Std::is_placeholder (3) Linux Manual Page

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 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>
(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
(public static member constant)
[static]

Member functions

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

operator() returns value
(public member function)
(C++14)

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> {
};

} // namespace std
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

std::is_placeholder (3) Linux Manual Page