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<title>Boost Pool Interfaces</title>
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<img src="../../../boost.png" width="276" height="86" alt="C++ Boost">
<h1 align="center">Boost Pool Interfaces</h1>
<h2>Introduction</h2>
<p>There are several interfaces provided which allow users great flexibility
in how they want to use Pools. Review the <a href=
"concepts.html">concepts document</a> to get the basic understanding of how
Pools work.</p>
<h2>Terminology and Tradeoffs</h2>
<h3>Object Usage vs. Singleton Usage</h3>
<p><em>Object Usage</em> is the method where each Pool is an object that may
be created and destroyed. Destroying a Pool implicitly frees all chunks that
have been allocated from it.</p>
<p><em>Singleton Usage</em> is the method where each Pool is an object with
static duration; that is, it will not be destroyed until program exit. Pool
objects with Singleton Usage may be shared; thus, Singleton Usage implies
thread-safety as well. System memory allocated by Pool objects with
Singleton Usage may be freed through <span class=
"code">release_memory</span> or <span class="code">purge_memory</span>.</p>
<h3>Out-of-Memory Conditions: Exceptions vs. Null Return</h3>
<p>Some Pool interfaces throw exceptions when out-of-memory; others will
return 0. In general, unless mandated by the Standard, Pool interfaces will
always prefer to return 0 instead of throw an exception.</p>
<h2>The Interfaces</h2>
<h3>pool</h3>
<p>The <a href="interfaces/pool.html">pool interface</a> is a simple Object
Usage interface with Null Return.</p>
<p>Example:</p>
<pre class="code">
void func()
{
boost::pool<> p(sizeof(int));
for (int i = 0; i < 10000; ++i)
{
int * const t = p.malloc();
... // Do something with t; don't take the time to free() it
}
} // on function exit, p is destroyed, and all malloc()'ed ints are implicitly freed
</pre>
<h3>object_pool</h3>
<p>The <a href="interfaces/object_pool.html">object_pool interface</a> is an
Object Usage interface with Null Return, but is aware of the type of the
object for which it is allocating chunks. On destruction, any chunks that
have been allocated from that object_pool will have their destructors
called.</p>
<p>Example:</p>
<pre class="code">
struct X { ... }; // has destructor with side-effects
void func()
{
boost::object_pool<X> p;
for (int i = 0; i < 10000; ++i)
{
X * const t = p.malloc();
... // Do something with t; don't take the time to free() it
}
} // on function exit, p is destroyed, and all destructors for the X objects are called
</pre>
<h3>singleton_pool</h3>
<p>The <a href="interfaces/singleton_pool.html">singleton_pool interface</a>
is a Singleton Usage interface with Null Return. It's just the same as the
pool interface but with Singleton Usage instead.</p>
<p>Example:</p>
<pre class="code">
struct MyPoolTag { };
typedef boost::singleton_pool<MyPoolTag, sizeof(int)> my_pool;
void func()
{
for (int i = 0; i < 10000; ++i)
{
int * const t = my_pool::malloc();
... // Do something with t; don't take the time to free() it
}
// Explicitly free all malloc()'ed int's
my_pool::purge_memory();
}
</pre>
<h3>pool_alloc</h3>
<p>The <a href="interfaces/pool_alloc.html">pool_alloc interface</a> is a
Singleton Usage interface with Exceptions. It is built on the singleton_pool
interface, and provides a Standard Allocator-compliant class (for use in
containers, etc.).</p>
<p>Example:</p>
<pre class="code">
void func()
{
std::vector<int, boost::pool_allocator<int> > v;
for (int i = 0; i < 10000; ++i)
v.push_back(13);
} // Exiting the function does NOT free the system memory allocated by the pool allocator
// You must call
// boost::singleton_pool<boost::pool_allocator_tag, sizeof(int)>::release_memory()
// in order to force that
</pre>
<h2>Future Directions</h2>
<p>Another pool interface will be written: a base class for per-class pool
allocation. This "pool_base" interface will be Singleton Usage with
Exceptions, and built on the singleton_pool interface.</p>
<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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