bind1st(), bind2nd(), binder1st, binder2nd

Library:  General utilities

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Templatized utilities to bind values to function objects

#include <functional>

namespace std {
  template <class Operation>
  class binder1st;
  
  template <class Operation, class T>
  binder1st<Operation> bind1st(const Operation&, const T&);

  template <class Operation>
  class binder2nd ;

  template <class Operation, class T>
  binder2nd<Operation> bind2nd(const Operation&, const T&);
}

Because so many functions included in the C++ Standard Library take other functions as arguments, the library includes classes that let you build new function objects out of old ones. Both bind1st() and bind2nd() are functions that take as arguments a binary function object f and a value x, and return, respectively, classes binder1st and binder2nd. The underlying function object must be a subclass of binary_function.

Class binder1st binds the value to the first argument of the binary function, and binder2nd does the same thing for the second argument of the function. The resulting classes can be used in place of a unary predicate in other function calls.

For example, you could use the count_if() algorithm to count all elements in a vector that are less than 7, using the following:

std::vector<int> v (/* ... */);
int littleNums = std::count_if (v.begin (), v.end (), std::bind1st (std::less<int>(), 7));

The function counts the number of elements in the range [v.begin(), v.end()) as denoted by the first two iterator arguments that satisfy the predicate specified by the third argument and returns the result.

namespace std {

  // Class binder1st 
  template <class Operation>
  class binder1st
     : public unary_function<typename 
                             Operation::second_argument_type,
                             typename Operation::result_type>
  {
  public:

     binder1st(const Operation&,
               const typename Operation::first_argument_type&);
     
     typename Operation::result_type 
     operator() (const typename
                 Operation::second_argument_type&) const;
  };

  // Class binder2nd 
  template <class Operation>
  class binder2nd
     : public unary_function<typename 
                         Operation::first_argument_type, 
                         typename Operation::result_type>
  {
  public:

     binder2nd(const Operation&,
               const typename Operation::second_argument_type&);
     typename Operation::result_type 
     operator()(const typename 
                Operation::first_argument_type&) const;
  };

  // Creator bind1st
  template <class Operation, class T>
  binder1st<Operation> bind1st(const Operation&, const T&);

  // Creator bind2nd 
  template<class Operation, class T>
  binder2nd <Operation> bind2nd(const Operation&, const T&);
}

//
// binders.cpp
//

#include <algorithm>   // for find_if
#include <functional>  // for equal_to, bind1st, bind2nd
#include <iostream>    // for cout
#include <iterator>    // for ostream_iterator
#include <vector>      // for vector


int main ()
{
    typedef std::vector<int>                  Vector;
    typedef std::equal_to<Vector::value_type> EqualTo;

    const Vector::value_type arr [] = { 1, 2, 3, 4, 5 };

    // Initialize a vector with the array elements.
    const Vector v1 (arr, arr + sizeof arr / sizeof *arr);

    // Value to look for.
    const Vector::value_type x (3);

    // Create an 'equal to 3' unary predicate by binding the value
    // 3 to the EqualTo binary predicate.
    const std::binder1st<EqualTo> equal_to_3 =
        std::bind1st (EqualTo (), x);

    // Now use this new predicate in a call to find_if.
    const Vector::const_iterator it1 =
        std::find_if (v1.begin (), v1.end (), equal_to_3);

    // Even better, construct the new predicate on the fly.
    const Vector::const_iterator it2 =
        std::find_if (v1.begin (), v1.end (), std::bind1st (EqualTo (), x));

    // And now the same thing using bind2nd.
    // Same result since EqualTo is commutative.
    const Vector::const_iterator it3 =
        std::find_if (v1.begin (), v1.end (), std::bind2nd (EqualTo (), x));

    // Use the same predicate to count the number of elements
    // equal to 3.
    const Vector::size_type n =
        std::count_if (v1.begin (), v1.end (), std::bind2nd (EqualTo (), x));

    // Output results.
    std::ostream_iterator<Vector::value_type> out (std::cout, " ");

    std::cout << "The vector { ";
    std::copy (v1.begin (), v1.end (), out);

    std::cout << "} contains " << n << " element equal to "
              << x <<  " at offset " <<
              it1 - v1.begin () << ".\n";
    
    // Exit with status of 0 on success, 1 on failure.
    const bool success = 1 == n && it1 == it2 && it1 == it2 && *it1 == x;

    return success ? 0 : 1;
}

Program Output:
The vector { 1 2 3 4 5 } contains 1 element equal to 3 at offset 2.

Function Objects

ISO/IEC 14882:1998 -- International Standard for Information Systems -- Programming Language C++, Section 20.3.6