2005-12-06 23:17:32 +00:00
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2006-03-18 14:58:20 +00:00
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<title>pointer_to_other.hpp</title>
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<h1><img src="../../boost.png" alt="boost.png (6897 bytes)" align="middle" WIDTH="277" HEIGHT="86">Header
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<a href="../../boost/pointer_to_other.hpp">boost/pointer_to_other.hpp</a></h1>
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<p>
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The pointer to other utility provides a way, given a source pointer type,
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to obtain a pointer of the same type to another pointee type. The utility is
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defined in <cite><a href="../../boost/pointer_to_other.hpp">boost/pointer_to_other.hpp</a>.</cite></p>
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<p>There is test/example code in <cite><a href="test/pointer_to_other_test.cpp">pointer_to_other_test.cpp</a></cite>.</p>
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<h2><a name="contents">Contents</a></h2>
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<ul>
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<li>
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<a href="#rationale">Rationale</a>
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<li>
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<a href="#synopsis">Synopsis</a>
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<li>
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<a href="#example">Example</a></li>
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</ul>
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<h2><a name="rationale">Rationale</a></h2>
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<p>When building pointer independent classes, like memory managers, allocators, or
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containers, there is often a need to define pointers generically, so that if a
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template parameter represents a pointer (for example, a raw or smart pointer to
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an int), we can define another pointer of the same type to another pointee (a
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raw or smart pointer to a float.)</p>
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<pre>template <class IntPtr>
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class FloatPointerHolder
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{
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<em>// Let's define a pointer to a float</em>
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typedef typename boost::pointer_to_other
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<IntPtr, float>::type float_ptr_t;
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float_ptr_t float_ptr;
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};</pre>
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<h2><a name="synopsis">Synopsis</a></h2>
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<pre>
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namespace boost {
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template<class T, class U>
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struct pointer_to_other;
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template<class T, class U, template <class> class Sp>
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struct pointer_to_other< Sp<T>, U >
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{
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typedef Sp<U> type;
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};
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template<class T, class T2, class U,
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template <class, class> class Sp>
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struct pointer_to_other< Sp<T, T2>, U >
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{
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typedef Sp<U, T2> type;
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};
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template<class T, class T2, class T3, class U,
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template <class, class, class> class Sp>
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struct pointer_to_other< Sp<T, T2, T3>, U >
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{
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typedef Sp<U, T2, T3> type;
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};
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template<class T, class U>
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struct pointer_to_other< T*, U >
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{
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typedef U* type;
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};
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} <em>// namespace boost</em></pre>
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<p>If these definitions are not correct for a specific smart pointer, we can define
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a specialization of pointer_to_other.</p>
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<h2><a name="example">Example</a></h2>
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<pre><em>// Let's define a memory allocator that can
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// work with raw and smart pointers</em>
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#include <boost/pointer_to_other.hpp>
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template <class VoidPtr>
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class memory_allocator
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{<em>
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// Predefine a memory_block </em>
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struct block;<em>
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// Define a pointer to a memory_block from a void pointer
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// If VoidPtr is void *, block_ptr_t is block*
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// If VoidPtr is smart_ptr<void>, block_ptr_t is smart_ptr<block></em>
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typedef typename boost::pointer_to_other
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<VoidPtr, block>::type block_ptr_t;
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struct block
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{
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std::size_t size;
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block_ptr_t next_block;
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};
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block_ptr_t free_blocks;
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};</pre>
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2006-03-18 14:58:20 +00:00
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<p>As we can see, using pointer_to_other we can create pointer independent code.</p>
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<hr>
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<p>Last revised: $Date$</p>
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<p><small>Copyright 2005, 2006 Ion Gazta<74>aga and Peter Dimov. Use, modification,
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and distribution are subject to the Boost Software License, Version 1.0.<br>
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(See accompanying file <a href="../../LICENSE_1_0.txt">LICENSE_1_0.txt</a> or a
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copy at < <a href="http://www.boost.org/LICENSE_1_0.txt">http://www.boost.org/LICENSE_1_0.txt</a>>.)</small></p>
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</body>
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2005-12-06 23:17:32 +00:00
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</html>
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