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| <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"><html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /><title>Design Issues</title><meta name="generator" content="DocBook XSL Stylesheets Vsnapshot" /><meta name="keywords" content="ISO C++, allocator" /><meta name="keywords" content="ISO C++, library" /><meta name="keywords" content="ISO C++, runtime, library" /><link rel="home" href="../index.html" title="The GNU C++ Library" /><link rel="up" href="mt_allocator.html" title="Chapter 19. The mt_allocator" /><link rel="prev" href="mt_allocator.html" title="Chapter 19. The mt_allocator" /><link rel="next" href="mt_allocator_impl.html" title="Implementation" /></head><body><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="3" align="center">Design Issues</th></tr><tr><td width="20%" align="left"><a accesskey="p" href="mt_allocator.html">Prev</a> </td><th width="60%" align="center">Chapter 19. The mt_allocator</th><td width="20%" align="right"> <a accesskey="n" href="mt_allocator_impl.html">Next</a></td></tr></table><hr /></div><div class="section"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a id="allocator.mt.design_issues"></a>Design Issues</h2></div></div></div><div class="section"><div class="titlepage"><div><div><h3 class="title"><a id="allocator.mt.overview"></a>Overview</h3></div></div></div><p> There are three general components to the allocator: a datum |
| describing the characteristics of the memory pool, a policy class |
| containing this pool that links instantiation types to common or |
| individual pools, and a class inheriting from the policy class that is |
| the actual allocator. |
| </p><p>The datum describing pools characteristics is |
| </p><pre class="programlisting"> |
| template<bool _Thread> |
| class __pool |
| </pre><p> This class is parametrized on thread support, and is explicitly |
| specialized for both multiple threads (with <code class="code">bool==true</code>) |
| and single threads (via <code class="code">bool==false</code>.) It is possible to |
| use a custom pool datum instead of the default class that is provided. |
| </p><p> There are two distinct policy classes, each of which can be used |
| with either type of underlying pool datum. |
| </p><pre class="programlisting"> |
| template<bool _Thread> |
| struct __common_pool_policy |
| |
| template<typename _Tp, bool _Thread> |
| struct __per_type_pool_policy |
| </pre><p> The first policy, <code class="code">__common_pool_policy</code>, implements a |
| common pool. This means that allocators that are instantiated with |
| different types, say <code class="code">char</code> and <code class="code">long</code> will both |
| use the same pool. This is the default policy. |
| </p><p> The second policy, <code class="code">__per_type_pool_policy</code>, implements |
| a separate pool for each instantiating type. Thus, <code class="code">char</code> |
| and <code class="code">long</code> will use separate pools. This allows per-type |
| tuning, for instance. |
| </p><p> Putting this all together, the actual allocator class is |
| </p><pre class="programlisting"> |
| template<typename _Tp, typename _Poolp = __default_policy> |
| class __mt_alloc : public __mt_alloc_base<_Tp>, _Poolp |
| </pre><p> This class has the interface required for standard library allocator |
| classes, namely member functions <code class="code">allocate</code> and |
| <code class="code">deallocate</code>, plus others. |
| </p></div></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="mt_allocator.html">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="mt_allocator.html">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="mt_allocator_impl.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">Chapter 19. The mt_allocator </td><td width="20%" align="center"><a accesskey="h" href="../index.html">Home</a></td><td width="40%" align="right" valign="top"> Implementation</td></tr></table></div></body></html> |
