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<H2>bind1st, bind2nd, binder1st, binder2nd</H2>
<HR><PRE> Function Object</PRE><HR>
<A NAME="Summary"><H3>Summary</H3></A>
<P>Templatized utilities to bind values to function objects</P>
<PRE></PRE>
<H3>Contents</H3>
<UL>
<A HREF="#Synopsis"><LI>Synopsis</LI></A>
<A HREF="#Description"><LI>Description</LI></A>
<A HREF="#Interface"><LI>Interface</LI></A>
<A HREF="#Example"><LI>Example</LI></A>
<A HREF="#Warnings"><LI>Warnings</LI></A>
<A HREF="#See Also"><LI>See Also</LI></A>
</UL>
<A NAME="Synopsis"><H3>Synopsis</H3></A>
<PRE>#include <functional></PRE>
<PRE>
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&);
</PRE>
<A NAME="Description"><H3>Description</H3></A>
<P>Because so many functions provided by the standard library take other functions as arguments, the library includes classes that let you build new function objects out of old ones. Both <SAMP>bind1st()</SAMP> and <SAMP>bind2nd()</SAMP> are functions that take as arguments a binary function object <SAMP>f</SAMP> and a value <SAMP>x,</SAMP> and return, respectively, classes <B><I>binder1st</B></I> and <B><I>binder2nd</B></I>. The underlying function object must be a subclass of <A HREF="bin_7851.htm"><B><I>binary_function</B></I></A>.</P>
<P>Class <B><I>binder1st</B></I> binds the value to the first argument of the binary function, and <B><I>binder2nd</B></I> 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.</P>
<P>For example, you could use the <A HREF="cou_3676.htm"><B><I>count_if</B></I></A> algorithm to count all elements in a vector that are less than or equal to 7, using the following:</P>
<PRE>count_if (v.begin, v.end, bind1st(greater<int> (),7), littleNums)</PRE>
<PRE></PRE><P>This function adds one to <SAMP>littleNums</SAMP> each time the predicate is <SAMP>true</SAMP>, i.e., each time 7 is greater than the element.</P>
<A NAME="Interface"><H3>Interface</H3></A>
<PRE>// Class binder1st </PRE>
<PRE> template <class Operation>
class binder1st
: public unary_function<typename
Operation::second_argument_type,
typename Operation::result_type>
{
public:
typedef typename unary_function<typename
Operation::second_argument_type, typename
Operation::result_type>::argument_type argument_type;
typedef typename unary_function<typename
Operation::second_argument_type, typename
Operation::result_type>::result_type result_type;
binder1st(const Operation&,
const typename Operation::first_argument_type&);
result_type operator() (const argument_type&) const;
};
// Class binder2nd
template <class Operation>
class binder2nd
: public unary_function<typename
Operation::first_argument_type,
typename Operation::result_type>
{
public:
typedef typename unary_function<typename
Operation::first_argument_type, typename
Operation::result_type>::argument_type argument_type;
typedef typename unary_function<typename
Operation::first_argument_type, typename
Operation::result_type>::result_type result_type;
binder2nd(const Operation&,
const typename Operation::second_argument_type&);
result_type operator() (const 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&);
</PRE>
<A NAME="Example"><H3>Example</H3></A>
<PRE>//
// binders.cpp
//
#include <functional>
#include <algorithm>
#include <vector>
#include <iostream.h>
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;
}
Output : 3 3 3</PRE>
<A NAME="Warnings"><H3>Warnings</H3></A>
<P>If your compiler does not support default template parameters then you need to always supply the <SAMP>Allocator</SAMP> template argument. For instance you'll have to write:</P>
<PRE>vector<int,allocator></PRE>
<P>instead of:</P>
<PRE>vector<int></PRE>
<A NAME="See Also"><H3>See Also</H3></A>
<P><A HREF="Fun_7437.htm"><B><I>Function Objects</B></I></A></P>
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