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

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

Inherited from std::integral_constant

Member constants

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

Member functions

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

Member types

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


#include <iostream>
#include <type_traits>
#include <functional>
struct My_2 {
} my_2;
namespace std {
    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';


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

See also

binds one or more arguments to a function object
(function template)
placeholders for the unbound arguments in a std::bind expression

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