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<title>Header boost/utility.hpp Documentation</title>
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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>
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Class templates supporting the <a href="base_from_member.html">base-from-member
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idiom</a></li>
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<li>
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Function templates <a href="#checked_delete">checked_delete() and
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checked_array_delete()</a></li>
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<li>
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Function templates <a href="#functions_next_prior">next() and prior()</a></li>
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<li>
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Class <a href="#Class_noncopyable">noncopyable</a></li>
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<li>
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Function template <a href="#addressof">addressof()</a></li>
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</ul>
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<h2>
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Function templates <a name="checked_delete">checked_delete</a>() and
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checked_array_delete()</h2>
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<p>See <a href="checked_delete.html">separate documentation</a>.</p>
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<h2>
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<a name="functions_next_prior">Function</a> templates next() and prior()</h2>
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<p>Certain data types, such as the C++ Standard Library's forward and bidirectional
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iterators, do not provide addition and subtraction via operator+() or
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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
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a 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 T, class Distance>
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T next(T x, Distance n)
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{
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std::advance(x, n);
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return x;
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}
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template <class T>
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T prior(T x) { return --x; }
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template <class T, class Distance>
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T prior(T x, Distance n)
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{
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std::advance(x, -n);
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return x;
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}</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);
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const std::list<T>::iterator next = boost::next(prev, 2);</pre>
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</blockquote>
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<p>The distance from the given iterator should be supplied as an absolute value. For
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example, the iterator four iterators prior to the given iterator <code>p</code>
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may be obtained by <code>prior(p, 4)</code>.</p>
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<p>Contributed by <a href="../../people/dave_abrahams.htm">Dave Abrahams</a>. Two-argument versions by Daniel Walker.</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
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from <strong>noncopyable</strong> when you want to prohibit copy construction
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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
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class that doesn't need to be copied just yet and you don't want to take the
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time to write the appropriate functions. Deriving from <b>noncopyable</b>
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will prevent the otherwise implicitly-generated functions (which don't have the
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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
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and copy assignment, and then document why this is done. But deriving
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from <b>noncopyable</b> is simpler and clearer, and doesn't require additional
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documentation.</p>
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<p>The program <a href="noncopyable_test.cpp">noncopyable_test.cpp</a> can be used
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to verify class <b>noncopyable</b> works as expected. It has have been run
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successfully under GCC 2.95, Metrowerks CodeWarrior 5.0, and Microsoft Visual
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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 emphasize
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that it is to be used only as a base class. Dave Abrahams notes concern
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about the effect on compiler optimization of adding (even trivial inline)
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destructor declarations. He says "Probably this concern is misplaced,
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because noncopyable will be used mostly for classes which own resources and
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thus have non-trivial destruction semantics."</p>
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<h2><a name="addressof">Function template addressof()</a></h2>
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<p>Function <strong>addressof()</strong> returns the address of an object.</p>
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<blockquote>
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<pre>template <typename T> inline T* addressof(T& v);
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template <typename T> inline const T* addressof(const T& v);
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template <typename T> inline volatile T* addressof(volatile T& v);
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template <typename T> inline const volatile T* addressof(const volatile T& v);
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</pre>
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</blockquote>
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<p>C++ allows programmers to replace the unary <strong>operator&()</strong> class
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member used to get the address of an object. Getting the real address of an
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object requires ugly casting tricks to avoid invoking the overloaded <strong>operator&()</strong>.
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Function <strong>addressof()</strong> provides a wrapper around the necessary
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code to make it easy to get an object's real address.
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</p>
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<p>The program <a href="addressof_test.cpp">addressof_test.cpp</a> can be used to
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verify that <b>addressof()</b> works as expected.</p>
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<p>Contributed by Brad King based on ideas from discussion with Doug Gregor.</p>
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<h3>Example</h3>
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<blockquote>
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<pre>#include <boost/utility.hpp>
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struct useless_type {};
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class nonaddressable {
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useless_type operator&() const;
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};
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void f() {
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nonaddressable x;
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nonaddressable* xp = boost::addressof(x);
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// nonaddressable* xpe = &x; /* error */
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}</pre>
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</blockquote>
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<h2>Class templates for the Base-from-Member Idiom</h2>
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<p>See <a href="base_from_member.html">separate documentation</a>.</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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-->23 December, 2003<!--webbot bot="Timestamp" endspan i-checksum="38582"
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-->
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</p>
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<p>© Copyright boost.org 1999-2003. Permission to copy, use, modify, sell and distribute
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this document is granted provided this copyright notice appears in all copies.
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This document is provided "as is" without express or implied
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warranty, and with no claim as to its suitability for any purpose.</p>
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