libstdc++-v3/doc/xml/manual/allocator.xml

<section xmlns="http://docbook.org/ns/docbook" version="5.0"
	 xml:id="std.util.memory.allocator" xreflabel="Allocator">
<?dbhtml filename="allocator.html"?>

<info><title>Allocators</title>
  <keywordset>
    <keyword>ISO C++</keyword>
    <keyword>allocator</keyword>
  </keywordset>
</info>



<para>
 Memory management for Standard Library entities is encapsulated in a
 class template called <classname>allocator</classname>. The
 <classname>allocator</classname> abstraction is used throughout the
 library in <classname>string</classname>, container classes,
 algorithms, and parts of iostreams. This class, and base classes of
 it, are the superset of available free store (<quote>heap</quote>)
 management classes.
</para>

<section xml:id="allocator.req"><info><title>Requirements</title></info>


  <para>
    The C++ standard only gives a few directives in this area:
  </para>
   <itemizedlist>
     <listitem>
      <para>
       When you add elements to a container, and the container must
       allocate more memory to hold them, the container makes the
       request via its <type>Allocator</type> template
       parameter, which is usually aliased to
       <type>allocator_type</type>.  This includes adding chars
       to the string class, which acts as a regular STL container in
       this respect.
      </para>
     </listitem>
     <listitem>
       <para>
       The default <type>Allocator</type> argument of every
       container-of-T is <classname>allocator&lt;T&gt;</classname>.
       </para>
     </listitem>
     <listitem>
       <para>
       The interface of the <classname>allocator&lt;T&gt;</classname> class is
	 extremely simple.  It has about 20 public declarations (nested
	 typedefs, member functions, etc), but the two which concern us most
	 are:
       </para>
<programlisting>
	 T*    allocate   (size_type n, const void* hint = 0);
	 void  deallocate (T* p, size_type n);
</programlisting>

       <para>
	 The <varname>n</varname> arguments in both those
	 functions is a <emphasis>count</emphasis> of the number of
	 <type>T</type>'s to allocate space for, <emphasis>not their
	 total size</emphasis>.
	 (This is a simplification; the real signatures use nested typedefs.)
       </para>
     </listitem>
     <listitem>
       <para>
	 The storage is obtained by calling <function>::operator
	 new</function>, but it is unspecified when or how
	 often this function is called.  The use of the
	 <varname>hint</varname> is unspecified, but intended as an
	 aid to locality if an implementation so
	 desires. <constant>[20.4.1.1]/6</constant>
       </para>
      </listitem>
   </itemizedlist>

   <para>
     Complete details can be found in the C++ standard, look in
     <constant>[20.4 Memory]</constant>.
   </para>

</section>

<section xml:id="allocator.design_issues"><info><title>Design Issues</title></info>


  <para>
    The easiest way of fulfilling the requirements is to call
    <function>operator new</function> each time a container needs
    memory, and to call <function>operator delete</function> each time
    the container releases memory. This method may be <link xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="http://gcc.gnu.org/ml/libstdc++/2001-05/msg00105.html">slower</link>
    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 <classname>__gnu_cxx::new_allocator</classname>
    implements the simple operator new and operator delete semantics,
    while <classname>__gnu_cxx::malloc_allocator</classname>
    implements much the same thing, only with the C language functions
    <function>std::malloc</function> and <function>std::free</function>.
  </para>

  <para>
    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 <classname>std::vector&lt;int&gt;</classname> gets
  cut in half and frees a bunch of its storage, that memory can be
  reused by the private
  <classname>std::list&lt;WonkyWidget&gt;</classname> brought in from
  a KDE library that you linked against.  And operators
  <function>new</function> and <function>delete</function> are not
  always called to pass the memory on, either, which is a speed
  bonus. Examples of allocators that use these techniques are
  <classname>__gnu_cxx::bitmap_allocator</classname>,
  <classname>__gnu_cxx::pool_allocator</classname>, and
  <classname>__gnu_cxx::__mt_alloc</classname>.
  </para>

  <para>
    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
    <function>abi::__cxa_atexit</function> is not recommended.
  </para>

</section>

<section xml:id="allocator.impl"><info><title>Implementation</title></info>


  <section xml:id="allocator.interface"><info><title>Interface Design</title></info>

   <para>
     The only allocator interface that
     is supported is the standard C++ interface. As such, all STL
     containers have been adjusted, and all external allocators have
     been modified to support this change.
   </para>

   <para>
     The class <classname>allocator</classname> 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.
   </para>

   <para>
     The choice of base class that <classname>allocator</classname>
     is derived from is fixed at the time when GCC is built,
     and the different choices are not ABI compatible.
</para>

  </section>

  <section xml:id="allocator.default"><info><title>Selecting Default Allocation Policy</title></info>

   <para>
     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.
   </para>

   <para>
     Three synthetic benchmarks have been created that provide data
     that is used to compare different C++ allocators. These tests are:
   </para>

   <orderedlist>
     <listitem>
       <para>
       Insertion.
       </para>
       <para>
       Over multiple iterations, various STL container
     objects have elements inserted to some maximum amount. A variety
     of allocators are tested.
     Test source for <link xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="https://gcc.gnu.org/cgit/gcc/tree/libstdc++-v3/testsuite/performance/23_containers/insert/sequence.cc">sequence</link>
     and <link xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="https://gcc.gnu.org/cgit/gcc/tree/libstdc++-v3/testsuite/performance/23_containers/insert/associative.cc">associative</link>
     containers.
       </para>

     </listitem>

     <listitem>
       <para>
       Insertion and erasure in a multi-threaded environment.
       </para>
       <para>
       This test shows the ability of the allocator to reclaim memory
     on a per-thread basis, as well as measuring thread contention
     for memory resources.
     Test source
    <link xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="https://gcc.gnu.org/cgit/gcc/tree/libstdc++-v3/testsuite/performance/23_containers/insert_erase/associative.cc">here</link>.
       </para>
     </listitem>

     <listitem>
       <para>
	 A threaded producer/consumer model.
       </para>
       <para>
       Test source for
     <link xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="https://gcc.gnu.org/cgit/gcc/tree/libstdc++-v3/testsuite/performance/23_containers/producer_consumer/sequence.cc">sequence</link>
     and
     <link xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="https://gcc.gnu.org/cgit/gcc/tree/libstdc++-v3/testsuite/performance/23_containers/producer_consumer/associative.cc">associative</link>
     containers.
     </para>
     </listitem>
   </orderedlist>

   <para>
     Since GCC 12 the default choice for
     <classname>allocator</classname> is
     <classname>std::__new_allocator</classname>.
     Before GCC 12 it was the <classname>__gnu_cxx::new_allocator</classname>
     extension (which has identical behaviour).
   </para>

  </section>

  <section xml:id="allocator.caching"><info><title>Disabling Memory Caching</title></info>

    <para>
      In use, <classname>allocator</classname> may allocate and
      deallocate using implementation-specific strategies and
      heuristics. Because of this, a given call to an allocator object's
      <function>allocate</function> member function may not actually
      call the global <code>operator new</code> and a given call to
      to the <function>deallocate</function> member function may not
      call <code>operator delete</code>.
    </para>

   <para>
     This can be confusing.
   </para>

   <para>
     In particular, this can make debugging memory errors more
     difficult, especially when using third-party tools like valgrind or
     debug versions of <function>new</function>.
   </para>

   <para>
     There are various ways to solve this problem. One would be to use
     a custom allocator that just called operators
     <function>new</function> and <function>delete</function>
     directly, for every allocation. (See the default allocator,
     <filename>include/ext/new_allocator.h</filename>, for instance.)
     However, that option may 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 <function>allocate</function> and
     directly deallocate with every call of
     <function>deallocate</function>, regardless of efficiency. As it
     turns out, this last option is also available.
   </para>


   <para>
     To globally disable memory caching within the library for some of
     the optional non-default allocators, merely set
     <constant>GLIBCXX_FORCE_NEW</constant> (with any value) in the
     system's environment before running the program. If your program
     crashes with <constant>GLIBCXX_FORCE_NEW</constant> in the
     environment, it likely means that you linked against objects
     built against the older library (objects which might still using the
     cached allocations...).
  </para>

  </section>

</section>

<section xml:id="allocator.using"><info><title>Using a Specific Allocator</title></info>


   <para>
     You can specify different memory management schemes on a
     per-container basis, by overriding the default
     <type>Allocator</type> template parameter.  For example, an easy
      (but non-portable) method of specifying that only <function>malloc</function> or <function>free</function>
      should be used instead of the default node allocator is:
   </para>
<programlisting>
    std::list &lt;int, __gnu_cxx::malloc_allocator&lt;int&gt; &gt;  malloc_list;</programlisting>
    <para>
      Likewise, a debugging form of whichever allocator is currently in use:
    </para>
<programlisting>
    std::deque &lt;int, __gnu_cxx::debug_allocator&lt;std::allocator&lt;int&gt; &gt; &gt;  debug_deque;
</programlisting>
</section>

<section xml:id="allocator.custom"><info><title>Custom Allocators</title></info>


  <para>
    Writing a portable C++ allocator would dictate that the interface
    would look much like the one specified for
    <classname>allocator</classname>. Additional member functions, but
    not subtractions, would be permissible.
  </para>

   <para>
     Probably the best place to start would be to copy one of the
   extension allocators: say a simple one like
   <classname>new_allocator</classname>.
   </para>

   <para>
     Since C++11 the minimal interface require for an allocator is
     much smaller, as <classname>std::allocator_traits</classname>
     can provide default for much of the interface.
   </para>


</section>

<section xml:id="allocator.ext"><info><title>Extension Allocators</title></info>


  <para>
    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.
    <xref linkend="table.extension_allocators"/> tracks the changes.
  </para>

  <para>
    More details on each of these extension allocators follows.
  </para>
   <orderedlist>
     <listitem>
       <para>
       <classname>new_allocator</classname>
       </para>
       <para>
	 Simply wraps <function>::operator new</function>
	 and <function>::operator delete</function>.
       </para>
     </listitem>
     <listitem>
       <para>
       <classname>malloc_allocator</classname>
       </para>
       <para>
	 Simply wraps <function>malloc</function> and
	 <function>free</function>. There is also a hook for an
	 out-of-memory handler (for
	 <function>new</function>/<function>delete</function> this is
	 taken care of elsewhere).
       </para>
     </listitem>
     <listitem>
       <para>
       <classname>debug_allocator</classname>
       </para>
       <para>
	 A wrapper around an arbitrary allocator <classname>A</classname>.
	 It passes on slightly increased size requests to <classname>A</classname>,
	 and uses the extra memory to store size information.
	 When a pointer is passed
	 to <function>deallocate()</function>, the stored size is
	 checked, and <function>assert()</function> is used to
	 guarantee they match.
       </para>
     </listitem>
      <listitem>
	<para>
	<classname>throw_allocator</classname>
	</para>
	<para>
	  Includes memory tracking and marking abilities as well as hooks for
	  throwing exceptions at configurable intervals (including random,
	  all, none).
	</para>
      </listitem>
     <listitem>
       <para>
       <classname>__pool_alloc</classname>
       </para>
       <para>
	 A high-performance, single pool allocator.  The reusable
	 memory is shared among identical instantiations of this type.
	 It calls through <function>::operator new</function> 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 <function>::operator new</function>
	 directly.
       </para>

       <para>
	 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.
       </para>

       <para>
	 (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.)
       </para>
     </listitem>

     <listitem>
       <para>
       <classname>__mt_alloc</classname>
       </para>
       <para>
	 A high-performance fixed-size allocator with
	 exponentially-increasing allocations. It has its own
	 <link linkend="manual.ext.allocator.mt">chapter</link>
         in the documentation.
       </para>
     </listitem>

     <listitem>
       <para>
       <classname>bitmap_allocator</classname>
       </para>
       <para>
	 A high-performance allocator that uses a bit-map to keep track
	 of the used and unused memory locations. It has its own
	 <link linkend="manual.ext.allocator.bitmap">chapter</link>
         in the documentation.
       </para>
     </listitem>
   </orderedlist>
</section>


<bibliography xml:id="allocator.biblio"><info><title>Bibliography</title></info>


  <biblioentry>
    <citetitle>
    ISO/IEC 14882:1998 Programming languages - C++
    </citetitle>
    <abbrev>
      isoc++_1998
    </abbrev>
    <pagenums>20.4 Memory</pagenums>
  </biblioentry>

  <biblioentry>
    <title>
      <link xmlns:xlink="http://www.w3.org/1999/xlink"
	    xlink:href="https://web.archive.org/web/20190622154249/http://www.drdobbs.com/the-standard-librarian-what-are-allocato/184403759">
      The Standard Librarian: What Are Allocators Good For?
      </link>
    </title>

    <author><personname><firstname>Matt</firstname><surname>Austern</surname></personname></author>
    <publisher>
      <publishername>
	C/C++ Users Journal
      </publishername>
    </publisher>
    <pubdate>2000-12</pubdate>
  </biblioentry>

  <biblioentry>
      <title>
	<link xmlns:xlink="http://www.w3.org/1999/xlink"
	      xlink:href="http://hoard.org">
      The Hoard Memory Allocator
	</link>
      </title>

    <author><personname><firstname>Emery</firstname><surname>Berger</surname></personname></author>
  </biblioentry>

  <biblioentry>
      <title>
	<link xmlns:xlink="http://www.w3.org/1999/xlink"
	      xlink:href="https://people.cs.umass.edu/~emery/pubs/berger-oopsla2002.pdf">
      Reconsidering Custom Memory Allocation
	</link>
      </title>

    <author><personname><firstname>Emery</firstname><surname>Berger</surname></personname></author>
    <author><personname><firstname>Ben</firstname><surname>Zorn</surname></personname></author>
    <author><personname><firstname>Kathryn</firstname><surname>McKinley</surname></personname></author>
    <copyright>
      <year>2002</year>
      <holder>OOPSLA</holder>
    </copyright>
  </biblioentry>


  <biblioentry>
      <title>
	<link xmlns:xlink="http://www.w3.org/1999/xlink"
	      xlink:href="https://angelikalanger.com/Articles/C++Report/Allocators/Allocators.html">
      Allocator Types
	</link>
      </title>


    <author><personname><firstname>Klaus</firstname><surname>Kreft</surname></personname></author>
    <author><personname><firstname>Angelika</firstname><surname>Langer</surname></personname></author>
    <publisher>
      <publishername>
	C/C++ Users Journal
      </publishername>
    </publisher>
  </biblioentry>

  <biblioentry>
    <citetitle>The C++ Programming Language</citetitle>
    <author><personname><firstname>Bjarne</firstname><surname>Stroustrup</surname></personname></author>
    <copyright>
      <year>2000</year>
      <holder/>
    </copyright>
    <pagenums>19.4 Allocators</pagenums>
    <publisher>
      <publishername>
	Addison Wesley
      </publishername>
    </publisher>
  </biblioentry>

  <biblioentry>
    <citetitle>Yalloc: A Recycling C++ Allocator</citetitle>
    <author><personname><firstname>Felix</firstname><surname>Yen</surname></personname></author>
  </biblioentry>
</bibliography>

</section>