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std::binder1st, std::binder2nd

Defined in header <functional> 
template< class Fn >
class binder1st :
    public std::unary_function<typename Fn::second_argument_type, 
                               typename Fn::result_type> {
protected:
    Fn op;
    typename Fn::first_argument_type value;
public:
    binder1st(const Fn& fn, 
              const typename Fn::first_argument_type& value);

    typename Fn::result_type 
        operator()(const typename Fn::second_argument_type& x) const;

    typename Fn::result_type 
        operator()(typename Fn::second_argument_type& x) const;
};
 (1) (deprecated in C++11)
(removed in C++17) 
template< class Fn >
class binder2nd :
    public std::unary_function<typename Fn::first_argument_type,
                               typename Fn::result_type> {
protected:
    Fn op;
    typename Fn::second_argument_type value;
public:
    binder2nd(const Fn& fn, 
              const typename Fn::second_argument_type& value);

    typename Fn::result_type 
        operator()(const typename Fn::first_argument_type& x) const;

    typename Fn::result_type 
        operator()(typename Fn::first_argument_type& x) const;
};
 (2) (deprecated in C++11)
(removed in C++17)

A function object that binds an argument to a binary function.

The value of the parameter is passed to the object at the construction time and stored within the object. Whenever the function object is invoked though operator(), the stored value is passed as one of the arguments, the other argument is passed as an argument of operator(). The resulting function object is a unary function.

1) Binds the first parameter to the value value given at the construction of the object.
2) Binds the second parameter to the value value given at the construction of the object.

Example

#include <cmath>
#include <functional>
#include <iostream>
#include <vector>
 
const double pi = std::acos(-1); // use std::numbers::pi in C++20
 
int main()
{
    // deprecated in C++11, removed in C++17
    auto f1 = std::bind1st(std::multiplies<double>(), pi / 180.0);
 
    // C++11 replacement
    auto f2 = [](double a) { return a * pi / 180.0; };
 
    for (double n : {0, 30, 45, 60, 90, 180})
        std::cout << n << "°\t" << std::fixed << "= "
                  << f1(n) << " rad (using binder)\t= "
                  << f2(n) << " rad (using lambda)\n"
                  << std::defaultfloat;
}

Output:

0°        = 0.000000 rad (using binder)        = 0.000000 rad (using lambda)
30°        = 0.523599 rad (using binder)        = 0.523599 rad (using lambda)
45°        = 0.785398 rad (using binder)        = 0.785398 rad (using lambda)
60°        = 1.047198 rad (using binder)        = 1.047198 rad (using lambda)
90°        = 1.570796 rad (using binder)        = 1.570796 rad (using lambda)
180°        = 3.141593 rad (using binder)        = 3.141593 rad (using lambda)

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR Applied to Behavior as published Correct behavior
LWG 109 C++98 operator() could not modify to argument passed to it added overloads to handle this

See also

(deprecated in C++11)(removed in C++17)
 binds one argument to a binary function
(function template)

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