Standard C++ Library
Copyright 1998, Rogue Wave Software, Inc.
NAME
bind1st, bind2nd, binder1st, binder2nd
- Templatized utilities to bind values to function objects.
SYNOPSIS
#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&);
DESCRIPTION
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.
INTERFACE
// 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&);
EXAMPLE
//
// 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
WARNINGS
If your compiler does not support default template parame-
ters, 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.
SEE ALSO
Function_Objects