_1 Object

Visual Studio 2015
 

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Placeholders for replaceable arguments.

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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.

C++
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// 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);   
    }
Output
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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

bind Function
is_placeholder Class

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