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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"
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<link href="../pool.css" rel="stylesheet" type="text/css">
<title>Pool</title>
</head>
<body>
<img src="../../../../boost.png" width="276" height="86" alt="C++ Boost">
<h1 align="center">User Allocators</h1>
<h2>Introduction</h2>
<p>Pool objects need to request memory blocks from the system, which the
Pool then splits into chunks to allocate to the user. By specifying a
<span class="code">UserAllocator</span> template parameter to various Pool
interfaces, users can control how those system memory blocks are
allocated.</p>
<h2>Semantics</h2>
<table border align="center" summary="">
<caption>
<em>Symbol Table</em>
</caption>
<tr>
<th>Symbol</th>
<th>Meaning</th>
</tr>
<tr>
<td class="code">UserAllocator</td>
<td>A User Allocator type</td>
</tr>
<tr>
<td class="code">block</td>
<td>value of type <span class="code">char *</span></td>
</tr>
<tr>
<td class="code">n</td>
<td>value of type <span class=
"code">UserAllocator::size_type</span></td>
</tr>
</table><br>
<table border align="center" summary="">
<caption>
<em>Typedefs</em>
</caption>
<tr>
<th>Expression</th>
<th>Type</th>
</tr>
<tr>
<td class="code">UserAllocator::size_type</td>
<td>An unsigned integral type that can represent the size of the
largest object to be allocated</td>
</tr>
<tr>
<td class="code">UserAllocator::difference_type</td>
<td>A signed integral type that can represent the difference of any two
pointers</td>
</tr>
</table><br>
<table border align="center" summary="">
<caption>
<em>Allocation and Deallocation</em>
</caption>
<tr>
<th>Expression</th>
<th>Return Type</th>
<th>Pre-Condition/Notes</th>
</tr>
<tr>
<td class="code">UserAllocator::malloc(n)</td>
<td class="code">char *</td>
<td>Attempts to allocate <span class="code">n</span> bytes from the
system. Returns 0 if out-of-memory.</td>
</tr>
<tr>
<td class="code">UserAllocator::free(block)</td>
<td class="code">void</td>
<td><span class="code">block</span> must have been previously returned
from a call to <span class="code">UserAllocator::malloc</span>.</td>
</tr>
</table>
<h2>Provided Implementations</h2>
<p>There are two <span class="code">UserAllocator</span> classes provided.
Both of them are in pool.hpp (see <a href="pool.html">pool</a>). The
default value for the template parameter <span class=
"code">UserAllocator</span> is always <span class=
"code">default_user_allocator_new_delete</span>.</p>
<h3>Synopsis</h3>
<pre class="code">
struct default_user_allocator_new_delete
{
typedef std::size_t size_type;
typedef std::ptrdiff_t difference_type;
static char * malloc(const size_type bytes)
{ return new (std::nothrow) char[bytes]; }
static void free(char * const block)
{ delete [] block; }
};
struct default_user_allocator_malloc_free
{
typedef std::size_t size_type;
typedef std::ptrdiff_t difference_type;
static char * malloc(const size_type bytes)
{ return reinterpret_cast<char *>(std::malloc(bytes)); }
static void free(char * const block)
{ std::free(block); }
};
</pre>
<hr>
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<p>Revised
<!--webbot bot="Timestamp" s-type="EDITED" s-format="%d %B, %Y" startspan -->05
December, 2006<!--webbot bot="Timestamp" endspan i-checksum="38516" --></p>
<p><i>Copyright © 2000, 2001 Stephen Cleary (scleary AT jerviswebb DOT
com)</i></p>
<p><i>Distributed under the Boost Software License, Version 1.0. (See
accompanying file <a href="../../../../LICENSE_1_0.txt">LICENSE_1_0.txt</a>
or copy at <a href=
"http://www.boost.org/LICENSE_1_0.txt">http://www.boost.org/LICENSE_1_0.txt</a>)</i></p>
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