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 type which should be treated by std::bind as if it was a placeholder for unbound arguments.
Contents |
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) |
Member types
| Type | Definition |
| value_type | int |
| type | std::integral_constant<int, value> |
[edit] 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) << '\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) |
| (C++11) |
placeholders for the unbound arguments in a std::bind expression (constant) |
