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<H2>bind1st(), bind2nd(), binder1st, binder2nd</H2>
<P><B>Library:</B>&nbsp;&nbsp;<A HREF="2-4.html">General utilities</A></P>

<PRE><HR><B><I>Function</I></B><HR></PRE>

<UL>
<LI><A HREF="#sec1">Local Index</A></LI>
<LI><A HREF="#sec2">Summary</A></LI>
<LI><A HREF="#sec3">Synopsis</A></LI>
<LI><A HREF="#sec4">Description</A></LI>
<LI><A HREF="#sec5">Interface</A></LI>
<LI><A HREF="#sec6">Example</A></LI>
<LI><A HREF="#sec7">See Also</A></LI>
<LI><A HREF="#sec8">Standards Conformance</A></LI>
</UL>
<A NAME="sec1"><H3>Local Index</H3></A>
No Entries
<A NAME="sec2"><H3>Summary</H3></A>
<P>Templatized utilities to bind values to function objects</P>
<A NAME="sec3"><H3>Synopsis</H3></A>

<PRE>#include &lt;functional&gt;

namespace std {
  template &lt;class Operation&gt;
  class binder1st;
  
  template &lt;class Operation, class T&gt;
  binder1st&lt;Operation&gt; bind1st(const Operation&amp;, const T&amp;);

  template &lt;class Operation&gt;
  class binder2nd ;

  template &lt;class Operation, class T&gt;
  binder2nd&lt;Operation&gt; bind2nd(const Operation&amp;, const T&amp;);
}
</PRE>
<A NAME="sec4"><H3>Description</H3></A>
<P>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 <SAMP><A HREF="bind1st.html">bind1st()</A></SAMP> and <SAMP><A HREF="bind1st.html">bind2nd()</A></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><A HREF="bind1st.html">binder1st</A></I></B> and <B><I><A HREF="bind1st.html">binder2nd</A></I></B>. The underlying function object must be a subclass of <B><I><A HREF="binary-function.html">binary_function</A></I></B>.</P>
<P>Class <B><I><A HREF="bind1st.html">binder1st</A></I></B> binds the value to the first argument of the binary function, and <B><I><A HREF="bind1st.html">binder2nd</A></I></B> 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 <SAMP><A HREF="count.html">count_if()</A></SAMP> algorithm to count all elements in a <B><I><A HREF="vector.html">vector</A></I></B> that are less than 7, using the following:</P>

<UL><PRE>
std::vector&lt;int&gt; v (/* ... */);
int littleNums = std::count_if (v.begin (), v.end (), std::bind1st (std::less&lt;int&gt;(), 7));
</PRE></UL>
<P>The function counts the number of elements in the range [<SAMP>v.begin()</SAMP>, <SAMP>v.end()</SAMP>) as denoted by the first two iterator arguments that satisfy the predicate specified by the third argument and returns the result.</P>
<A NAME="sec5"><H3>Interface</H3></A>

<UL><PRE>namespace std {

  // Class binder1st 
  template &lt;class Operation&gt;
  class binder1st
     : public unary_function&lt;typename 
                             Operation::second_argument_type,
                             typename Operation::result_type&gt;
  {
  public:

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

  // Class binder2nd 
  template &lt;class Operation&gt;
  class binder2nd
     : public unary_function&lt;typename 
                         Operation::first_argument_type, 
                         typename Operation::result_type&gt;
  {
  public:

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

  // Creator bind1st
  template &lt;class Operation, class T&gt;
  binder1st&lt;Operation&gt; bind1st(const Operation&amp;, const T&amp;);

  // Creator bind2nd 
  template&lt;class Operation, class T&gt;
  binder2nd &lt;Operation&gt; bind2nd(const Operation&amp;, const T&amp;);
}
</PRE></UL>
<A NAME="sec6"><H3>Example</H3></A>

<UL><PRE>//
// binders.cpp
//

#include &lt;algorithm&gt;   // for find_if
#include &lt;functional&gt;  // for equal_to, bind1st, bind2nd
#include &lt;iostream&gt;    // for cout
#include &lt;iterator&gt;    // for ostream_iterator
#include &lt;vector&gt;      // for vector


int main ()
{
    typedef std::vector&lt;int&gt;                  Vector;
    typedef std::equal_to&lt;Vector::value_type&gt; 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&lt;EqualTo&gt; 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&lt;Vector::value_type&gt; out (std::cout, " ");

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

    std::cout &lt;&lt; "} contains " &lt;&lt; n &lt;&lt; " element equal to "
              &lt;&lt; x &lt;&lt;  " at offset " &lt;&lt;
              it1 - v1.begin () &lt;&lt; ".\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.
</PRE></UL>
<UL><PRE></PRE></UL>
<A NAME="sec7"><H3>See Also</H3></A>
<P><A HREF="functionobjects.html">Function Objects</A></P>
<A NAME="sec8"><H3>Standards Conformance</H3></A>
<P><I>ISO/IEC 14882:1998 -- International Standard for Information Systems -- Programming Language C++, Section 20.3.6</I></P>

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