std:: 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.

Helper variable template

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';
}

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

See also