_1 Object
The latest version of this topic can be found at _1 Object.
Placeholders for replaceable arguments.
Syntax
namespace placeholders {
extern unspecified _1,
_2, ... _M
} // namespace placeholders (within std)
Remarks
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.
Example
// 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
Requirements
Header: <functional>
Namespace: std