1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
|
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=US-ASCII">
<title>Custom Memory Allocation</title>
<link rel="stylesheet" href="../../../../../doc/src/boostbook.css" type="text/css">
<meta name="generator" content="DocBook XSL Stylesheets V1.79.1">
<link rel="home" href="../../../boost_asio.html" title="Boost.Asio">
<link rel="up" href="../core.html" title="Core Concepts and Functionality">
<link rel="prev" href="line_based.html" title="Line-Based Operations">
<link rel="next" href="handler_tracking.html" title="Handler Tracking">
</head>
<body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF">
<table cellpadding="2" width="100%"><tr>
<td valign="top"><img alt="Boost C++ Libraries" width="277" height="86" src="../../../../../boost.png"></td>
<td align="center"><a href="../../../../../index.html">Home</a></td>
<td align="center"><a href="../../../../../libs/libraries.htm">Libraries</a></td>
<td align="center"><a href="http://www.boost.org/users/people.html">People</a></td>
<td align="center"><a href="http://www.boost.org/users/faq.html">FAQ</a></td>
<td align="center"><a href="../../../../../more/index.htm">More</a></td>
</tr></table>
<hr>
<div class="spirit-nav">
<a accesskey="p" href="line_based.html"><img src="../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../core.html"><img src="../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../../../boost_asio.html"><img src="../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="handler_tracking.html"><img src="../../../../../doc/src/images/next.png" alt="Next"></a>
</div>
<div class="section">
<div class="titlepage"><div><div><h4 class="title">
<a name="boost_asio.overview.core.allocation"></a><a class="link" href="allocation.html" title="Custom Memory Allocation">Custom Memory
Allocation</a>
</h4></div></div></div>
<p>
Many asynchronous operations need to allocate an object to store state
associated with the operation. For example, a Win32 implementation needs
<code class="computeroutput"><span class="identifier">OVERLAPPED</span></code>-derived objects
to pass to Win32 API functions.
</p>
<p>
Furthermore, programs typically contain easily identifiable chains of asynchronous
operations. A half duplex protocol implementation (e.g. an HTTP server)
would have a single chain of operations per client (receives followed by
sends). A full duplex protocol implementation would have two chains executing
in parallel. Programs should be able to leverage this knowledge to reuse
memory for all asynchronous operations in a chain.
</p>
<p>
Given a copy of a user-defined <code class="computeroutput"><span class="identifier">Handler</span></code>
object <code class="computeroutput"><span class="identifier">h</span></code>, if the implementation
needs to allocate memory associated with that handler it will obtain an
allocator using the <code class="computeroutput"><span class="identifier">get_associated_allocator</span></code>
function. For example:
</p>
<pre class="programlisting"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">asio</span><span class="special">::</span><span class="identifier">associated_allocator_t</span><span class="special"><</span><span class="identifier">Handler</span><span class="special">></span> <span class="identifier">a</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">asio</span><span class="special">::</span><span class="identifier">get_associated_allocator</span><span class="special">(</span><span class="identifier">h</span><span class="special">);</span>
</pre>
<p>
The associated allocator must satisfy the standard Allocator requirements.
</p>
<p>
By default, handlers use the standard allocator (which is implemented in
terms of <code class="computeroutput"><span class="special">::</span><span class="keyword">operator</span>
<span class="keyword">new</span><span class="special">()</span></code>
and <code class="computeroutput"><span class="special">::</span><span class="keyword">operator</span>
<span class="keyword">delete</span><span class="special">()</span></code>).
The allocator may be customised for a particular handler type by specifying
a nested type <code class="computeroutput"><span class="identifier">allocator_type</span></code>
and member function <code class="computeroutput"><span class="identifier">get_allocator</span><span class="special">()</span></code>:
</p>
<pre class="programlisting"><span class="keyword">class</span> <span class="identifier">my_handler</span>
<span class="special">{</span>
<span class="keyword">public</span><span class="special">:</span>
<span class="comment">// Custom implementation of Allocator type requirements.</span>
<span class="keyword">typedef</span> <span class="identifier">my_allocator</span> <span class="identifier">allocator_type</span><span class="special">;</span>
<span class="comment">// Return a custom allocator implementation.</span>
<span class="identifier">allocator_type</span> <span class="identifier">get_allocator</span><span class="special">()</span> <span class="keyword">const</span> <span class="keyword">noexcept</span>
<span class="special">{</span>
<span class="keyword">return</span> <span class="identifier">my_allocator</span><span class="special">();</span>
<span class="special">}</span>
<span class="keyword">void</span> <span class="keyword">operator</span><span class="special">()()</span> <span class="special">{</span> <span class="special">...</span> <span class="special">}</span>
<span class="special">};</span>
</pre>
<p>
In more complex cases, the <code class="computeroutput"><span class="identifier">associated_allocator</span></code>
template may be partially specialised directly:
</p>
<pre class="programlisting"><span class="keyword">namespace</span> <span class="identifier">boost</span> <span class="special">{</span> <span class="keyword">namespace</span> <span class="identifier">asio</span> <span class="special">{</span>
<span class="keyword">template</span> <span class="special"><</span><span class="keyword">typename</span> <span class="identifier">Allocator</span><span class="special">></span>
<span class="keyword">struct</span> <span class="identifier">associated_allocator</span><span class="special"><</span><span class="identifier">my_handler</span><span class="special">,</span> <span class="identifier">Allocator</span><span class="special">></span>
<span class="special">{</span>
<span class="comment">// Custom implementation of Allocator type requirements.</span>
<span class="keyword">typedef</span> <span class="identifier">my_allocator</span> <span class="identifier">type</span><span class="special">;</span>
<span class="comment">// Return a custom allocator implementation.</span>
<span class="keyword">static</span> <span class="identifier">type</span> <span class="identifier">get</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">my_handler</span><span class="special">&,</span>
<span class="keyword">const</span> <span class="identifier">Allocator</span><span class="special">&</span> <span class="identifier">a</span> <span class="special">=</span> <span class="identifier">Allocator</span><span class="special">())</span> <span class="keyword">noexcept</span>
<span class="special">{</span>
<span class="keyword">return</span> <span class="identifier">my_allocator</span><span class="special">();</span>
<span class="special">}</span>
<span class="special">};</span>
<span class="special">}</span> <span class="special">}</span> <span class="comment">// namespace boost::asio</span>
</pre>
<p>
The implementation guarantees that the deallocation will occur before the
associated handler is invoked, which means the memory is ready to be reused
for any new asynchronous operations started by the handler.
</p>
<p>
The custom memory allocation functions may be called from any user-created
thread that is calling a library function. The implementation guarantees
that, for the asynchronous operations included the library, the implementation
will not make concurrent calls to the memory allocation functions for that
handler. The implementation will insert appropriate memory barriers to
ensure correct memory visibility should allocation functions need to be
called from different threads.
</p>
<h6>
<a name="boost_asio.overview.core.allocation.h0"></a>
<span class="phrase"><a name="boost_asio.overview.core.allocation.see_also"></a></span><a class="link" href="allocation.html#boost_asio.overview.core.allocation.see_also">See
Also</a>
</h6>
<p>
<a class="link" href="../../reference/associated_allocator.html" title="associated_allocator">associated_allocator</a>,
<a class="link" href="../../reference/get_associated_allocator.html" title="get_associated_allocator">get_associated_allocator</a>,
<a class="link" href="../../examples/cpp03_examples.html#boost_asio.examples.cpp03_examples.allocation">custom memory
allocation example (C++03)</a>, <a class="link" href="../../examples/cpp11_examples.html#boost_asio.examples.cpp11_examples.allocation">custom
memory allocation example (C++11)</a>.
</p>
</div>
<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
<td align="left"></td>
<td align="right"><div class="copyright-footer">Copyright © 2003-2018 Christopher M. Kohlhoff<p>
Distributed under the Boost Software License, Version 1.0. (See accompanying
file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
</p>
</div></td>
</tr></table>
<hr>
<div class="spirit-nav">
<a accesskey="p" href="line_based.html"><img src="../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../core.html"><img src="../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../../../boost_asio.html"><img src="../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="handler_tracking.html"><img src="../../../../../doc/src/images/next.png" alt="Next"></a>
</div>
</body>
</html>
|
