| 1 | <!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"> |
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| 2 | <html> |
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| 3 | <head> |
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| 4 | <title>pointer_to_other.hpp</title> |
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| 5 | <meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1"> |
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| 6 | </head> |
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| 7 | <body bgcolor="#ffffff" text="#000000"> |
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| 8 | <h1><img src="../../boost.png" alt="boost.png (6897 bytes)" align="middle" WIDTH="277" HEIGHT="86">Header |
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| 9 | <a href="../../boost/pointer_to_other.hpp">boost/pointer_to_other.hpp</a></h1> |
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| 10 | <p> |
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| 11 | The pointer to other utility provides a way, given a source pointer type, |
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| 12 | to obtain a pointer of the same type to another pointee type. The utility is |
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| 13 | defined in <cite><a href="../../boost/pointer_to_other.hpp">boost/pointer_to_other.hpp</a>.</cite></p> |
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| 14 | <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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| 15 | <h2><a name="contents">Contents</a></h2> |
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| 16 | <ul> |
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| 17 | <li> |
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| 18 | <a href="#rationale">Rationale</a> |
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| 19 | <li> |
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| 20 | <a href="#synopsis">Synopsis</a> |
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| 21 | <li> |
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| 22 | <a href="#example">Example</a></li> |
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| 23 | </ul> |
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| 24 | <h2><a name="rationale">Rationale</a></h2> |
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| 25 | <p>When building pointer independent classes, like memory managers, allocators, or |
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| 26 | containers, there is often a need to define pointers generically, so that if a |
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| 27 | template parameter represents a pointer (for example, a raw or smart pointer to |
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| 28 | an int), we can define another pointer of the same type to another pointee (a |
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| 29 | raw or smart pointer to a float.)</p> |
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| 30 | <pre>template <class IntPtr> |
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| 31 | class FloatPointerHolder |
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| 32 | { |
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| 33 | <em>// Let's define a pointer to a float</em> |
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| 34 | typedef typename boost::pointer_to_other |
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| 35 | <IntPtr, float>::type float_ptr_t; |
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| 36 | float_ptr_t float_ptr; |
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| 37 | };</pre> |
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| 38 | <h2><a name="synopsis">Synopsis</a></h2> |
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| 39 | <pre> |
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| 40 | namespace boost { |
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| 41 | |
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| 42 | template<class T, class U> |
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| 43 | struct pointer_to_other; |
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| 44 | |
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| 45 | template<class T, class U, template <class> class Sp> |
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| 46 | struct pointer_to_other< Sp<T>, U > |
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| 47 | { |
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| 48 | typedef Sp<U> type; |
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| 49 | }; |
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| 50 | |
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| 51 | template<class T, class T2, class U, |
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| 52 | template <class, class> class Sp> |
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| 53 | struct pointer_to_other< Sp<T, T2>, U > |
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| 54 | { |
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| 55 | typedef Sp<U, T2> type; |
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| 56 | }; |
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| 57 | |
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| 58 | template<class T, class T2, class T3, class U, |
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| 59 | template <class, class, class> class Sp> |
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| 60 | struct pointer_to_other< Sp<T, T2, T3>, U > |
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| 61 | { |
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| 62 | typedef Sp<U, T2, T3> type; |
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| 63 | }; |
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| 64 | |
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| 65 | template<class T, class U> |
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| 66 | struct pointer_to_other< T*, U > |
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| 67 | { |
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| 68 | typedef U* type; |
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| 69 | }; |
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| 70 | |
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| 71 | } <em>// namespace boost</em></pre> |
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| 72 | <p>If these definitions are not correct for a specific smart pointer, we can define |
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| 73 | a specialization of pointer_to_other.</p> |
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| 74 | <h2><a name="example">Example</a></h2> |
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| 75 | <pre><em>// Let's define a memory allocator that can |
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| 76 | // work with raw and smart pointers</em> |
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| 77 | |
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| 78 | #include <boost/pointer_to_other.hpp> |
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| 79 | |
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| 80 | template <class VoidPtr> |
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| 81 | class memory_allocator |
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| 82 | {<em> |
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| 83 | // Predefine a memory_block </em> |
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| 84 | struct block;<em> |
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| 85 | |
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| 86 | // Define a pointer to a memory_block from a void pointer |
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| 87 | // If VoidPtr is void *, block_ptr_t is block* |
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| 88 | // If VoidPtr is smart_ptr<void>, block_ptr_t is smart_ptr<block></em> |
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| 89 | typedef typename boost::pointer_to_other |
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| 90 | <VoidPtr, block>::type block_ptr_t; |
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| 91 | |
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| 92 | struct block |
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| 93 | { |
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| 94 | std::size_t size; |
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| 95 | block_ptr_t next_block; |
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| 96 | }; |
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| 97 | |
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| 98 | block_ptr_t free_blocks; |
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| 99 | };</pre> |
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| 100 | <p>As we can see, using pointer_to_other we can create pointer independent code.</p> |
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| 101 | <hr> |
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| 102 | <p>Last revised: $Date: 2006/03/18 14:58:20 $</p> |
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| 103 | <p><small>Copyright 2005, 2006 Ion Gaztañaga and Peter Dimov. Use, modification, |
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| 104 | and distribution are subject to the Boost Software License, Version 1.0.<br> |
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| 105 | (See accompanying file <a href="../../LICENSE_1_0.txt">LICENSE_1_0.txt</a> or a |
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| 106 | 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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| 107 | </body> |
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| 108 | </html> |
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