is_placeholder Class

Tests if type is a placeholder.

Syntax

struct is_placeholder {
   static const int value;
};

Remarks

The constant value value is 0 if the type Ty is not a placeholder; otherwise, its value is the position of the function call argument that it binds to. You use it to determine the value N for the Nth placeholder _N.

Example

// std__functional__is_placeholder.cpp
// compile with: /EHsc
#include <functional>
#include <iostream>

using namespace std::placeholders;

template<class Expr>
void test_for_placeholder(const Expr&)
{
    std::cout << std::is_placeholder<Expr>::value << std::endl;
}

int main()
{
    test_for_placeholder(3.0);
    test_for_placeholder(_3);

    return (0);
}

0
3