std::is_placeholder (3) - Linux Manuals

std::is_placeholder: std::is_placeholder

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>
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> {};
  }

  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