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| <h1><img src="../../boost.png" alt="boost.png (6897 bytes)" align="middle" WIDTH="277" HEIGHT="86">Header |
| <a href="../../boost/pointer_to_other.hpp">boost/pointer_to_other.hpp</a></h1> |
| <p> |
| The pointer to other utility provides a way, given a source pointer type, |
| to obtain a pointer of the same type to another pointee type. The utility is |
| defined in <cite><a href="../../boost/pointer_to_other.hpp">boost/pointer_to_other.hpp</a>.</cite></p> |
| <p>There is test/example code in <cite><a href="test/pointer_to_other_test.cpp">pointer_to_other_test.cpp</a></cite>.</p> |
| <h2><a name="contents">Contents</a></h2> |
| <ul> |
| <li> |
| <a href="#rationale">Rationale</a> |
| <li> |
| <a href="#synopsis">Synopsis</a> |
| <li> |
| <a href="#example">Example</a></li> |
| </ul> |
| <h2><a name="rationale">Rationale</a></h2> |
| <p>When building pointer independent classes, like memory managers, allocators, or |
| containers, there is often a need to define pointers generically, so that if a |
| template parameter represents a pointer (for example, a raw or smart pointer to |
| an int), we can define another pointer of the same type to another pointee (a |
| raw or smart pointer to a float.)</p> |
| <pre>template <class IntPtr> |
| class FloatPointerHolder |
| { |
| <em>// Let's define a pointer to a float</em> |
| typedef typename boost::pointer_to_other |
| <IntPtr, float>::type float_ptr_t; |
| float_ptr_t float_ptr; |
| };</pre> |
| <h2><a name="synopsis">Synopsis</a></h2> |
| <pre> |
| namespace boost { |
| |
| template<class T, class U> |
| struct pointer_to_other; |
| |
| template<class T, class U, template <class> class Sp> |
| struct pointer_to_other< Sp<T>, U > |
| { |
| typedef Sp<U> type; |
| }; |
| |
| template<class T, class T2, class U, |
| template <class, class> class Sp> |
| struct pointer_to_other< Sp<T, T2>, U > |
| { |
| typedef Sp<U, T2> type; |
| }; |
| |
| template<class T, class T2, class T3, class U, |
| template <class, class, class> class Sp> |
| struct pointer_to_other< Sp<T, T2, T3>, U > |
| { |
| typedef Sp<U, T2, T3> type; |
| }; |
| |
| template<class T, class U> |
| struct pointer_to_other< T*, U > |
| { |
| typedef U* type; |
| }; |
| |
| } <em>// namespace boost</em></pre> |
| <p>If these definitions are not correct for a specific smart pointer, we can define |
| a specialization of pointer_to_other.</p> |
| <h2><a name="example">Example</a></h2> |
| <pre><em>// Let's define a memory allocator that can |
| // work with raw and smart pointers</em> |
| |
| #include <boost/pointer_to_other.hpp> |
| |
| template <class VoidPtr> |
| class memory_allocator |
| {<em> |
| // Predefine a memory_block </em> |
| struct block;<em> |
| |
| // Define a pointer to a memory_block from a void pointer |
| // If VoidPtr is void *, block_ptr_t is block* |
| // If VoidPtr is smart_ptr<void>, block_ptr_t is smart_ptr<block></em> |
| typedef typename boost::pointer_to_other |
| <VoidPtr, block>::type block_ptr_t; |
| |
| struct block |
| { |
| std::size_t size; |
| block_ptr_t next_block; |
| }; |
| |
| block_ptr_t free_blocks; |
| };</pre> |
| <p>As we can see, using pointer_to_other we can create pointer independent code.</p> |
| <hr> |
| <p>Last revised: $Date: 2006-03-18 10:58:20 -0400 (Sat, 18 Mar 2006) $</p> |
| <p><small>Copyright 2005, 2006 Ion Gaztañaga and Peter Dimov. Use, modification, |
| and distribution are subject to the Boost Software License, Version 1.0.<br> |
| (See accompanying file <a href="../../LICENSE_1_0.txt">LICENSE_1_0.txt</a> or a |
| 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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