is_placeholder Class

 

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Tests if type is a placeholder.

struct is_placeholder {
static const int value;
};

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.

// std_tr1__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  

Header: <functional>

Namespace: std

_1 Object

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