std::is_placeholder
Defined in header <functional> | ? | ? |
|---|---|---|
template< class T > struct is_placeholder; | ? | (since C++11) |
如果T是标准占位符%28的类型。[医]1，[医]2，[医]3，%29，然后从std::integral_constant<int,1>，，，std::integral_constant<int,2>，，，std::integral_constant<int,3>分别。
如果T不是标准占位符类型，此模板派生自std::integral_constant<int,0>
模板可以专门用于任何用户定义的。T类型：专业化必须满足UnaryTypeTrait带着基本特征成std::integral_constant<int, N>带着N > 0以表明T应视为N%第二十七占位符类型。
std::bind使用std::is_placeholder若要检测未绑定参数的占位符，请执行以下操作。
辅助变量模板
template< class T > inline constexpr bool is_placeholder_v = is_placeholder<T>::value; | ? | (since C++17) |
|---|
继承自STD：积分[医]常量
成员常数
value static | placeholder value or 0 for non-placeholder types (public static member constant) |
|---|
成员函数
operator int | converts the object to int, returns value (public member function) |
|---|---|
operator() (C++14) | returns value (public member function) |
成员类型
Type | Definition |
|---|---|
value_type | int |
type | std::integral_constant<int, value> |
例
二次
#include <iostream>
#include <type_traits>
#include <functional>
struct My_2 {
} my_2;
namespace std {
template<>
struct is_placeholder<My_2> : public integral_constant<int, 2> {};
}
int f(int n1, int n2)
{
return n1+n2;
}
int main()
{
std::cout << "Standard placeholder _5 is for the argument number "
<< std::is_placeholder<decltype(std::placeholders::_5)>::value
<< '\n';
auto b = std::bind(f, my_2, 2);
std::cout << "Adding 2 to 11 selected with a custom placeholder gives "
<< b(10, 11) // the first argument, namely 10, is ignored
<< '\n';
}二次
产出：
二次
Standard placeholder _5 is for the argument number 5
Adding 2 to 11 selected with a custom placeholder gives 13二次
另见
bind (C++11) | binds one or more arguments to a function object (function template) |
|---|---|
_1, _2, _3, _4, ... (C++11) | placeholders for the unbound arguments in a std::bind expression (constant) |
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