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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
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<title>pointer_to_other</title>
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<h1><img height="86" alt="boost.png (6897 bytes)" src="../../boost.png"
width="277" align="middle" border="0">pointer_to_other</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>$Date$</p>
<p><small>Copyright 2005, 2006 Ion Gaztaaga 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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