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std::declval

STD：：解密

Defined in header ​​
template<class T> typename std::add_rvalue_reference<T>::type declval(​(since C++11)

转换任何类型T的引用类型，从而可以在decltype表达式，不需要遍历构造函数。

declval通常在模板中使用，其中可接受的模板参数可能没有共同的构造函数，但具有相同的成员函数，其返回类型是必需的。

请注意，因为没有定义declval，它只能用于未评估上下文；计算包含此函数的表达式是错误的。形式上，如果此函数是ODR-使用...

参数

%280%29

返回值

无法调用，因此永远不会返回值。返回类型是T&&除非T%28可能是cv-合格%29void，在这种情况下，返回类型是T...

例外

noexcept规格：

noexcept

例

二次

#include <utility>
#include <iostream>
 
struct Default { int foo() const { return 1; } };
 
struct NonDefault
{
    NonDefault(const NonDefault&) { }
    int foo() const { return 1; }
};
 
int main()
{
    decltype(Default().foo()) n1 = 1;                   // type of n1 is int
//  decltype(NonDefault().foo()) n2 = n1;               // error: no default constructor
    decltype(std::declval<NonDefault>().foo()) n2 = n1; // type of n2 is int
    std::cout << "n1 = " << n1 << '\n'
              << "n2 = " << n2 << '\n';
}

二次

产出：

二次

n1 = 1
n2 = 1

二次

另见

decltype specifierdefines a type equivalent to the type of an expression (C++11)
result_ofinvoke_result (C++11)(deprecated in C++17)(C++17)deduces the result type of invoking a callable object with a set of arguments (class template)

 © cppreference.com

在CreativeCommonsAttribution下授权-ShareAlike未移植许可v3.0。

http：//en.cppreference.com/w/cpp/实用程序/decval

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