std::common_type

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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_move_constructible
is_trivially_move_constructible
is_nothrow_move_constructible		（C++11）
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is_assignable
is_trivially_assignable
is_nothrow_assignable		（C++11）
（C++11）
（C++11）
is_copy_assignable
is_trivially_copy_assignable
is_nothrow_copy_assignable		（C++11）
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is_move_assignable
is_trivially_move_assignable
is_nothrow_move_assignable		（C++11）
（C++11）
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remove_const
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类型转换
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Type transformations
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aligned_storage（C++11）
aligned_union（C++11）
decay（C++11）
enable_if（C++11）
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common_type（C++11）
underlying_type（C++11）
result_of（C++11）
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 >
struct common_type;
 （C++11 起）
确定各类型T...常见的类型，这是所有T...可以隐式类型转换为.
Original:
Determines the common type among all types T..., that is the type all T... can be implicitly converted to.
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  • std::common_type可能是专门为自定义类型，例如std::chrono::time_point.
    Original:
    std::common_type may be specialized for custom types, e.g. std::chrono::time_point.
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  • 对于非专门std::common_type，每对之间的常见的类型的规则用于确定T1, T2是完全相同的规则用于确定返回类型的三元条件运算符T1和T2是它的第二个和第三个操作数的类型.
    Original:
    For non-specialized std::common_type, the rules for determining the common type between every pair T1, T2 are exactly the rules for determining the return type of the 三元条件运算符 where T1 and T2 are the types of its second and the third operands.
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  • 对于算术类型，常见的类型也可以被视为类型的（可能是混合模式）的算术表达式，如T0() + T1() + ... + Tn().
    Original:
    For arithmetic types, the common type may also be viewed as the type of the (possibly mixed-mode) arithmetic expression such as T0() + T1() + ... + Tn().
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目录

[编辑] 会员类型

姓名
Original:
Name
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 Definition
type
所有T...常见的类型
Original:
the common type for all T...
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[编辑] 专业化

自定义的类型特征std::common_type专业是允许的。以下专业的标准库已经提供了
Original:
Custom specializations of the type trait std::common_type are allowed. The following specializations are already provided by the standard library:
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专业的std::common_type特征
Original:
specializes the std::common_type trait
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（类模板专业化） [edit]
专业的std::common_type特征
Original:
specializes the std::common_type trait
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（类模板专业化） [edit]

[编辑] 注释

随着一个单独的模板参数，std::common_type<T>::type正是T，这使得它在C + +相当于boost::identity<T>
Original:
With a single template argument, std::common_type<T>::type is exactly T, which makes it the C++ equivalent of boost::identity<T>
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[编辑] 可能的实现

template<class ...T> struct common_type;
 
template<class T>
struct common_type<T> {
    typedef T type;
};
 
template<class T, class U>
struct common_type<T, U> {
    typedef decltype(true ? declval<T>() : declval<U>()) type;
};
 
template<class T, class U, class... V>
struct common_type<T, U, V...> {
    typedef typename common_type<typename common_type<T, U>::type, V...>::type type;
};

[编辑] 为例

一个用户定义的类演示混合模式运算
Original:
Demonstrates mixed-mode arithmetic on a user-defined class
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#include <iostream>
#include <type_traits>
 
template<class T>
struct Number { T n; };
 
template<class T, class U>
Number<typename std::common_type<T, U>::type> operator+(const Number<T>& lhs,
                                                        const Number<U>& rhs)
{
    return {lhs.n + rhs.n};
}
 
int main()
{
    Number<int> i1 = {1}, i2 = {2};
    Number<double> d1 = {2.3}, d2 = {3.5};
    std::cout << "i1i2: " << (i1 + i2).n << "\ni1d2: " << (i1 + d2).n << '\n'
              << "d1i2: " << (d1 + i2).n << "\nd1d2: " << (d1 + d2).n << '\n';
}

Output: 

i1i2: 3
i1d2: 4.5
d1i2: 4.3
d1d2: 5.8
来自“http://zh.cppreference.com/mwiki/index.php?title=cpp/types/common_type&oldid=27649”