std::is_placeholder
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is_placeholder
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Defined in header
<functional>
template< class T >
struct is_placeholder;
(since C++11)
struct is_placeholder;
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
[edit] Helper variable template
template< class T >
constexpr int is_placeholder_v = is_placeholder<T>::value;
(since C++17)
constexpr int is_placeholder_v = is_placeholder<T>::value;
Inherited from std::integral_constant
Member constants
value
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(public static member constant)
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operator int
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operator()
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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