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<html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /><title>Chapter 11. Memory</title><meta name="generator" content="DocBook XSL Stylesheets V1.74.0" /><meta name="keywords" content=" ISO C++ , library " /><link rel="home" href="../spine.html" title="The GNU C++ Library Documentation" /><link rel="up" href="utilities.html" title="Part IV. Utilities" /><link rel="prev" href="bk01pt04ch10.html" title="Chapter 10. Pairs" /><link rel="next" href="auto_ptr.html" title="auto_ptr" /></head><body><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="3" align="center">Chapter 11. Memory</th></tr><tr><td width="20%" align="left"><a accesskey="p" href="bk01pt04ch10.html">Prev</a> </td><th width="60%" align="center">Part IV. Utilities</th><td width="20%" align="right"> <a accesskey="n" href="auto_ptr.html">Next</a></td></tr></table><hr /></div><div class="chapter" lang="en" xml:lang="en"><div class="titlepage"><div><div><h2 class="title"><a id="manual.util.memory"></a>Chapter 11. Memory</h2></div></div></div><div class="toc"><p><b>Table of Contents</b></p><dl><dt><span class="sect1"><a href="bk01pt04ch11.html#manual.util.memory.allocator">Allocators</a></span></dt><dd><dl><dt><span class="sect2"><a href="bk01pt04ch11.html#allocator.req">Requirements</a></span></dt><dt><span class="sect2"><a href="bk01pt04ch11.html#allocator.design_issues">Design Issues</a></span></dt><dt><span class="sect2"><a href="bk01pt04ch11.html#allocator.impl">Implementation</a></span></dt><dt><span class="sect2"><a href="bk01pt04ch11.html#allocator.using">Using a Specific Allocator</a></span></dt><dt><span class="sect2"><a href="bk01pt04ch11.html#allocator.custom">Custom Allocators</a></span></dt><dt><span class="sect2"><a href="bk01pt04ch11.html#allocator.ext">Extension Allocators</a></span></dt></dl></dd><dt><span class="sect1"><a href="auto_ptr.html">auto_ptr</a></span></dt><dd><dl><dt><span class="sect2"><a href="auto_ptr.html#auto_ptr.limitations">Limitations</a></span></dt><dt><span class="sect2"><a href="auto_ptr.html#auto_ptr.using">Use in Containers</a></span></dt></dl></dd><dt><span class="sect1"><a href="shared_ptr.html">shared_ptr</a></span></dt><dd><dl><dt><span class="sect2"><a href="shared_ptr.html#shared_ptr.req">Requirements</a></span></dt><dt><span class="sect2"><a href="shared_ptr.html#shared_ptr.design_issues">Design Issues</a></span></dt><dt><span class="sect2"><a href="shared_ptr.html#shared_ptr.impl">Implementation</a></span></dt><dt><span class="sect2"><a href="shared_ptr.html#shared_ptr.using">Use</a></span></dt><dt><span class="sect2"><a href="shared_ptr.html#shared_ptr.ack">Acknowledgments</a></span></dt></dl></dd></dl></div><p>
Memory contains three general areas. First, function and operator
calls via <code class="function">new</code> and <code class="function">delete</code>
operator or member function calls. Second, allocation via
<code class="classname">allocator</code>. And finally, smart pointer and
intelligent pointer abstractions.
</p><div class="sect1" lang="en" xml:lang="en"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a id="manual.util.memory.allocator"></a>Allocators</h2></div></div></div><p>
Memory management for Standard Library entities is encapsulated in a
class template called <code class="classname">allocator</code>. The
<code class="classname">allocator</code> abstraction is used throughout the
library in <code class="classname">string</code>, container classes,
algorithms, and parts of iostreams. This class, and base classes of
it, are the superset of available free store (“<span class="quote">heap</span>”)
management classes.
</p><div class="sect2" lang="en" xml:lang="en"><div class="titlepage"><div><div><h3 class="title"><a id="allocator.req"></a>Requirements</h3></div></div></div><p>
The C++ standard only gives a few directives in this area:
</p><div class="itemizedlist"><ul type="disc"><li><p>
When you add elements to a container, and the container must
allocate more memory to hold them, the container makes the
request via its <span class="type">Allocator</span> template
parameter, which is usually aliased to
<span class="type">allocator_type</span>. This includes adding chars
to the string class, which acts as a regular STL container in
this respect.
</p></li><li><p>
The default <span class="type">Allocator</span> argument of every
container-of-T is <code class="classname">allocator<T></code>.
</p></li><li><p>
The interface of the <code class="classname">allocator<T></code> class is
extremely simple. It has about 20 public declarations (nested
typedefs, member functions, etc), but the two which concern us most
are:
</p><pre class="programlisting">
T* allocate (size_type n, const void* hint = 0);
void deallocate (T* p, size_type n);
</pre><p>
The <code class="varname">n</code> arguments in both those
functions is a <span class="emphasis"><em>count</em></span> of the number of
<span class="type">T</span>'s to allocate space for, <span class="emphasis"><em>not their
total size</em></span>.
(This is a simplification; the real signatures use nested typedefs.)
</p></li><li><p>
The storage is obtained by calling <code class="function">::operator
new</code>, but it is unspecified when or how
often this function is called. The use of the
<code class="varname">hint</code> is unspecified, but intended as an
aid to locality if an implementation so
desires. <code class="constant">[20.4.1.1]/6</code>
</p></li></ul></div><p>
Complete details cam be found in the C++ standard, look in
<code class="constant">[20.4 Memory]</code>.
</p></div><div class="sect2" lang="en" xml:lang="en"><div class="titlepage"><div><div><h3 class="title"><a id="allocator.design_issues"></a>Design Issues</h3></div></div></div><p>
The easiest way of fulfilling the requirements is to call
<code class="function">operator new</code> each time a container needs
memory, and to call <code class="function">operator delete</code> each time
the container releases memory. This method may be <a class="ulink" href="http://gcc.gnu.org/ml/libstdc++/2001-05/msg00105.html" target="_top">slower</a>
than caching the allocations and re-using previously-allocated
memory, but has the advantage of working correctly across a wide
variety of hardware and operating systems, including large
clusters. The <code class="classname">__gnu_cxx::new_allocator</code>
implements the simple operator new and operator delete semantics,
while <code class="classname">__gnu_cxx::malloc_allocator</code>
implements much the same thing, only with the C language functions
<code class="function">std::malloc</code> and <code class="function">free</code>.
</p><p>
Another approach is to use intelligence within the allocator
class to cache allocations. This extra machinery can take a variety
of forms: a bitmap index, an index into an exponentially increasing
power-of-two-sized buckets, or simpler fixed-size pooling cache.
The cache is shared among all the containers in the program: when
your program's <code class="classname">std::vector<int></code> gets
cut in half and frees a bunch of its storage, that memory can be
reused by the private
<code class="classname">std::list<WonkyWidget></code> brought in from
a KDE library that you linked against. And operators
<code class="function">new</code> and <code class="function">delete</code> are not
always called to pass the memory on, either, which is a speed
bonus. Examples of allocators that use these techniques are
<code class="classname">__gnu_cxx::bitmap_allocator</code>,
<code class="classname">__gnu_cxx::pool_allocator</code>, and
<code class="classname">__gnu_cxx::__mt_alloc</code>.
</p><p>
Depending on the implementation techniques used, the underlying
operating system, and compilation environment, scaling caching
allocators can be tricky. In particular, order-of-destruction and
order-of-creation for memory pools may be difficult to pin down
with certainty, which may create problems when used with plugins
or loading and unloading shared objects in memory. As such, using
caching allocators on systems that do not support
<code class="function">abi::__cxa_atexit</code> is not recommended.
</p></div><div class="sect2" lang="en" xml:lang="en"><div class="titlepage"><div><div><h3 class="title"><a id="allocator.impl"></a>Implementation</h3></div></div></div><div class="sect3" lang="en" xml:lang="en"><div class="titlepage"><div><div><h4 class="title"><a id="id495307"></a>Interface Design</h4></div></div></div><p>
The only allocator interface that
is support is the standard C++ interface. As such, all STL
containers have been adjusted, and all external allocators have
been modified to support this change.
</p><p>
The class <code class="classname">allocator</code> just has typedef,
constructor, and rebind members. It inherits from one of the
high-speed extension allocators, covered below. Thus, all
allocation and deallocation depends on the base class.
</p><p>
The base class that <code class="classname">allocator</code> is derived from
may not be user-configurable.
</p></div><div class="sect3" lang="en" xml:lang="en"><div class="titlepage"><div><div><h4 class="title"><a id="id462480"></a>Selecting Default Allocation Policy</h4></div></div></div><p>
It's difficult to pick an allocation strategy that will provide
maximum utility, without excessively penalizing some behavior. In
fact, it's difficult just deciding which typical actions to measure
for speed.
</p><p>
Three synthetic benchmarks have been created that provide data
that is used to compare different C++ allocators. These tests are:
</p><div class="orderedlist"><ol type="1"><li><p>
Insertion.
</p><p>
Over multiple iterations, various STL container
objects have elements inserted to some maximum amount. A variety
of allocators are tested.
Test source for <a class="ulink" href="http://gcc.gnu.org/viewcvs/trunk/libstdc%2B%2B-v3/testsuite/performance/23_containers/insert/sequence.cc?view=markup" target="_top">sequence</a>
and <a class="ulink" href="http://gcc.gnu.org/viewcvs/trunk/libstdc%2B%2B-v3/testsuite/performance/23_containers/insert/associative.cc?view=markup" target="_top">associative</a>
containers.
</p></li><li><p>
Insertion and erasure in a multi-threaded environment.
</p><p>
This test shows the ability of the allocator to reclaim memory
on a pre-thread basis, as well as measuring thread contention
for memory resources.
Test source
<a class="ulink" href="http://gcc.gnu.org/viewcvs/trunk/libstdc%2B%2B-v3/testsuite/performance/23_containers/insert_erase/associative.cc?view=markup" target="_top">here</a>.
</p></li><li><p>
A threaded producer/consumer model.
</p><p>
Test source for
<a class="ulink" href="http://gcc.gnu.org/viewcvs/trunk/libstdc%2B%2B-v3/testsuite/performance/23_containers/producer_consumer/sequence.cc?view=markup" target="_top">sequence</a>
and
<a class="ulink" href="http://gcc.gnu.org/viewcvs/trunk/libstdc%2B%2B-v3/testsuite/performance/23_containers/producer_consumer/associative.cc?view=markup" target="_top">associative</a>
containers.
</p></li></ol></div><p>
The current default choice for
<code class="classname">allocator</code> is
<code class="classname">__gnu_cxx::new_allocator</code>.
</p></div><div class="sect3" lang="en" xml:lang="en"><div class="titlepage"><div><div><h4 class="title"><a id="id531789"></a>Disabling Memory Caching</h4></div></div></div><p>
In use, <code class="classname">allocator</code> may allocate and
deallocate using implementation-specified strategies and
heuristics. Because of this, every call to an allocator object's
<code class="function">allocate</code> member function may not actually
call the global operator new. This situation is also duplicated
for calls to the <code class="function">deallocate</code> member
function.
</p><p>
This can be confusing.
</p><p>
In particular, this can make debugging memory errors more
difficult, especially when using third party tools like valgrind or
debug versions of <code class="function">new</code>.
</p><p>
There are various ways to solve this problem. One would be to use
a custom allocator that just called operators
<code class="function">new</code> and <code class="function">delete</code>
directly, for every allocation. (See
<code class="filename">include/ext/new_allocator.h</code>, for instance.)
However, that option would involve changing source code to use
a non-default allocator. Another option is to force the
default allocator to remove caching and pools, and to directly
allocate with every call of <code class="function">allocate</code> and
directly deallocate with every call of
<code class="function">deallocate</code>, regardless of efficiency. As it
turns out, this last option is also available.
</p><p>
To globally disable memory caching within the library for the
default allocator, merely set
<code class="constant">GLIBCXX_FORCE_NEW</code> (with any value) in the
system's environment before running the program. If your program
crashes with <code class="constant">GLIBCXX_FORCE_NEW</code> in the
environment, it likely means that you linked against objects
built against the older library (objects which might still using the
cached allocations...).
</p></div></div><div class="sect2" lang="en" xml:lang="en"><div class="titlepage"><div><div><h3 class="title"><a id="allocator.using"></a>Using a Specific Allocator</h3></div></div></div><p>
You can specify different memory management schemes on a
per-container basis, by overriding the default
<span class="type">Allocator</span> template parameter. For example, an easy
(but non-portable) method of specifying that only <code class="function">malloc</code> or <code class="function">free</code>
should be used instead of the default node allocator is:
</p><pre class="programlisting">
std::list <int, __gnu_cxx::malloc_allocator<int> > malloc_list;</pre><p>
Likewise, a debugging form of whichever allocator is currently in use:
</p><pre class="programlisting">
std::deque <int, __gnu_cxx::debug_allocator<std::allocator<int> > > debug_deque;
</pre></div><div class="sect2" lang="en" xml:lang="en"><div class="titlepage"><div><div><h3 class="title"><a id="allocator.custom"></a>Custom Allocators</h3></div></div></div><p>
Writing a portable C++ allocator would dictate that the interface
would look much like the one specified for
<code class="classname">allocator</code>. Additional member functions, but
not subtractions, would be permissible.
</p><p>
Probably the best place to start would be to copy one of the
extension allocators: say a simple one like
<code class="classname">new_allocator</code>.
</p></div><div class="sect2" lang="en" xml:lang="en"><div class="titlepage"><div><div><h3 class="title"><a id="allocator.ext"></a>Extension Allocators</h3></div></div></div><p>
Several other allocators are provided as part of this
implementation. The location of the extension allocators and their
names have changed, but in all cases, functionality is
equivalent. Starting with gcc-3.4, all extension allocators are
standard style. Before this point, SGI style was the norm. Because of
this, the number of template arguments also changed. Here's a simple
chart to track the changes.
</p><p>
More details on each of these extension allocators follows.
</p><div class="orderedlist"><ol type="1"><li><p>
<code class="classname">new_allocator</code>
</p><p>
Simply wraps <code class="function">::operator new</code>
and <code class="function">::operator delete</code>.
</p></li><li><p>
<code class="classname">malloc_allocator</code>
</p><p>
Simply wraps <code class="function">malloc</code> and
<code class="function">free</code>. There is also a hook for an
out-of-memory handler (for
<code class="function">new</code>/<code class="function">delete</code> this is
taken care of elsewhere).
</p></li><li><p>
<code class="classname">array_allocator</code>
</p><p>
Allows allocations of known and fixed sizes using existing
global or external storage allocated via construction of
<code class="classname">std::tr1::array</code> objects. By using this
allocator, fixed size containers (including
<code class="classname">std::string</code>) can be used without
instances calling <code class="function">::operator new</code> and
<code class="function">::operator delete</code>. This capability
allows the use of STL abstractions without runtime
complications or overhead, even in situations such as program
startup. For usage examples, please consult the testsuite.
</p></li><li><p>
<code class="classname">debug_allocator</code>
</p><p>
A wrapper around an arbitrary allocator A. It passes on
slightly increased size requests to A, and uses the extra
memory to store size information. When a pointer is passed
to <code class="function">deallocate()</code>, the stored size is
checked, and <code class="function">assert()</code> is used to
guarantee they match.
</p></li><li><p>
<code class="classname">throw_allocator</code>
</p><p>
Includes memory tracking and marking abilities as well as hooks for
throwing exceptions at configurable intervals (including random,
all, none).
</p></li><li><p>
<code class="classname">__pool_alloc</code>
</p><p>
A high-performance, single pool allocator. The reusable
memory is shared among identical instantiations of this type.
It calls through <code class="function">::operator new</code> to
obtain new memory when its lists run out. If a client
container requests a block larger than a certain threshold
size, then the pool is bypassed, and the allocate/deallocate
request is passed to <code class="function">::operator new</code>
directly.
</p><p>
Older versions of this class take a boolean template
parameter, called <code class="varname">thr</code>, and an integer template
parameter, called <code class="varname">inst</code>.
</p><p>
The <code class="varname">inst</code> number is used to track additional memory
pools. The point of the number is to allow multiple
instantiations of the classes without changing the semantics at
all. All three of
</p><pre class="programlisting">
typedef __pool_alloc<true,0> normal;
typedef __pool_alloc<true,1> private;
typedef __pool_alloc<true,42> also_private;
</pre><p>
behave exactly the same way. However, the memory pool for each type
(and remember that different instantiations result in different types)
remains separate.
</p><p>
The library uses <span class="emphasis"><em>0</em></span> in all its instantiations. If you
wish to keep separate free lists for a particular purpose, use a
different number.
</p><p>The <code class="varname">thr</code> boolean determines whether the
pool should be manipulated atomically or not. When
<code class="varname">thr</code> = <code class="constant">true</code>, the allocator
is is thread-safe, while <code class="varname">thr</code> =
<code class="constant">false</code>, and is slightly faster but unsafe for
multiple threads.
</p><p>
For thread-enabled configurations, the pool is locked with a
single big lock. In some situations, this implementation detail
may result in severe performance degradation.
</p><p>
(Note that the GCC thread abstraction layer allows us to provide
safe zero-overhead stubs for the threading routines, if threads
were disabled at configuration time.)
</p></li><li><p>
<code class="classname">__mt_alloc</code>
</p><p>
A high-performance fixed-size allocator with
exponentially-increasing allocations. It has its own
documentation, found <a class="link" href="bk01pt12ch32.html#manual.ext.allocator.mt" title="mt_allocator">here</a>.
</p></li><li><p>
<code class="classname">bitmap_allocator</code>
</p><p>
A high-performance allocator that uses a bit-map to keep track
of the used and unused memory locations. It has its own
documentation, found <a class="link" href="bitmap_allocator.html" title="bitmap_allocator">here</a>.
</p></li></ol></div></div><div class="bibliography"><div class="titlepage"><div><div><h3 class="title"><a id="allocator.biblio"></a>Bibliography</h3></div></div></div><div class="biblioentry"><a id="id505299"></a><p><span class="title"><i>
ISO/IEC 14882:1998 Programming languages - C++
</i>. </span>
isoc++_1998
<span class="pagenums">20.4 Memory. </span></p></div><div class="biblioentry"><a id="id465950"></a><p><span class="title"><i>The Standard Librarian: What Are Allocators Good
</i>. </span>
austernm
<span class="author"><span class="firstname">Matt</span> <span class="surname">Austern</span>. </span><span class="publisher"><span class="publishername">
C/C++ Users Journal
. </span></span><span class="biblioid">
<a class="ulink" href="http://www.cuj.com/documents/s=8000/cujcexp1812austern/" target="_top">
</a>
. </span></p></div><div class="biblioentry"><a id="id515746"></a><p><span class="title"><i>The Hoard Memory Allocator</i>. </span>
emeryb
<span class="author"><span class="firstname">Emery</span> <span class="surname">Berger</span>. </span><span class="biblioid">
<a class="ulink" href="http://www.cs.umass.edu/~emery/hoard/" target="_top">
</a>
. </span></p></div><div class="biblioentry"><a id="id507784"></a><p><span class="title"><i>Reconsidering Custom Memory Allocation</i>. </span>
bergerzorn
<span class="author"><span class="firstname">Emery</span> <span class="surname">Berger</span>. </span><span class="author"><span class="firstname">Ben</span> <span class="surname">Zorn</span>. </span><span class="author"><span class="firstname">Kathryn</span> <span class="surname">McKinley</span>. </span><span class="copyright">Copyright © 2002 OOPSLA. </span><span class="biblioid">
<a class="ulink" href="http://www.cs.umass.edu/~emery/pubs/berger-oopsla2002.pdf" target="_top">
</a>
. </span></p></div><div class="biblioentry"><a id="id467782"></a><p><span class="title"><i>Allocator Types</i>. </span>
kreftlanger
<span class="author"><span class="firstname">Klaus</span> <span class="surname">Kreft</span>. </span><span class="author"><span class="firstname">Angelika</span> <span class="surname">Langer</span>. </span><span class="publisher"><span class="publishername">
C/C++ Users Journal
. </span></span><span class="biblioid">
<a class="ulink" href="http://www.langer.camelot.de/Articles/C++Report/Allocators/Allocators.html" target="_top">
</a>
. </span></p></div><div class="biblioentry"><a id="id470766"></a><p><span class="title"><i>The C++ Programming Language</i>. </span>
tcpl
<span class="author"><span class="firstname">Bjarne</span> <span class="surname">Stroustrup</span>. </span><span class="copyright">Copyright © 2000 . </span><span class="pagenums">19.4 Allocators. </span><span class="publisher"><span class="publishername">
Addison Wesley
. </span></span></p></div><div class="biblioentry"><a id="id468765"></a><p><span class="title"><i>Yalloc: A Recycling C++ Allocator</i>. </span>
yenf
<span class="author"><span class="firstname">Felix</span> <span class="surname">Yen</span>. </span><span class="copyright">Copyright © . </span><span class="biblioid">
<a class="ulink" href="http://home.earthlink.net/~brimar/yalloc/" target="_top">
</a>
. </span></p></div></div></div></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="bk01pt04ch10.html">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="utilities.html">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="auto_ptr.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">Chapter 10. Pairs </td><td width="20%" align="center"><a accesskey="h" href="../spine.html">Home</a></td><td width="40%" align="right" valign="top"> auto_ptr</td></tr></table></div></body></html>
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