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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="pairs.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="memory.html#manual.util.memory.allocator">Allocators</a></span></dt><dd><dl><dt><span class="sect2"><a href="memory.html#allocator.req">Requirements</a></span></dt><dt><span class="sect2"><a href="memory.html#allocator.design_issues">Design Issues</a></span></dt><dt><span class="sect2"><a href="memory.html#allocator.impl">Implementation</a></span></dt><dt><span class="sect2"><a href="memory.html#allocator.using">Using a Specific Allocator</a></span></dt><dt><span class="sect2"><a href="memory.html#allocator.custom">Custom Allocators</a></span></dt><dt><span class="sect2"><a href="memory.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="id419128"></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="id410525"></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="id457524"></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="ext_allocators.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="id455580"></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="id408540"></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="id411757"></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="id392744"></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="id422908"></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="id395999"></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="id398620"></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="pairs.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. 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