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C++
应用 | Utilities

std::is_placeholder

STD::是[医]占位符

Defined in header
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::value;(since C++17)

继承自STD:积分[医]常量

成员常数

value staticplaceholder value or 0 for non-placeholder types (public static member constant)

成员函数

operator intconverts the object to int, returns value (public member function)
operator() (C++14)returns value (public member function)

成员类型

TypeDefinition
value_typeint
typestd::integral_constant<int, value>

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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