| 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 > constexpr bool 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 <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
| (C++11) | binds one or more arguments to a function object (function template) |
| (C++11) | placeholders for the unbound arguments in a std::bind expression (constant) |
| std::experimental::is_placeholder_v
(library fundamentals TS) | variable template alias of std::is_placeholder::value (variable template) |
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