bind1st, bind2nd, binder1st, binder2nd

Function Object

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• Synopsis
• Description
• Interface
• Example
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Templatized utilities to bind values to function objects.

None

#include <functional>
template <class Operation>
class binder1st : public unary_function<typename 
                   Operation::second_argument_type,
                   typename Operation::result_type> ;

template <class Operation, class T>
binder1st<Operation> bind1st (const Operation&, const T&);
template <class Operation>
class binder2nd : public unary_function<typename            
                   Operation::first_argument_type,
                   typename Operation::result_type> ;

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

Because so many functions included in the 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 or equal to 7, using the following:

count_if (v.begin, v.end, bind1st(greater<int> (),7),
          littleNums)

This function adds one to littleNums each time the predicate is true, in other words, each time 7 is greater than the element.

// 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 <functional>
 #include <algorithm>
 #include <vector>
 #include <iostream>
 using namespace std;
 int main()
 {
   typedef vector<int>::iterator iterator;
   int d1[4] = {1,2,3,4};
   //
   // Set up a vector
   //
   vector<int> v1(d1,d1 + 4);
   //
   // Create an 'equal to 3' unary predicate by binding 3 to
   // the equal_to binary predicate.
   //
   binder1st<equal_to<int> > equal_to_3 = 
      bind1st(equal_to<int>(),3);
   //
   // Now use this new predicate in a call to find_if
   //
   iterator it1 = find_if(v1.begin(),v1.end(),equal_to_3);
   //
   // Even better, construct the new predicate on the fly
   //
   iterator it2 = 
      find_if(v1.begin(),v1.end(),bind1st(equal_to<int>(),3)); 
   //
   // And now the same thing using bind2nd
   // Same result since == is commutative
   //
   iterator it3 = 
      find_if(v1.begin(),v1.end(),bind2nd(equal_to<int>(),3)); 
   //
   // it3 = v1.begin() + 2
   //
   // Output results
   //
   cout << *it1 << " " << *it2 << " " << *it3 << endl;
   return 0;
 }

Program Output

3 3 3

If your compiler does not support default template parameters, then you always need to supply the Allocator template argument. For instance, you have to write:

vector<int,allocator<int> >

instead of:

vector<int>

If your compiler does not support namespaces, then you do not need the using declaration for std.

Function Objects