ceph/src/boost/libs/smart_ptr/pointer_to_other.html

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   8         <h1><img height="86" alt="boost.png (6897 bytes)" src="../../boost.png"
   9             width="277" align="middle" border="0">pointer_to_other</h1>
  10         <p>
  11             The pointer to other utility provides a way, given a source pointer type,
  12             to obtain a pointer of the same type to another pointee type. The utility is
  13             defined in <cite><a href="../../boost/pointer_to_other.hpp">boost/pointer_to_other.hpp</a>.</cite></p>
  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>
  15         <h2><a name="contents">Contents</a></h2>
  16         <ul>
  17             <li>
  18                 <a href="#rationale">Rationale</a>
  19             <li>
  20                 <a href="#synopsis">Synopsis</a>
  21             <li>
  22                 <a href="#example">Example</a></li>
  23         </ul>
  24         <h2><a name="rationale">Rationale</a></h2>
  25         <p>When building pointer independent classes, like memory managers, allocators, or
  26             containers, there is often a need to define pointers generically, so that if a
  27             template parameter represents a pointer (for example, a raw or smart pointer to
  28             an int), we can define another pointer of the same type to another pointee (a
  29             raw or smart pointer to a float.)</p>
  30         <pre>template &lt;class IntPtr&gt;
  31 class FloatPointerHolder
  32 {
  33    <em>// Let's define a pointer to a float</em>
  34    typedef typename boost::pointer_to_other
  35       &lt;IntPtr, float&gt;::type float_ptr_t;
  36    float_ptr_t float_ptr;
  37 };</pre>
  38         <h2><a name="synopsis">Synopsis</a></h2>
  39         <pre>
  40 namespace boost {
  41
  42 template&lt;class T, class U&gt;
  43    struct pointer_to_other;
  44
  45 template&lt;class T, class U, template &lt;class&gt; class Sp&gt;
  46    struct pointer_to_other&lt; Sp&lt;T&gt;, U &gt;
  47 {
  48    typedef Sp&lt;U&gt; type;
  49 };
  50
  51 template&lt;class T, class T2, class U,
  52         template &lt;class, class&gt; class Sp&gt;
  53    struct pointer_to_other&lt; Sp&lt;T, T2&gt;, U &gt;
  54 {
  55    typedef Sp&lt;U, T2&gt; type;
  56 };
  57
  58 template&lt;class T, class T2, class T3, class U,
  59         template &lt;class, class, class&gt; class Sp&gt;
  60 struct pointer_to_other&lt; Sp&lt;T, T2, T3&gt;, U &gt;
  61 {
  62    typedef Sp&lt;U, T2, T3&gt; type;
  63 };
  64
  65 template&lt;class T, class U&gt;
  66 struct pointer_to_other&lt; T*, U &gt;
  67 {
  68    typedef U* type;
  69 };
  70
  71 } <em>// namespace boost</em></pre>
  72         <p>If these definitions are not correct for a specific smart pointer, we can define
  73             a specialization of pointer_to_other.</p>
  74         <h2><a name="example">Example</a></h2>
  75         <pre><em>// Let's define a memory allocator that can
  76 // work with raw and smart pointers</em>
  77
  78 #include &lt;boost/pointer_to_other.hpp&gt;
  79
  80 template &lt;class VoidPtr&gt;
  81 class memory_allocator
  82 {<em>
  83    // Predefine a memory_block </em>
  84    struct block;<em>
  85
  86    // Define a pointer to a memory_block from a void pointer
  87    // If VoidPtr is void *, block_ptr_t is block*
  88    // If VoidPtr is smart_ptr&lt;void&gt;, block_ptr_t is smart_ptr&lt;block&gt;</em>
  89    typedef typename boost::pointer_to_other
  90             &lt;VoidPtr, block&gt;::type block_ptr_t;
  91
  92    struct block
  93    {
  94       std::size_t size;
  95       block_ptr_t next_block;
  96    };
  97
  98    block_ptr_t free_blocks;
  99 };</pre>
 100         <p>As we can see, using pointer_to_other we can create pointer independent code.</p>
 101         <hr>
 102         <p>$Date$</p>
 103         <p><small>Copyright 2005, 2006 Ion Gaztañaga and Peter Dimov. Use, modification,
 104             and distribution are subject to the Boost Software License, Version 1.0.<br>
 105             (See accompanying file <a href="../../LICENSE_1_0.txt">LICENSE_1_0.txt</a> or a
 106             copy at &lt; <a href="http://www.boost.org/LICENSE_1_0.txt">http://www.boost.org/LICENSE_1_0.txt</a>&gt;.)</small></p>
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