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>

The template may be specialized for any user-defined T type: the specialization must
satisfy UnaryTypeTrait with base characteristic of std::integral_constant<int, N>
with N > 0 to indicate that T should be treated as N'th placeholder type.

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

Helper variable template

template< class T > (since C++17)
inline constexpr int is_placeholder_v = is_placeholder<T>::value;

Inherited from std::integral_constant

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 <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';
}

Output:

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