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<title>Header boost/utility.hpp Documentation</title>
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<body bgcolor="#FFFFFF" text="#000000">
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<h1><img src="../../c++boost.gif" alt="c++boost.gif (8819 bytes)" align="center" WIDTH="277" HEIGHT="86">Header
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<a href="../../boost/utility.hpp">boost/utility.hpp</a></h1>
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<p>The entire contents of the header <code><a href="../../boost/utility.hpp"><boost/utility.hpp></a></code>
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are in <code>namespace boost</code>.</p>
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<h2>Contents</h2>
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<ul>
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<li>Function templates <a href="#functions next">next() and prior()</a></li>
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<li>Class <a href="#Class noncopyable">noncopyable</a></li>
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<li>Function template <a href="tie.html">tie()</a> and supporting class tied.</li>
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</ul>
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<h2> <a name="functions next">Function</a> templates next() and prior()</h2>
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<p>Certain data types, such as the C++ Standard Library's forward and
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bidirectional iterators, do not provide addition and subtraction via operator+()
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or operator-(). This means that non-modifying computation of the next or
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prior value requires a temporary, even though operator++() or operator--() is
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provided. It also means that writing code like <code>itr+1</code> inside a
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template restricts the iterator category to random access iterators.</p>
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<p>The next() and prior() functions provide a simple way around these problems:</p>
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<blockquote>
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<pre>template <class T>
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T next(T x) { return ++x; }
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template <class X>
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T prior(T x) { return --x; }</pre>
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</blockquote>
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<p>Usage is simple:</p>
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<blockquote>
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<pre>const std::list<T>::iterator p = get_some_iterator();
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const std::list<T>::iterator prev = boost::prior(p);</pre>
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</blockquote>
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<p>Contributed by <a href="../../people/dave_abrahams.htm">Dave Abrahams</a>.</p>
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<h2><a name="Class noncopyable">Class noncopyable</a></h2>
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<p>Class <strong>noncopyable</strong> is a base class. Derive your own class from <strong>noncopyable</strong>
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when you want to prohibit copy construction and copy assignment.</p>
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<p>Some objects, particularly those which hold complex resources like files or
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network connections, have no sensible copy semantics. Sometimes there are
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possible copy semantics, but these would be of very limited usefulness and be
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very difficult to implement correctly. Sometimes you're implementing a class that doesn't need to be copied
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just yet and you don't want to take the time to write the appropriate functions.
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Deriving from <b> noncopyable</b> will prevent the otherwise implicitly-generated
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functions (which don't have the proper semantics) from becoming a trap for other programmers.</p>
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<p>The traditional way to deal with these is to declare a private copy constructor and copy assignment, and then
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document why this is done. But deriving from <b>noncopyable</b> is simpler
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and clearer, and doesn't require additional documentation.</p>
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<p>The program <a href="noncopyable_test.cpp">noncopyable_test.cpp</a> can be
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used to verify class <b>noncopyable</b> works as expected. It has have been run successfully under
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GCC 2.95, Metrowerks
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CodeWarrior 5.0, and Microsoft Visual C++ 6.0 sp 3.</p>
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<p>Contributed by <a href="../../people/dave_abrahams.htm">Dave Abrahams</a>.</p>
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<h3>Example</h3>
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<blockquote>
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<pre>// inside one of your own headers ...
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#include <boost/utility.hpp>
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class ResourceLadenFileSystem : boost::noncopyable {
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...</pre>
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</blockquote>
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<h3>Rationale</h3>
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<p>Class noncopyable has protected constructor and destructor members to
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emphasize that it is to be used only as a base class. Dave Abrahams notes
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concern about the effect on compiler optimization of adding (even trivial inline)
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destructor declarations. He says "Probably this concern is misplaced, because
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noncopyable will be used mostly for classes which own resources and thus have non-trivial destruction semantics."</p>
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<hr>
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<p>Revised <!--webbot bot="Timestamp" S-Type="EDITED" S-Format="%d %B, %Y" startspan
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-->28 September, 2000<!--webbot bot="Timestamp" endspan i-checksum="39343"
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-->
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</p>
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<p><EFBFBD> Copyright boost.org 1999. Permission to copy, use, modify, sell and
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distribute this document is granted provided this copyright notice appears in
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all copies. This document is provided "as is" without express or
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implied warranty, and with no claim as to its suitability for any purpose.</p>
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</body>
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