std::integral_constant

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C++
 
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类型的支持
基本类型
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Basic types
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基本类型
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is_constructible
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is_assignable
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is_copy_assignable
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is_move_assignable
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is_nothrow_move_assignable		（C++11）
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键入特征常数
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Type trait constants
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integral_constant（C++11）
 
Defined in header <type_traits>
template< class T, T v >
struct integral_constant;
 （C++11 起）
std::integral_constant包装指定类型的静态常量。它的基类是C + +的类型特征.
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std::integral_constant wraps a static constant of specified type. It is the base class for the C++ type traits.
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两个专业的类型bool
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Two specializations for the type bool are provided:
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Defined in header <type_traits>
类型
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Type
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 Definition
true_type std::integral_constant<bool, true>
false_type std::integral_constant<bool, false>

目录

[编辑] 会员类型

类型
Original:
Type
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 Definition
value_type T
type std::integral_constant<T,v>

[编辑] 会员常数

姓名
Original:
Name
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 Value
value
静态的常量表达式的值类型Tv
Original:
static constant expression of type T with value v
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[编辑] 成员函数

operator value_type
转换的对象value_type，返回value
Original:
converts the object to value_type, returns value
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（公共成员函数）

[编辑] 可能的实现

template<class T, T v>
struct integral_constant {
    static constexpr T value = v;
    typedef T value_type;
    typedef integral_constant type;
    constexpr operator value_type() const { return value; }
};

[编辑] 为例

#include <iostream>
#include <type_traits>
 
int main()
{
    typedef std::integral_constant<int, 2> two_t;
    typedef std::integral_constant<int, 4> four_t;
 
//  static_assert(std::is_same<two_t, four_t>::value,
//                "two_t and four_t are not equal!");
//  error: static assertion failed: "two_t and four_t are not a same types"
 
    static_assert(two_t::value*2 == four_t::value,
       "2*2 != 4"
    );
 
    enum class my_e {
       e1,
       e2
    };
    typedef std::integral_constant<my_e, my_e::e1> my_e_e1;
    typedef std::integral_constant<my_e, my_e::e2> my_e_e2;
 
//  static_assert(my_e_e1::value == my_e::e2,
//               "my_e_e1::value != my_e::e2");
//  error: static assertion failed: "my_e_e1::value != my_e::e2"
 
    static_assert(std::is_same<my_e_e2, my_e_e2>::value,
                  "my_e_e2 != my_e_e2");
}


来自“http://zh.cppreference.com/mwiki/index.php?title=cpp/types/integral_constant&oldid=27598”