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_1 Object

Placeholders for replaceable arguments.

namespace placeholders {
  extern unspecified _1, _2, ... _M
  } // namespace placeholders (within std)

The objects _1, _2, ... _M are placeholders designating the first, second, ..., Mth argument, respectively in a function call to an object returned by bind Function. You use _N to specify where the Nth argument should be inserted when the bind expression is evaluated.

In this implementation the value of M is 20.

 

// std__functional_placeholder.cpp 
// compile with: /EHsc 
#include <functional> 
#include <algorithm> 
#include <iostream> 
 
using namespace std::placeholders; 
 
void square(double x) 
    { 
    std::cout << x << "^2 == " << x * x << std::endl; 
    } 
 
void product(double x, double y) 
    { 
    std::cout << x << "*" << y << " == " << x * y << std::endl; 
    } 
 
int main() 
    { 
    double arg[] = {1, 2, 3}; 
 
    std::for_each(&arg[0], &arg[3], square); 
    std::cout << std::endl; 
 
    std::for_each(&arg[0], &arg[3], std::bind(product, _1, 2)); 
    std::cout << std::endl; 
 
    std::for_each(&arg[0], &arg[3], std::bind(square, _1)); 
 
    return (0); 
    } 
 
1^2 == 1
2^2 == 4
3^2 == 9

1*2 == 2
2*2 == 4
3*2 == 6

1^2 == 1
2^2 == 4
3^2 == 9

Header: <functional>

Namespace: std

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