std::is_placeholder - cppreference.com (original) (raw)

| | | | | -------------------------------------------- | | ------------- | | template< class T > struct is_placeholder; | | (since C++11) |

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 Nth placeholder type.

std::bind uses std::is_placeholder to detect placeholders for unbound arguments.

Contents

• 1 Helper variable template
• 2 Inherited from std::integral_constant
  • 2.1 Member constants
  • 2.2 Member functions
  • 2.3 Member types
  • 2.4 Example
  • 2.5 See also

[edit] Helper variable template

| template< class T > constexpr int is_placeholder_v = is_placeholder<T>::value; | | (since C++17) | | ------------------------------------------------------------------------------------ | | ------------- |

Inherited from std::integral_constant

Member constants

| | placeholder value or ​0​ for non-placeholder types (public static member constant) | | ------------------------------------------------------------------------------------- |

Member functions

| | converts the object to int, returns value (public member function) | | --------------------------------------------------------------------- | | | returns value (public member function) |

Member types

[edit] Example

#include #include #include   struct My_2 {} my_2;   namespace std { template<> struct is_placeholder : 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

[edit] See also

| | binds one or more arguments to a function object (function template) [edit] | | ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | | | placeholders for the unbound arguments in a std::bind expression (constant) [edit] |