std::is_placeholder

Since C++11

Header: <functional>

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.

# Declarations

template< class T >
struct is_placeholder;

(since C++11)

# Example

#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';
}

# See also