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<boostcompressed_pair.hpp>
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<h2><img src="../../boost.png" width="276" height="86">Header <<a href="../../boost/detail/compressed_pair.hpp">boost/compressed_pair.hpp</a>></h2>
<p>All of the contents of <boost/compressed_pair.hpp> 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 <class T1, class T2>
class compressed_pair
{
public:
typedef T1 first_type;
typedef T2 second_type;
typedef typename call_traits<first_type>::param_type first_param_type;
typedef typename call_traits<second_type>::param_type second_param_type;
typedef typename call_traits<first_type>::reference first_reference;
typedef typename call_traits<second_type>::reference second_reference;
typedef typename call_traits<first_type>::const_reference first_const_reference;
typedef typename call_traits<second_type>::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& operator=(const compressed_pair&);
first_reference first();
first_const_reference first() const;
second_reference second();
second_const_reference second() const;
void swap(compressed_pair& 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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