std::is_placeholder(3)

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>.
   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.

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 <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';
 }

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

See also
   bind binds one or more arguments to a function object
   (C++11) (function template)
   _1, _2, _3, _4, ... placeholders for the unbound arguments in a std::bind expression
   (C++11) (constant)