std::is_placeholder (3) - Linux Man Pages

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

See also


bind binds one or more arguments to a function object
                    (function template)
(C++11)


_1,__2,__3,__4,_... placeholders for the unbound arguments in a std::bind expression
                    (constant)
(C++11)