boost_1_45_0/libs/utility/compressed_pair.htm

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      <boostcompressed_pair.hpp>
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      <h2><img src="../../boost.png" width="276" height="86">Header &lt;<a href="../../boost/detail/compressed_pair.hpp">boost/compressed_pair.hpp</a>&gt;</h2>
      <p>All of the contents of &lt;boost/compressed_pair.hpp&gt; are defined inside 
         namespace boost.</p>
      <p>The class compressed pair is very similar to std::pair, but if either of the 
         template arguments are empty classes, then the "empty base-class optimisation" 
         is applied to compress the size of the pair.</p>
      <pre>template &lt;class T1, class T2&gt;
class compressed_pair
{
public:
	typedef T1                                                 first_type;
	typedef T2                                                 second_type;
	typedef typename call_traits&lt;first_type&gt;::param_type       first_param_type;
	typedef typename call_traits&lt;second_type&gt;::param_type      second_param_type;
	typedef typename call_traits&lt;first_type&gt;::reference        first_reference;
	typedef typename call_traits&lt;second_type&gt;::reference       second_reference;
	typedef typename call_traits&lt;first_type&gt;::const_reference  first_const_reference;
	typedef typename call_traits&lt;second_type&gt;::const_reference second_const_reference;

	         compressed_pair() : base() {}
	         compressed_pair(first_param_type x, second_param_type y);
	explicit compressed_pair(first_param_type x);
	explicit compressed_pair(second_param_type y);

	compressed_pair&amp; operator=(const compressed_pair&amp;);

	first_reference       first();
	first_const_reference first() const;

	second_reference       second();
	second_const_reference second() const;

	void swap(compressed_pair&amp; y);
};</pre>
      <p>The two members of the pair can be accessed using the member functions first() 
         and second(). Note that not all member functions can be instantiated for all 
         template parameter types. In particular compressed_pair can be instantiated for 
         reference and array types, however in these cases the range of constructors 
         that can be used are limited. If types T1 and T2 are the same type, then there 
         is only one version of the single-argument constructor, and this constructor 
         initialises both values in the pair to the passed value.</p>
      <P>Note that if either member is a POD type, then that member is not 
         zero-initialized by the compressed_pair default constructor: it's up to you to 
         supply an initial value for these types if you want them to have a default 
         value.</P>
      <p>Note that compressed_pair can not be instantiated if either of the template 
         arguments is a union type, unless there is compiler support for 
         boost::is_union, or if boost::is_union is specialised for the union type.</p>
      <p>Finally, a word of caution for Visual C++ 6 users: if either argument is an 
         empty type, then assigning to that member will produce memory corruption, 
         unless the empty type has a "do nothing" assignment operator defined. This is 
         due to a bug in the way VC6 generates implicit assignment operators.</p>
      <h3>Acknowledgements</h3>
      <p>Based on contributions by Steve Cleary, Beman Dawes, Howard Hinnant and John 
         Maddock.</p>
      <p>Maintained by <a href="mailto:john@johnmaddock.co.uk">John Maddock</a>, the 
         latest version of this file can be found at <a href="http://www.boost.org">www.boost.org</a>, 
         and the boost discussion list at <a href="http://www.yahoogroups.com/list/boost">www.yahoogroups.com/list/boost</a>.</p>
      <hr>
      <p>Revised
      <!--webbot bot="Timestamp" S-Type="EDITED" S-Format="%d %B %Y" startspan -->07 November 2007<!--webbot bot="Timestamp" endspan i-checksum="40338" --></p>
      <p>© Copyright Beman Dawes, 2000.</p>
<p>Distributed under the Boost Software License, Version 1.0. See
<a href="http://www.boost.org/LICENSE_1_0.txt">www.boost.org/LICENSE_1_0.txt</a></p>

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