NAME¶

std::is_placeholder - std::is_placeholder

Synopsis¶

Defined in header <functional>
template< class T > (since C++11)
struct is_placeholder;

If T is the type of a standard placeholder (_1, _2, _3, ...), then this template is
derived from std::integral_constant<int, 1>, std::integral_constant<int, 2>,
std::integral_constant<int, 3>, respectively.

If T is not a standard placeholder type, this template is derived from
std::integral_constant<int, 0>.

A program may specialize this template for a program-defined type T to implement
UnaryTypeTrait with base characteristic of std::integral_constant<int, N> with
positive N to indicate that T should be treated as N^th placeholder type.

std::bind uses std::is_placeholder to detect placeholders for unbound arguments.

Member constants¶

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

Member functions¶

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

Member types¶

Type Definition
value_type int
type std::integral_constant<int, value>

Example¶

// Run this code

#include <functional>
#include <iostream>
#include <type_traits>

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_v<decltype(std::placeholders::_5)>
<< '\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';
}

Output:¶

Standard placeholder _5 is for the argument number 5
Adding 2 to 11 selected with a custom placeholder gives 13

See also¶

bind binds one or more arguments to a function object
(C++11) (function template)
_1, _2, _3, _4, ... placeholders for the unbound arguments in a std::bind expression
(C++11) (constant)