::value>
struct __is_copy_constructible_impl;

template<typename _Tp>
struct __is_copy_constructible_impl<_Tp, false>
        : public false_type { };

template<typename _Tp>
struct __is_copy_constructible_impl<_Tp, true>
        : public is_constructible<_Tp, const _Tp&>
{ };

// 判断拷贝构造函数
/// is_copy_constructible
template<typename _Tp>
struct is_copy_constructible
        : public __is_copy_constructible_impl<_Tp>
{ };

template<typename _Tp, bool = __is_referenceable<_Tp>::value>
struct __is_move_constructible_impl;

template<typename _Tp>
struct __is_move_constructible_impl<_Tp, false>
        : public false_type { };

template<typename _Tp>
struct __is_move_constructible_impl<_Tp, true>
        : public is_constructible<_Tp, _Tp&&>
{ };

// 判断移动拷贝构造函数
/// is_move_constructible
template<typename _Tp>
struct is_move_constructible
        : public __is_move_constructible_impl<_Tp>
{ };

// 判断非抛出异常默认构造函数
template<typename _Tp>
struct __is_nt_default_constructible_atom
        : public integral_constant<bool, noexcept(_Tp())>
{ };

// 对数组特化
template<typename _Tp, bool = is_array<_Tp>::value>
struct __is_nt_default_constructible_impl;

// 对数组特化: 已知数组大小
template<typename _Tp>
struct __is_nt_default_constructible_impl<_Tp, true>
        : public __and_<__is_array_known_bounds<_Tp>,
        __is_nt_default_constructible_atom<typename
        remove_all_extents<_Tp>::type>>::type
{ };

template<typename _Tp>
struct __is_nt_default_constructible_impl<_Tp, false>
        : public __is_nt_default_constructible_atom<_Tp>
{ };

// 非抛出异常的默认构造函数
/// is_nothrow_default_constructible
template<typename _Tp>
struct is_nothrow_default_constructible
        : public __and_<is_default_constructible<_Tp>,
        __is_nt_default_constructible_impl<_Tp>>::type
{ };

// 非抛出异常的构造函数
template<typename _Tp, typename... _Args>
struct __is_nt_constructible_impl
        : public integral_constant<bool, noexcept(_Tp(declval<_Args>()...))>
{ };

template<typename _Tp, typename _Arg>
struct __is_nt_constructible_impl<_Tp, _Arg>
        : public integral_constant<bool,
        noexcept(static_cast<_Tp>(declval<_Arg>()))>
{ };

template<typename _Tp>
struct __is_nt_constructible_impl<_Tp>
        : public is_nothrow_default_constructible<_Tp>
{ };

// 判断非抛出异常的构造函数
/// is_nothrow_constructible
template<typename _Tp, typename... _Args>
struct is_nothrow_constructible
        : public __and_<is_constructible<_Tp, _Args...>,
        __is_nt_constructible_impl<_Tp, _Args...>>::type
{ };

template<typename _Tp, bool = __is_referenceable<_Tp>::value>
struct __is_nothrow_copy_constructible_impl;

template<typename _Tp>
struct __is_nothrow_copy_constructible_impl<_Tp, false>
        : public false_type { };

template<typename _Tp>
struct __is_nothrow_copy_constructible_impl<_Tp, true>
        : public is_nothrow_constructible<_Tp, const _Tp&>
{ };

// 非抛出异常的拷贝构造函数
/// is_nothrow_copy_constructible
template<typename _Tp>
struct is_nothrow_copy_constructible
        : public __is_nothrow_copy_constructible_impl<_Tp>
{ };

template<typename _Tp, bool = __is_referenceable<_Tp>::value>
struct __is_nothrow_move_constructible_impl;

template<typename _Tp>
struct __is_nothrow_move_constructible_impl<_Tp, false>
        : public false_type { };

template<typename _Tp>
struct __is_nothrow_move_constructible_impl<_Tp, true>
        : public is_nothrow_constructible<_Tp, _Tp&&>
{ };

// 判断非抛出异常的移动的拷贝构造函数
/// is_nothrow_move_constructible
template<typename _Tp>
struct is_nothrow_move_constructible
        : public __is_nothrow_move_constructible_impl<_Tp>
{ };

template<typename _Tp, typename _Up>
class __is_assignable_helper
{
    template<typename _Tp1, typename _Up1,
             typename = decltype(declval<_Tp1>() = declval<_Up1>())>
    static true_type
    __test(int);

    template<typename, typename>
    static false_type
    __test(...);

public:
    typedef decltype(__test<_Tp, _Up>(0)) type;
};

// 判断是否可以赋值
/// is_assignable
template<typename _Tp, typename _Up>
struct is_assignable
        : public __is_assignable_helper<_Tp, _Up>::type
{ };

template<typename _Tp, bool = __is_referenceable<_Tp>::value>
struct __is_copy_assignable_impl;

template<typename _Tp>
struct __is_copy_assignable_impl<_Tp, false>
        : public false_type { };

template<typename _Tp>
struct __is_copy_assignable_impl<_Tp, true>
        : public is_assignable<_Tp&, const _Tp&>
{ };

// 判断是否可以拷贝赋值
/// is_copy_assignable
template<typename _Tp>
struct is_copy_assignable
        : public __is_copy_assignable_impl<_Tp>
{ };

template<typename _Tp, bool = __is_referenceable<_Tp>::value>
struct __is_move_assignable_impl;

template<typename _Tp>
struct __is_move_assignable_impl<_Tp, false>
        : public false_type { };

template<typename _Tp>
struct __is_move_assignable_impl<_Tp, true>
        : public is_assignable<_Tp&, _Tp&&>
{ };

// 判断是否移动赋值
/// is_move_assignable
template<typename _Tp>
struct is_move_assignable
        : public __is_move_assignable_impl<_Tp>
{ };

template<typename _Tp, typename _Up>
struct __is_nt_assignable_impl
        : public integral_constant<bool, noexcept(declval<_Tp>() = declval<_Up>())>
{ };

// 判断非抛出异常的赋值
/// is_nothrow_assignable
template<typename _Tp, typename _Up>
struct is_nothrow_assignable
        : public __and_<is_assignable<_Tp, _Up>,
        __is_nt_assignable_impl<_Tp, _Up>>::type
{ };

template<typename _Tp, bool = __is_referenceable<_Tp>::value>
struct __is_nt_copy_assignable_impl;

template<typename _Tp>
struct __is_nt_copy_assignable_impl<_Tp, false>
        : public false_type { };

template<typename _Tp>
struct __is_nt_copy_assignable_impl<_Tp, true>
        : public is_nothrow_assignable<_Tp&, const _Tp&>
{ };

// 判断非抛出异常的拷贝赋值
/// is_nothrow_copy_assignable
template<typename _Tp>
struct is_nothrow_copy_assignable
        : public __is_nt_copy_assignable_impl<_Tp>
{ };

template<typename _Tp, bool = __is_referenceable<_Tp>::value>
struct __is_nt_move_assignable_impl;

template<typename _Tp>
struct __is_nt_move_assignable_impl<_Tp, false>
        : public false_type { };

template<typename _Tp>
struct __is_nt_move_assignable_impl<_Tp, true>
        : public is_nothrow_assignable<_Tp&, _Tp&&>
{ };

// 判断非抛出异常的移动拷贝赋值
/// is_nothrow_move_assignable
template<typename _Tp>
struct is_nothrow_move_assignable
        : public __is_nt_move_assignable_impl<_Tp>
{ };

// 判断是否为平凡的拷贝构造
/// is_trivially_constructible
template<typename _Tp, typename... _Args>
struct is_trivially_constructible
        : public __and_<is_constructible<_Tp, _Args...>, integral_constant<bool,
        __is_trivially_constructible(_Tp, _Args...)>>::type
{ };

// 判断是否为平凡的默认拷贝构造
/// is_trivially_default_constructible
template<typename _Tp>
struct is_trivially_default_constructible
        : public is_trivially_constructible<_Tp>::type
{ };

// 判断是否为平凡的拷贝构造
/// is_trivially_copy_constructible
template<typename _Tp>
struct is_trivially_copy_constructible
        : public __and_<is_copy_constructible<_Tp>,
        integral_constant<bool,
        __is_trivially_constructible(_Tp, const _Tp&)>>::type
{ };

// 判断是否为平凡的移动拷贝构造
/// is_trivially_move_constructible
template<typename _Tp>
struct is_trivially_move_constructible
        : public __and_<is_move_constructible<_Tp>,
        integral_constant<bool,
        __is_trivially_constructible(_Tp, _Tp&&)>>::type
{ };

// 判断是否为平凡的赋值
/// is_trivially_assignable
template<typename _Tp, typename _Up>
struct is_trivially_assignable
        : public __and_<is_assignable<_Tp, _Up>,
        integral_constant<bool,
        __is_trivially_assignable(_Tp, _Up)>>::type
{ };

// 判断是否为平凡的拷贝赋值
/// is_trivially_copy_assignable
template<typename _Tp>
struct is_trivially_copy_assignable
        : public __and_<is_copy_assignable<_Tp>,
        integral_constant<bool,
        __is_trivially_assignable(_Tp&, const _Tp&)>>::type
{ };

// 判断是否为平凡的移动拷贝赋值
/// is_trivially_move_assignable
template<typename _Tp>
struct is_trivially_move_assignable
        : public __and_<is_move_assignable<_Tp>,
        integral_constant<bool,
        __is_trivially_assignable(_Tp&, _Tp&&)>>::type
{ };

// 判断是否为平凡的析构
/// is_trivially_destructible
template<typename _Tp>
struct is_trivially_destructible
        : public __and_<is_destructible<_Tp>, integral_constant<bool,
        __has_trivial_destructor(_Tp)>>::type
{ };

// 判断是否有平凡的默认构造
/// has_trivial_default_constructor (temporary legacy)
template<typename _Tp>
struct has_trivial_default_constructor
        : public integral_constant<bool, __has_trivial_constructor(_Tp)>
{ } _GLIBCXX_DEPRECATED;

// 判断是否有平凡的拷贝构造
/// has_trivial_copy_constructor (temporary legacy)
template<typename _Tp>
struct has_trivial_copy_constructor
        : public integral_constant<bool, __has_trivial_copy(_Tp)>
{ } _GLIBCXX_DEPRECATED;

// 判断是否有平凡的拷贝赋值
/// has_trivial_copy_assign (temporary legacy)
template<typename _Tp>
struct has_trivial_copy_assign
        : public integral_constant<bool, __has_trivial_assign(_Tp)>
{ } _GLIBCXX_DEPRECATED;

// 判断是否有虚析构
/// has_virtual_destructor
template<typename _Tp>
struct has_virtual_destructor
        : public integral_constant<bool, __has_virtual_destructor(_Tp)>
{ };


// type property queries.

// 判断是否对齐
/// alignment_of
template<typename _Tp>
struct alignment_of
        : public integral_constant<std::size_t, __alignof__(_Tp)> { };

// 判断数据的秩: 可以和维度类似，例如单一类型为0, 数组为1, 二维数组为2等
/// rank
template<typename>
struct rank
        : public integral_constant<std::size_t, 0> { };

template<typename _Tp, std::size_t _Size>
struct rank<_Tp[_Size]>
        : public integral_constant<std::size_t, 1 + rank<_Tp>::value> { };

template<typename _Tp>
struct rank<_Tp[]>
        : public integral_constant<std::size_t, 1 + rank<_Tp>::value> { };

// 判断是否为拓展类型， 单一类型为0, 数组返回数组的大小
/// extent
template<typename, unsigned _Uint>
struct extent
        : public integral_constant<std::size_t, 0> { };

template<typename _Tp, unsigned _Uint, std::size_t _Size>
struct extent<_Tp[_Size], _Uint>
        : public integral_constant<std::size_t,
        _Uint == 0 ? _Size : extent<_Tp,
        _Uint - 1>::value>
{ };

template<typename _Tp, unsigned _Uint>
struct extent<_Tp[], _Uint>
        : public integral_constant<std::size_t,
        _Uint == 0 ? 0 : extent<_Tp,
        _Uint - 1>::value>
{ };


// Type relations.
// 是否相同类型，默认为false
/// is_same
template<typename, typename>
struct is_same
        : public false_type { };

template<typename _Tp>
struct is_same<_Tp, _Tp>
        : public true_type { };

// 判断是否为继承关系
/// is_base_of
template<typename _Base, typename _Derived>
struct is_base_of
        : public integral_constant<bool, __is_base_of(_Base, _Derived)>
{ };

// from为void/to为函数/to为数组特化 => void
template<typename _From, typename _To,
         bool = __or_<is_void<_From>, is_function<_To>,
                      is_array<_To>>::value>
struct __is_convertible_helper
{ typedef typename is_void<_To>::type type; };

template<typename _From, typename _To>
class __is_convertible_helper<_From, _To, false>
{
    template<typename _To1>
    static void __test_aux(_To1);

    // 这里判断From1的变量能否作为__test_aux(To1)的参数，如果可以，则可以进行类型转换
    template<typename _From1, typename _To1,
             typename = decltype(__test_aux<_To1>(std::declval<_From1>()))>
    static true_type
    __test(int);

    template<typename, typename>
    static false_type
    __test(...);

public:
    typedef decltype(__test<_From, _To>(0)) type;
};

// 判断是否可以进行类型转换
/// is_convertible
template<typename _From, typename _To>
struct is_convertible
        : public __is_convertible_helper<_From, _To>::type
{ };


// Const-volatile modifications.
// 移除const修饰
/// remove_const
template<typename _Tp>
struct remove_const
{ typedef _Tp     type; };

template<typename _Tp>
struct remove_const<_Tp const>
{ typedef _Tp     type; };
// 移除volatile修饰
/// remove_volatile
template<typename _Tp>
struct remove_volatile
{ typedef _Tp     type; };

template<typename _Tp>
struct remove_volatile<_Tp volatile>
{ typedef _Tp     type; };

// 移除const + volatile修饰
/// remove_cv
template<typename _Tp>
struct remove_cv
{
    typedef typename
    remove_const<typename remove_volatile<_Tp>::type>::type     type;
};

// 添加const修饰
/// add_const
template<typename _Tp>
struct add_const
{ typedef _Tp const     type; };

// 添加volatile修饰
/// add_volatile
template<typename _Tp>
struct add_volatile
{ typedef _Tp volatile     type; };

// 添加const + volatile修饰
/// add_cv
template<typename _Tp>
struct add_cv
{
    typedef typename
    add_const<typename add_volatile<_Tp>::type>::type     type;
};

#if __cplusplus > 201103L

#define __cpp_lib_transformation_trait_aliases 201304

/// Alias template for remove_const
template<typename _Tp>
using remove_const_t = typename remove_const<_Tp>::type;

/// Alias template for remove_volatile
template<typename _Tp>
using remove_volatile_t = typename remove_volatile<_Tp>::type;

/// Alias template for remove_cv
template<typename _Tp>
using remove_cv_t = typename remove_cv<_Tp>::type;

/// Alias template for add_const
template<typename _Tp>
using add_const_t = typename add_const<_Tp>::type;

/// Alias template for add_volatile
template<typename _Tp>
using add_volatile_t = typename add_volatile<_Tp>::type;

/// Alias template for add_cv
template<typename _Tp>
using add_cv_t = typename add_cv<_Tp>::type;
#endif

// Reference transformations.
// 移除引用(包含左值引用+右值引用)
/// remove_reference
template<typename _Tp>
struct remove_reference
{ typedef _Tp   type; };

template<typename _Tp>
struct remove_reference<_Tp&>
{ typedef _Tp   type; };

template<typename _Tp>
struct remove_reference<_Tp&&>
{ typedef _Tp   type; };

template<typename _Tp, bool = __is_referenceable<_Tp>::value>
struct __add_lvalue_reference_helper
{ typedef _Tp   type; };

template<typename _Tp>
struct __add_lvalue_reference_helper<_Tp, true>
{ typedef _Tp&   type; };

// 添加左值引用
/// add_lvalue_reference
template<typename _Tp>
struct add_lvalue_reference
        : public __add_lvalue_reference_helper<_Tp>
{ };

template<typename _Tp, bool = __is_referenceable<_Tp>::value>
struct __add_rvalue_reference_helper
{ typedef _Tp   type; };

template<typename _Tp>
struct __add_rvalue_reference_helper<_Tp, true>
{ typedef _Tp&&   type; };

// 添加右值引用
/// add_rvalue_reference
template<typename _Tp>
struct add_rvalue_reference
        : public __add_rvalue_reference_helper<_Tp>
{ };

#if __cplusplus > 201103L
/// Alias template for remove_reference
template<typename _Tp>
using remove_reference_t = typename remove_reference<_Tp>::type;

/// Alias template for add_lvalue_reference
template<typename _Tp>
using add_lvalue_reference_t = typename add_lvalue_reference<_Tp>::type;

/// Alias template for add_rvalue_reference
template<typename _Tp>
using add_rvalue_reference_t = typename add_rvalue_reference<_Tp>::type;
#endif

// Sign modifications.

// Utility for constructing identically cv-qualified types.
// 添加 const + volatile 修饰
template<typename _Unqualified, bool _IsConst, bool _IsVol>
struct __cv_selector;

template<typename _Unqualified>
struct __cv_selector<_Unqualified, false, false>
{ typedef _Unqualified __type; };

template<typename _Unqualified>
struct __cv_selector<_Unqualified, false, true>
{ typedef volatile _Unqualified __type; };

template<typename _Unqualified>
struct __cv_selector<_Unqualified, true, false>
{ typedef const _Unqualified __type; };

template<typename _Unqualified>
struct __cv_selector<_Unqualified, true, true>
{ typedef const volatile _Unqualified __type; };

// 根据Qualified的属性，对Unqulified选择添加的属性，保证添加: const / volatile / const + volatile / 无添加
template<typename _Qualified, typename _Unqualified,
         bool _IsConst = is_const<_Qualified>::value,
         bool _IsVol = is_volatile<_Qualified>::value>
class __match_cv_qualifiers
{
    typedef __cv_selector<_Unqualified, _IsConst, _IsVol> __match;

public:
    typedef typename __match::__type __type;
};

// Utility for finding the unsigned versions of signed integral types.
// 对整型添加unsigned修饰
template<typename _Tp>
struct __make_unsigned
{ typedef _Tp __type; };

template<>
struct __make_unsigned<char>
{ typedef unsigned char __type; };

template<>
struct __make_unsigned<signed char>
{ typedef unsigned char __type; };

template<>
struct __make_unsigned<short>
{ typedef unsigned short __type; };

template<>
struct __make_unsigned<int>
{ typedef unsigned int __type; };

template<>
struct __make_unsigned<long>
{ typedef unsigned long __type; };

template<>
struct __make_unsigned<long long>
{ typedef unsigned long long __type; };

#if defined(_GLIBCXX_USE_WCHAR_T) && !defined(__WCHAR_UNSIGNED__)
template<>
struct __make_unsigned<wchar_t> : __make_unsigned<__WCHAR_TYPE__>
{ };
#endif

#if defined(__GLIBCXX_TYPE_INT_N_0)
template<>
struct __make_unsigned<__GLIBCXX_TYPE_INT_N_0>
{ typedef unsigned __GLIBCXX_TYPE_INT_N_0 __type; };
#endif
#if defined(__GLIBCXX_TYPE_INT_N_1)
template<>
struct __make_unsigned<__GLIBCXX_TYPE_INT_N_1>
{ typedef unsigned __GLIBCXX_TYPE_INT_N_1 __type; };
#endif
#if defined(__GLIBCXX_TYPE_INT_N_2)
template<>
struct __make_unsigned<__GLIBCXX_TYPE_INT_N_2>
{ typedef unsigned __GLIBCXX_TYPE_INT_N_2 __type; };
#endif
#if defined(__GLIBCXX_TYPE_INT_N_3)
template<>
struct __make_unsigned<__GLIBCXX_TYPE_INT_N_3>
{ typedef unsigned __GLIBCXX_TYPE_INT_N_3 __type; };
#endif

// Select between integral and enum: not possible to be both.
// 在整型和枚举型之间的选择
template<typename _Tp,
         bool _IsInt = is_integral<_Tp>::value,
         bool _IsEnum = is_enum<_Tp>::value>
class __make_unsigned_selector;

// 整形时 => unsigned 整型
template<typename _Tp>
class __make_unsigned_selector<_Tp, true, false>
{
    typedef __make_unsigned<typename remove_cv<_Tp>::type> __unsignedt;
    typedef typename __unsignedt::__type __unsigned_type;
    typedef __match_cv_qualifiers<_Tp, __unsigned_type> __cv_unsigned;

public:
    typedef typename __cv_unsigned::__type __type;
};

// 枚举型时 => unsigned 整型
template<typename _Tp>
class __make_unsigned_selector<_Tp, false, true>
{
    // With -fshort-enums, an enum may be as small as a char.
    typedef unsigned char __smallest;
    static const bool __b0 = sizeof(_Tp) <= sizeof(__smallest);
    static const bool __b1 = sizeof(_Tp) <= sizeof(unsigned short);
    static const bool __b2 = sizeof(_Tp) <= sizeof(unsigned int);
    typedef conditional<__b2, unsigned int, unsigned long> __cond2;
    typedef typename __cond2::type __cond2_type;
    typedef conditional<__b1, unsigned short, __cond2_type> __cond1;
    typedef typename __cond1::type __cond1_type;

public:
    typedef typename conditional<__b0, __smallest, __cond1_type>::type __type;
};

// Given an integral/enum type, return the corresponding unsigned
// integer type.
// Primary template.
// 获取整型的有符号类型
/// make_unsigned
template<typename _Tp>
struct make_unsigned
{ typedef typename __make_unsigned_selector<_Tp>::__type type; };

// 特化布尔类型
// Integral, but don't define.
template<>
struct make_unsigned<bool>;


// Utility for finding the signed versions of unsigned integral types.
// 整型添加signed 修饰
template<typename _Tp>
struct __make_signed
{ typedef _Tp __type; };

template<>
struct __make_signed<char>
{ typedef signed char __type; };

template<>
struct __make_signed<unsigned char>
{ typedef signed char __type; };

template<>
struct __make_signed<unsigned short>
{ typedef signed short __type; };

template<>
struct __make_signed<unsigned int>
{ typedef signed int __type; };

template<>
struct __make_signed<unsigned long>
{ typedef signed long __type; };

template<>
struct __make_signed<unsigned long long>
{ typedef signed long long __type; };

#if defined(_GLIBCXX_USE_WCHAR_T) && defined(__WCHAR_UNSIGNED__)
template<>
struct __make_signed<wchar_t> : __make_signed<__WCHAR_TYPE__>
{ };
#endif

#ifdef _GLIBCXX_USE_C99_STDINT_TR1
template<>
struct __make_signed<char16_t> : __make_signed<uint_least16_t>
{ };
template<>
struct __make_signed<char32_t> : __make_signed<uint_least32_t>
{ };
#endif

#if defined(__GLIBCXX_TYPE_INT_N_0)
template<>
struct __make_signed<unsigned __GLIBCXX_TYPE_INT_N_0>
{ typedef __GLIBCXX_TYPE_INT_N_0 __type; };
#endif
#if defined(__GLIBCXX_TYPE_INT_N_1)
template<>
struct __make_signed<unsigned __GLIBCXX_TYPE_INT_N_1>
{ typedef __GLIBCXX_TYPE_INT_N_1 __type; };
#endif
#if defined(__GLIBCXX_TYPE_INT_N_2)
template<>
struct __make_signed<unsigned __GLIBCXX_TYPE_INT_N_2>
{ typedef __GLIBCXX_TYPE_INT_N_2 __type; };
#endif
#if defined(__GLIBCXX_TYPE_INT_N_3)
template<>
struct __make_signed<unsigned __GLIBCXX_TYPE_INT_N_3>
{ typedef __GLIBCXX_TYPE_INT_N_3 __type; };
#endif

// Select between integral and enum: not possible to be both.
template<typename _Tp,
         bool _IsInt = is_integral<_Tp>::value,
         bool _IsEnum = is_enum<_Tp>::value>
class __make_signed_selector;

template<typename _Tp>
class __make_signed_selector<_Tp, true, false>
{
    typedef __make_signed<typename remove_cv<_Tp>::type> __signedt;
    typedef typename __signedt::__type __signed_type;
    typedef __match_cv_qualifiers<_Tp, __signed_type> __cv_signed;

public:
    typedef typename __cv_signed::__type __type;
};

template<typename _Tp>
class __make_signed_selector<_Tp, false, true>
{
    // With -fshort-enums, an enum may be as small as a char.
    typedef signed char __smallest;
    static const bool __b0 = sizeof(_Tp) <= sizeof(__smallest);
    static const bool __b1 = sizeof(_Tp) <= sizeof(signed short);
    static const bool __b2 = sizeof(_Tp) <= sizeof(signed int);
    typedef conditional<__b2, signed int, signed long> __cond2;
    typedef typename __cond2::type __cond2_type;
    typedef conditional<__b1, signed short, __cond2_type> __cond1;
    typedef typename __cond1::type __cond1_type;

public:
    typedef typename conditional<__b0, __smallest, __cond1_type>::type __type;
};

// Given an integral/enum type, return the corresponding signed
// integer type.
// 获取整型对应的有符号类型
// Primary template.
/// make_signed
template<typename _Tp>
struct make_signed
{ typedef typename __make_signed_selector<_Tp>::__type type; };

// Integral, but don't define.
template<>
struct make_signed<bool>;

#if __cplusplus > 201103L
/// Alias template for make_signed
template<typename _Tp>
using make_signed_t = typename make_signed<_Tp>::type;

/// Alias template for make_unsigned
template<typename _Tp>
using make_unsigned_t = typename make_unsigned<_Tp>::type;
#endif

// Array modifications.
// 获取Tp/T[N]/Tp[]的类型
/// remove_extent
template<typename _Tp>
struct remove_extent
{ typedef _Tp     type; };

template<typename _Tp, std::size_t _Size>
struct remove_extent<_Tp[_Size]>
{ typedef _Tp     type; };

template<typename _Tp>
struct remove_extent<_Tp[]>
{ typedef _Tp     type; };

// 获取Tp/T[N]/Tp[]的类型: 如果Tp是数组时，还会递归获取
/// remove_all_extents
template<typename _Tp>
struct remove_all_extents
{ typedef _Tp     type; };

template<typename _Tp, std::size_t _Size>
struct remove_all_extents<_Tp[_Size]>
{ typedef typename remove_all_extents<_Tp>::type     type; };

template<typename _Tp>
struct remove_all_extents<_Tp[]>
{ typedef typename remove_all_extents<_Tp>::type     type; };

#if __cplusplus > 201103L
/// Alias template for remove_extent
template<typename _Tp>
using remove_extent_t = typename remove_extent<_Tp>::type;

/// Alias template for remove_all_extents
template<typename _Tp>
using remove_all_extents_t = typename remove_all_extents<_Tp>::type;
#endif

// Pointer modifications.

template<typename _Tp, typename>
struct __remove_pointer_helper
{ typedef _Tp     type; };

template<typename _Tp, typename _Up>
struct __remove_pointer_helper<_Tp, _Up*>
{ typedef _Up     type; };

// 获取指针的类型
/// remove_pointer
template<typename _Tp>
struct remove_pointer
        : public __remove_pointer_helper<_Tp, typename remove_cv<_Tp>::type>
{ };

// 添加指针类型
/// add_pointer
template<typename _Tp, bool = __or_<__is_referenceable<_Tp>,
                                    is_void<_Tp>>::value>
struct __add_pointer_helper
{ typedef _Tp     type; };

template<typename _Tp>
struct __add_pointer_helper<_Tp, true>
{ typedef typename remove_reference<_Tp>::type*     type; };

template<typename _Tp>
struct add_pointer
        : public __add_pointer_helper<_Tp>
{ };

#if __cplusplus > 201103L
/// Alias template for remove_pointer
template<typename _Tp>
using remove_pointer_t = typename remove_pointer<_Tp>::type;

/// Alias template for add_pointer
template<typename _Tp>
using add_pointer_t = typename add_pointer<_Tp>::type;
#endif

// 按照Len对齐
template<std::size_t _Len>
struct __aligned_storage_msa
{
    union __type
    {
        unsigned char __data[_Len];
        struct __attribute__((__aligned__)) { } __align;
    };
};

/**
   *  @brief Alignment type.
   *
   *  The value of _Align is a default-alignment which shall be the
   *  most stringent alignment requirement for any C++ object type
   *  whose size is no greater than _Len (3.9). The member typedef
   *  type shall be a POD type suitable for use as uninitialized
   *  storage for any object whose size is at most _Len and whose
   *  alignment is a divisor of _Align.
  */
// 按照Len内存对齐
template<std::size_t _Len, std::size_t _Align =
         __alignof__(typename __aligned_storage_msa<_Len>::__type)>
struct aligned_storage
{
    union type
    {
        unsigned char __data[_Len];
        struct __attribute__((__aligned__((_Align)))) { } __align;
    };
};

template <typename... _Types>
struct __strictest_alignment
{
    static const size_t _S_alignment = 0;
    static const size_t _S_size = 0;
};

template <typename _Tp, typename... _Types>
struct __strictest_alignment<_Tp, _Types...>
{
    static const size_t _S_alignment =
            alignof(_Tp) > __strictest_alignment<_Types...>::_S_alignment
            ? alignof(_Tp) : __strictest_alignment<_Types...>::_S_alignment;
    static const size_t _S_size =
            sizeof(_Tp) > __strictest_alignment<_Types...>::_S_size
            ? sizeof(_Tp) : __strictest_alignment<_Types...>::_S_size;
};

/**
   *  @brief Provide aligned storage for types.
   *
   *  [meta.trans.other]
   *
   *  Provides aligned storage for any of the provided types of at
   *  least size _Len.
   *
   *  @see aligned_storage
   */
// 按照Len对齐Type
template <size_t _Len, typename... _Types>
struct aligned_union
{
private:
    static_assert(sizeof...(_Types) != 0, "At least one type is required");

    using __strictest = __strictest_alignment<_Types...>;
    static const size_t _S_len = _Len > __strictest::_S_size
            ? _Len : __strictest::_S_size;
public:
    /// The value of the strictest alignment of _Types.
    static const size_t alignment_value = __strictest::_S_alignment;
    /// The storage.
    typedef typename aligned_storage<_S_len, alignment_value>::type type;
};

template <size_t _Len, typename... _Types>
const size_t aligned_union<_Len, _Types...>::alignment_value;

// Decay trait for arrays and functions, used for perfect forwarding
// in make_pair, make_tuple, etc.
template<typename _Up,
         bool _IsArray = is_array<_Up>::value,
         bool _IsFunction = is_function<_Up>::value>
struct __decay_selector;

// NB: DR 705.
template<typename _Up>
struct __decay_selector<_Up, false, false>
{ typedef typename remove_cv<_Up>::type __type; };

template<typename _Up>
struct __decay_selector<_Up, true, false>
{ typedef typename remove_extent<_Up>::type* __type; };

template<typename _Up>
struct __decay_selector<_Up, false, true>
{ typedef typename add_pointer<_Up>::type __type; };

// 非数组 & 非函数: 移除引用 + 移除const + volatile修饰
// 数组: 移除引用 + 获取数组第一层的类型
// 函数: 移除引用 + 添加指针类型
/// decay
template<typename _Tp>
class decay
{
    typedef typename remove_reference<_Tp>::type __remove_type;

public:
    typedef typename __decay_selector<__remove_type>::__type type;
};

template<typename _Tp>
class reference_wrapper;

// Helper which adds a reference to a type when given a reference_wrapper
template<typename _Tp>
struct __strip_reference_wrapper
{
    typedef _Tp __type;
};

template<typename _Tp>
struct __strip_reference_wrapper<reference_wrapper<_Tp> >
{
    typedef _Tp& __type;
};

template<typename _Tp>
struct __decay_and_strip
{
    typedef typename __strip_reference_wrapper<
    typename decay<_Tp>::type>::__type __type;
};

// 条件为true时，定义类型
// Primary template.
/// Define a member typedef @c type only if a boolean constant is true.
template<bool, typename _Tp = void>
struct enable_if
{ };

// Partial specialization for true.
template<typename _Tp>
struct enable_if<true, _Tp>
{ typedef _Tp type; };

// 多个条件
template<typename... _Cond>
using _Require = typename enable_if<__and_<_Cond...>::value>::type;

// 条件为true时，获取Iftrue的类型; false时，获取Iffalse的类型
// Primary template.
/// Define a member typedef @c type to one of two argument types.
template<bool _Cond, typename _Iftrue, typename _Iffalse>
struct conditional
{ typedef _Iftrue type; };

// Partial specialization for false.
template<typename _Iftrue, typename _Iffalse>
struct conditional<false, _Iftrue, _Iffalse>
{ typedef _Iffalse type; };

/// common_type
template<typename... _Tp>
struct common_type;

// Sfinae-friendly common_type implementation:

struct __do_common_type_impl
{
    template<typename _Tp, typename _Up>
    static __success_type<typename decay<decltype
    (true ? std::declval<_Tp>()
          : std::declval<_Up>())>::type> _S_test(int);

    template<typename, typename>
    static __failure_type _S_test(...);
};

template<typename _Tp, typename _Up>
struct __common_type_impl
        : private __do_common_type_impl
{
    typedef decltype(_S_test<_Tp, _Up>(0)) type;
};

struct __do_member_type_wrapper
{
    template<typename _Tp>
    static __success_type<typename _Tp::type> _S_test(int);

    template<typename>
    static __failure_type _S_test(...);
};

template<typename _Tp>
struct __member_type_wrapper
        : private __do_member_type_wrapper
{
    typedef decltype(_S_test<_Tp>(0)) type;
};

template<typename _CTp, typename... _Args>
struct __expanded_common_type_wrapper
{
    typedef common_type<typename _CTp::type, _Args...> type;
};

template<typename... _Args>
struct __expanded_common_type_wrapper<__failure_type, _Args...>
{ typedef __failure_type type; };

// 获取所有类型都兼容的类型
template<typename _Tp>
struct common_type<_Tp>
{ typedef typename decay<_Tp>::type type; };

template<typename _Tp, typename _Up>
struct common_type<_Tp, _Up>
        : public __common_type_impl<_Tp, _Up>::type
{ };

template<typename _Tp, typename _Up, typename... _Vp>
struct common_type<_Tp, _Up, _Vp...>
        : public __expanded_common_type_wrapper<typename __member_type_wrapper<
        common_type<_Tp, _Up>>::type, _Vp...>::type
{ };

// 枚举型的底层类型
/// The underlying type of an enum.
template<typename _Tp>
struct underlying_type
{
    typedef __underlying_type(_Tp) type;
};

template<typename _Tp>
struct __declval_protector
{
    static const bool __stop = false;
    static typename add_rvalue_reference<_Tp>::type __delegate();
};

// declval()函数，用于模板推导， 实际上就是返回Tp的右值引用
template<typename _Tp>
inline typename add_rvalue_reference<_Tp>::type
declval() noexcept
{
    static_assert(__declval_protector<_Tp>::__stop,
                  "declval() must not be used!");
    return __declval_protector<_Tp>::__delegate();
}

/// result_of
template<typename _Signature>
class result_of;

// Sfinae-friendly result_of implementation:

#define __cpp_lib_result_of_sfinae 201210

// [func.require] paragraph 1 bullet 1:
struct __result_of_memfun_ref_impl
{
    template<typename _Fp, typename _Tp1, typename... _Args>
    static __success_type<decltype(
            (std::declval<_Tp1>().*std::declval<_Fp>())(std::declval<_Args>()...)
            )> _S_test(int);

    template<typename...>
    static __failure_type _S_test(...);
};

template<typename _MemPtr, typename _Arg, typename... _Args>
struct __result_of_memfun_ref
        : private __result_of_memfun_ref_impl
{
    typedef decltype(_S_test<_MemPtr, _Arg, _Args...>(0)) type;
};

// [func.require] paragraph 1 bullet 2:
struct __result_of_memfun_deref_impl
{
    template<typename _Fp, typename _Tp1, typename... _Args>
    static __success_type<decltype(
            ((*std::declval<_Tp1>()).*std::declval<_Fp>())(std::declval<_Args>()...)
            )> _S_test(int);

    template<typename...>
    static __failure_type _S_test(...);
};

template<typename _MemPtr, typename _Arg, typename... _Args>
struct __result_of_memfun_deref
        : private __result_of_memfun_deref_impl
{
    typedef decltype(_S_test<_MemPtr, _Arg, _Args...>(0)) type;
};

// [func.require] paragraph 1 bullet 3:
struct __result_of_memobj_ref_impl
{
    template<typename _Fp, typename _Tp1>
    static __success_type<decltype(
            std::declval<_Tp1>().*std::declval<_Fp>()
            )> _S_test(int);

    template<typename, typename>
    static __failure_type _S_test(...);
};

template<typename _MemPtr, typename _Arg>
struct __result_of_memobj_ref
        : private __result_of_memobj_ref_impl
{
    typedef decltype(_S_test<_MemPtr, _Arg>(0)) type;
};

// [func.require] paragraph 1 bullet 4:
struct __result_of_memobj_deref_impl
{
    template<typename _Fp, typename _Tp1>
    static __success_type<decltype(
            (*std::declval<_Tp1>()).*std::declval<_Fp>()
            )> _S_test(int);

    template<typename, typename>
    static __failure_type _S_test(...);
};

template<typename _MemPtr, typename _Arg>
struct __result_of_memobj_deref
        : private __result_of_memobj_deref_impl
{
    typedef decltype(_S_test<_MemPtr, _Arg>(0)) type;
};

template<typename _MemPtr, typename _Arg>
struct __result_of_memobj;

template<typename _Res, typename _Class, typename _Arg>
struct __result_of_memobj<_Res _Class::*, _Arg>
{
    typedef typename remove_cv<typename remove_reference<
    _Arg>::type>::type _Argval;
    typedef _Res _Class::* _MemPtr;
    typedef typename conditional<__or_<is_same<_Argval, _Class>,
    is_base_of<_Class, _Argval>>::value,
    __result_of_memobj_ref<_MemPtr, _Arg>,
    __result_of_memobj_deref<_MemPtr, _Arg>
    >::type::type type;
};

template<typename _MemPtr, typename _Arg, typename... _Args>
struct __result_of_memfun;

template<typename _Res, typename _Class, typename _Arg, typename... _Args>
struct __result_of_memfun<_Res _Class::*, _Arg, _Args...>
{
    typedef typename remove_cv<typename remove_reference<
    _Arg>::type>::type _Argval;
    typedef _Res _Class::* _MemPtr;
    typedef typename conditional<__or_<is_same<_Argval, _Class>,
    is_base_of<_Class, _Argval>>::value,
    __result_of_memfun_ref<_MemPtr, _Arg, _Args...>,
    __result_of_memfun_deref<_MemPtr, _Arg, _Args...>
    >::type::type type;
};

template<bool, bool, typename _Functor, typename... _ArgTypes>
struct __result_of_impl
{
    typedef __failure_type type;
};

template<typename _MemPtr, typename _Arg>
struct __result_of_impl<true, false, _MemPtr, _Arg>
        : public __result_of_memobj<typename decay<_MemPtr>::type, _Arg>
{ };

template<typename _MemPtr, typename _Arg, typename... _Args>
struct __result_of_impl<false, true, _MemPtr, _Arg, _Args...>
        : public __result_of_memfun<typename decay<_MemPtr>::type, _Arg, _Args...>
{ };

// [func.require] paragraph 1 bullet 5:
struct __result_of_other_impl
{
    template<typename _Fn, typename... _Args>
    static __success_type<decltype(
            std::declval<_Fn>()(std::declval<_Args>()...)
            )> _S_test(int);

    template<typename...>
    static __failure_type _S_test(...);
};

template<typename _Functor, typename... _ArgTypes>
struct __result_of_impl<false, false, _Functor, _ArgTypes...>
        : private __result_of_other_impl
{
    typedef decltype(_S_test<_Functor, _ArgTypes...>(0)) type;
};

// 函数的返回值类型
template<typename _Functor, typename... _ArgTypes>
struct result_of<_Functor(_ArgTypes...)>
    : public __result_of_impl<
             is_member_object_pointer<
             typename remove_reference<_Functor>::type
             >::value,
             is_member_function_pointer<
             typename remove_reference<_Functor>::type
             >::value,
             _Functor, _ArgTypes...
             >::type
{ };

#if __cplusplus > 201103L
/// Alias template for aligned_storage
template<size_t _Len, size_t _Align =
         __alignof__(typename __aligned_storage_msa<_Len>::__type)>
using aligned_storage_t = typename aligned_storage<_Len, _Align>::type;

template <size_t _Len, typename... _Types>
using aligned_union_t = typename aligned_union<_Len, _Types...>::type;

/// Alias template for decay
template<typename _Tp>
using decay_t = typename decay<_Tp>::type;

/// Alias template for enable_if
template<bool _Cond, typename _Tp = void>
using enable_if_t = typename enable_if<_Cond, _Tp>::type;

/// Alias template for conditional
template<bool _Cond, typename _Iftrue, typename _Iffalse>
using conditional_t = typename conditional<_Cond, _Iftrue, _Iffalse>::type;

/// Alias template for common_type
template<typename... _Tp>
using common_type_t = typename common_type<_Tp...>::type;

/// Alias template for underlying_type
template<typename _Tp>
using underlying_type_t = typename underlying_type<_Tp>::type;

/// Alias template for result_of
template<typename _Tp>
using result_of_t = typename result_of<_Tp>::type;
#endif

template<typename...> using __void_t = void;

/// @} group metaprogramming

/**
   *  Use SFINAE to determine if the type _Tp has a publicly-accessible
   *  member type _NTYPE.
   */
#define _GLIBCXX_HAS_NESTED_TYPE(_NTYPE)				\
    template<typename _Tp, typename = __void_t<>>				\
    struct __has_##_NTYPE						\
    : false_type							\
{ };								\
    template<typename _Tp>						\
    struct __has_##_NTYPE<_Tp, __void_t<typename _Tp::_NTYPE>>		\
    : true_type								\
{ };

_GLIBCXX_END_NAMESPACE_VERSION
} // namespace std

#endif  // C++11

#endif  // _GLIBCXX_TYPE_TRAITS

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