# std::is_placeholder

From cppreference.com
< cpp‎ | utility‎ | functional

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 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 > 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

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 (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)

Retrieved from "https://en.cppreference.com/mwiki/index.php?title=cpp/utility/functional/is_placeholder&oldid=103312"