std::is_placeholder

From cppreference.com

Defined in header `<functional>`

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>.

The template may be specialized for any user-defined type which should be treated by std::bind as if it was a placeholder for unbound arguments.

Inherited from std::integral_constant

Member constants


value [static]	placeholder value or 0 for non-placeholder types (public static member constant)

Member functions


operator int	converts the object to int, returns `value` (public member function)

Member types


Type	Definition

`value_type`	`int`

`type`	std::integral_constant<int, value>

[edit] 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) 
              << '\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


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

_1, _2, _3, _4, ... (C++11)	placeholders for the unbound arguments in a `std::bind` expression (constant)

Language
Standard library headers
Concepts
Utilities library
Strings library
Containers library
Algorithms library
Iterators library
Numerics library
Input/output library
Localizations library
Regular expressions library (C++11)
Atomic operations library (C++11)
Thread support library (C++11)