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std::is_placeholder

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< cpp‎ | utility‎ | functional
 
 
Utilities library
General utilities
Relational operators (deprecated in C++20)
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(C++11)
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Function objects
(C++20)(C++23)
(C++11)
is_placeholder
(C++11)
(C++17)(C++23)
Identity function object
(C++20)
(until C++17*)
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(until C++17*)(until C++17*)
(until C++17*)(until C++17*)

(until C++17*)
(until C++17*)(until C++17*)(until C++17*)(until C++17*)
(until C++20*)
(until C++20*)
 
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>.

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.

[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

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>

[edit] Example

Run this code
#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

[edit] See also

(C++11)
binds one or more arguments to a function object
(function template) [edit]
placeholders for the unbound arguments in a std::bind expression
(constant) [edit]
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