std::is_placeholder
From cppreference.com
< cpp | utility | functional
| Defined in header <functional>
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||
| 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
| (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) |