libstdc++-v3/doc/html/manual/bk01pt03ch20s02.html

   1 <html><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8"><title>Design Issues</title><meta name="generator" content="DocBook XSL-NS Stylesheets V1.76.1"><meta name="keywords" content="
   2       ISO C++
   3     ,
   4       allocator
   5     "><meta name="keywords" content="
   6       ISO C++
   7     ,
   8       library
   9     "><meta name="keywords" content="
  10       ISO C++
  11     ,
  12       runtime
  13     ,
  14       library
  15     "><link rel="home" href="../index.html" title="The GNU C++ Library"><link rel="up" href="mt_allocator.html" title="Chapter 20. The mt_allocator"><link rel="prev" href="mt_allocator.html" title="Chapter 20. The mt_allocator"><link rel="next" href="bk01pt03ch20s03.html" title="Implementation"></head><body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"><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 20. The mt_allocator</th><td width="20%" align="right"> <a accesskey="n" href="bk01pt03ch20s03.html">Next</a></td></tr></table><hr></div><div class="section" title="Design Issues"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a name="allocator.mt.design_issues"></a>Design Issues</h2></div></div></div><div class="section" title="Overview"><div class="titlepage"><div><div><h3 class="title"><a name="allocator.mt.overview"></a>Overview</h3></div></div></div><p> There are three general components to the allocator: a datum
  16 describing the characteristics of the memory pool, a policy class
  17 containing this pool that links instantiation types to common or
  18 individual pools, and a class inheriting from the policy class that is
  19 the actual allocator.
  20 </p><p>The datum describing pools characteristics is
  21 </p><pre class="programlisting">
  22   template&lt;bool _Thread&gt;
  23     class __pool
  24 </pre><p> This class is parametrized on thread support, and is explicitly
  25 specialized for both multiple threads (with <code class="code">bool==true</code>)
  26 and single threads (via <code class="code">bool==false</code>.) It is possible to
  27 use a custom pool datum instead of the default class that is provided.
  28 </p><p> There are two distinct policy classes, each of which can be used
  29 with either type of underlying pool datum.
  30 </p><pre class="programlisting">
  31   template&lt;bool _Thread&gt;
  32     struct __common_pool_policy
  33
  34   template&lt;typename _Tp, bool _Thread&gt;
  35     struct __per_type_pool_policy
  36 </pre><p> The first policy, <code class="code">__common_pool_policy</code>, implements a
  37 common pool. This means that allocators that are instantiated with
  38 different types, say <code class="code">char</code> and <code class="code">long</code> will both
  39 use the same pool. This is the default policy.
  40 </p><p> The second policy, <code class="code">__per_type_pool_policy</code>, implements
  41 a separate pool for each instantiating type. Thus, <code class="code">char</code>
  42 and <code class="code">long</code> will use separate pools. This allows per-type
  43 tuning, for instance.
  44 </p><p> Putting this all together, the actual allocator class is
  45 </p><pre class="programlisting">
  46   template&lt;typename _Tp, typename _Poolp = __default_policy&gt;
  47     class __mt_alloc : public __mt_alloc_base&lt;_Tp&gt;,  _Poolp
  48 </pre><p> This class has the interface required for standard library allocator
  49 classes, namely member functions <code class="code">allocate</code> and
  50 <code class="code">deallocate</code>, plus others.
  51 </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="bk01pt03ch20s03.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">Chapter 20. 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>