table of contents
std::is_placeholder(3) | C++ Standard Libary | std::is_placeholder(3) |
NAME¶
std::is_placeholder - std::is_placeholder
Synopsis¶
Defined in header <functional>
template< class T > (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>
The template may be specialized for any user-defined T type: the
specialization must
satisfy UnaryTypeTrait with base characteristic 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 > (since C++17)
inline constexpr int is_placeholder_v = is_placeholder<T>::value;
Inherited from std::integral_constant
Member constants¶
value placeholder value or 0 for non-placeholder types
[static] (public static member constant)
Member functions¶
operator int converts the object to int, returns value
(public member function)
operator() returns value
(C++14) (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¶
bind binds one or more arguments to a function object
(C++11) (function template)
_1, _2, _3, _4, ... placeholders for the unbound arguments in a std::bind
expression
(C++11) (constant)
2022.07.31 | http://cppreference.com |