std::binder1st,std::binder2nd(3)

NAME
std::binder1st,std::binder2nd - std::binder1st,std::binder2nd

Synopsis
   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, (1) (deprecated in C++11)
                  const typename Fn::first_argument_type& (removed in C++17)
   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;
   };
   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, (2) (deprecated in C++11)
                  const typename Fn::second_argument_type& (removed in C++17)
   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;
   };
   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
// Run this code
 #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 added overloads to handle
                      passed to it this

See also
   bind1st binds one argument to a binary function
   bind2nd (function template)
   (deprecated in C++11)(removed in C++17)