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std::is_placeholder

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 T type: the specialization must satisfy UnaryTypeTrait with base characteristic 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 >
inline constexpr int is_placeholder_v = is_placeholder<T>::value;

(since C++17)

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)
operator() (C++14)	returns `value` (public member function)

Member types

Type	Definition
`value_type`	int
`type`	std::integral_constant<int, value>

Example

#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

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)