bind1st(3C++) - bind1st(3C++)
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 Output3 3 3WARNINGS
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.
SEE ALSO
Function_Objects
Rogue Wave Software 02 Apr 1998 bind1st(3C++)