template< class T >
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.
template< class T >
constexpr int is_placeholder_v = is_placeholder<T>::value;
#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[edit] See also binds one or more arguments to a function object
std::bind expression
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