<?xml version="1.0" encoding="UTF-8" standalone="no"?> <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"> <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="pairs.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="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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