gcc-4_9-branch/libstdc++-v3/doc/html/manual/memory.html

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   5 </th><td width="20%" align="right"> <a accesskey="n" href="traits.html">Next</a></td></tr></table><hr /></div><div class="section"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a id="std.util.memory"></a>Memory</h2></div></div></div><p>
   6     Memory contains three general areas. First, function and operator
   7     calls via <code class="function">new</code> and <code class="function">delete</code>
   8     operator or member function calls.  Second, allocation via
   9     <code class="classname">allocator</code>. And finally, smart pointer and
  10     intelligent pointer abstractions.
  11   </p><div class="section"><div class="titlepage"><div><div><h3 class="title"><a id="std.util.memory.allocator"></a>Allocators</h3></div></div></div><p>
  12  Memory management for Standard Library entities is encapsulated in a
  13  class template called <code class="classname">allocator</code>. The
  14  <code class="classname">allocator</code> abstraction is used throughout the
  15  library in <code class="classname">string</code>, container classes,
  16  algorithms, and parts of iostreams. This class, and base classes of
  17  it, are the superset of available free store (<span class="quote">“<span class="quote">heap</span>”</span>)
  18  management classes.
  19 </p><div class="section"><div class="titlepage"><div><div><h4 class="title"><a id="allocator.req"></a>Requirements</h4></div></div></div><p>
  20     The C++ standard only gives a few directives in this area:
  21   </p><div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem"><p>
  22        When you add elements to a container, and the container must
  23        allocate more memory to hold them, the container makes the
  24        request via its <span class="type">Allocator</span> template
  25        parameter, which is usually aliased to
  26        <span class="type">allocator_type</span>.  This includes adding chars
  27        to the string class, which acts as a regular STL container in
  28        this respect.
  29       </p></li><li class="listitem"><p>
  30        The default <span class="type">Allocator</span> argument of every
  31        container-of-T is <code class="classname">allocator&lt;T&gt;</code>.
  32        </p></li><li class="listitem"><p>
  33        The interface of the <code class="classname">allocator&lt;T&gt;</code> class is
  34          extremely simple.  It has about 20 public declarations (nested
  35          typedefs, member functions, etc), but the two which concern us most
  36          are:
  37        </p><pre class="programlisting">
  38          T*    allocate   (size_type n, const void* hint = 0);
  39          void  deallocate (T* p, size_type n);
  40        </pre><p>
  41          The <code class="varname">n</code> arguments in both those
  42          functions is a <span class="emphasis"><em>count</em></span> of the number of
  43          <span class="type">T</span>'s to allocate space for, <span class="emphasis"><em>not their
  44          total size</em></span>.
  45          (This is a simplification; the real signatures use nested typedefs.)
  46        </p></li><li class="listitem"><p>
  47          The storage is obtained by calling <code class="function">::operator
  48          new</code>, but it is unspecified when or how
  49          often this function is called.  The use of the
  50          <code class="varname">hint</code> is unspecified, but intended as an
  51          aid to locality if an implementation so
  52          desires. <code class="constant">[20.4.1.1]/6</code>
  53        </p></li></ul></div><p>
  54      Complete details can be found in the C++ standard, look in
  55      <code class="constant">[20.4 Memory]</code>.
  56    </p></div><div class="section"><div class="titlepage"><div><div><h4 class="title"><a id="allocator.design_issues"></a>Design Issues</h4></div></div></div><p>
  57     The easiest way of fulfilling the requirements is to call
  58     <code class="function">operator new</code> each time a container needs
  59     memory, and to call <code class="function">operator delete</code> each time
  60     the container releases memory. This method may be <a class="link" href="http://gcc.gnu.org/ml/libstdc++/2001-05/msg00105.html" target="_top">slower</a>
  61     than caching the allocations and re-using previously-allocated
  62     memory, but has the advantage of working correctly across a wide
  63     variety of hardware and operating systems, including large
  64     clusters. The <code class="classname">__gnu_cxx::new_allocator</code>
  65     implements the simple operator new and operator delete semantics,
  66     while <code class="classname">__gnu_cxx::malloc_allocator</code>
  67     implements much the same thing, only with the C language functions
  68     <code class="function">std::malloc</code> and <code class="function">std::free</code>.
  69   </p><p>
  70     Another approach is to use intelligence within the allocator
  71     class to cache allocations. This extra machinery can take a variety
  72     of forms: a bitmap index, an index into an exponentially increasing
  73     power-of-two-sized buckets, or simpler fixed-size pooling cache.
  74     The cache is shared among all the containers in the program: when
  75     your program's <code class="classname">std::vector&lt;int&gt;</code> gets
  76   cut in half and frees a bunch of its storage, that memory can be
  77   reused by the private
  78   <code class="classname">std::list&lt;WonkyWidget&gt;</code> brought in from
  79   a KDE library that you linked against.  And operators
  80   <code class="function">new</code> and <code class="function">delete</code> are not
  81   always called to pass the memory on, either, which is a speed
  82   bonus. Examples of allocators that use these techniques are
  83   <code class="classname">__gnu_cxx::bitmap_allocator</code>,
  84   <code class="classname">__gnu_cxx::pool_allocator</code>, and
  85   <code class="classname">__gnu_cxx::__mt_alloc</code>.
  86   </p><p>
  87     Depending on the implementation techniques used, the underlying
  88     operating system, and compilation environment, scaling caching
  89     allocators can be tricky. In particular, order-of-destruction and
  90     order-of-creation for memory pools may be difficult to pin down
  91     with certainty, which may create problems when used with plugins
  92     or loading and unloading shared objects in memory. As such, using
  93     caching allocators on systems that do not support
  94     <code class="function">abi::__cxa_atexit</code> is not recommended.
  95   </p></div><div class="section"><div class="titlepage"><div><div><h4 class="title"><a id="allocator.impl"></a>Implementation</h4></div></div></div><div class="section"><div class="titlepage"><div><div><h5 class="title"><a id="idm270864947328"></a>Interface Design</h5></div></div></div><p>
  96      The only allocator interface that
  97      is supported is the standard C++ interface. As such, all STL
  98      containers have been adjusted, and all external allocators have
  99      been modified to support this change.
 100    </p><p>
 101      The class <code class="classname">allocator</code> just has typedef,
 102    constructor, and rebind members. It inherits from one of the
 103    high-speed extension allocators, covered below. Thus, all
 104    allocation and deallocation depends on the base class.
 105    </p><p>
 106      The base class that <code class="classname">allocator</code> is derived from
 107      may not be user-configurable.
 108 </p></div><div class="section"><div class="titlepage"><div><div><h5 class="title"><a id="idm270864943776"></a>Selecting Default Allocation Policy</h5></div></div></div><p>
 109      It's difficult to pick an allocation strategy that will provide
 110    maximum utility, without excessively penalizing some behavior. In
 111    fact, it's difficult just deciding which typical actions to measure
 112    for speed.
 113    </p><p>
 114      Three synthetic benchmarks have been created that provide data
 115      that is used to compare different C++ allocators. These tests are:
 116    </p><div class="orderedlist"><ol class="orderedlist" type="1"><li class="listitem"><p>
 117        Insertion.
 118        </p><p>
 119        Over multiple iterations, various STL container
 120      objects have elements inserted to some maximum amount. A variety
 121      of allocators are tested.
 122      Test source for <a class="link" href="http://gcc.gnu.org/viewcvs/trunk/libstdc%2B%2B-v3/testsuite/performance/23_containers/insert/sequence.cc?view=markup" target="_top">sequence</a>
 123      and <a class="link" href="http://gcc.gnu.org/viewcvs/trunk/libstdc%2B%2B-v3/testsuite/performance/23_containers/insert/associative.cc?view=markup" target="_top">associative</a>
 124      containers.
 125        </p></li><li class="listitem"><p>
 126        Insertion and erasure in a multi-threaded environment.
 127        </p><p>
 128        This test shows the ability of the allocator to reclaim memory
 129      on a per-thread basis, as well as measuring thread contention
 130      for memory resources.
 131      Test source
 132     <a class="link" 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>.
 133        </p></li><li class="listitem"><p>
 134          A threaded producer/consumer model.
 135        </p><p>
 136        Test source for
 137      <a class="link" href="http://gcc.gnu.org/viewcvs/trunk/libstdc++-v3/testsuite/performance/23_containers/producer_consumer/sequence.cc?view=markup" target="_top">sequence</a>
 138      and
 139      <a class="link" href="http://gcc.gnu.org/viewcvs/trunk/libstdc++-v3/testsuite/performance/23_containers/producer_consumer/associative.cc?view=markup" target="_top">associative</a>
 140      containers.
 141      </p></li></ol></div><p>
 142      The current default choice for
 143      <code class="classname">allocator</code> is
 144      <code class="classname">__gnu_cxx::new_allocator</code>.
 145    </p></div><div class="section"><div class="titlepage"><div><div><h5 class="title"><a id="idm270864930480"></a>Disabling Memory Caching</h5></div></div></div><p>
 146       In use, <code class="classname">allocator</code> may allocate and
 147       deallocate using implementation-specific strategies and
 148       heuristics. Because of this, a given call to an allocator object's
 149       <code class="function">allocate</code> member function may not actually
 150       call the global <code class="code">operator new</code> and a given call to
 151       to the <code class="function">deallocate</code> member function may not
 152       call <code class="code">operator delete</code>.
 153     </p><p>
 154      This can be confusing.
 155    </p><p>
 156      In particular, this can make debugging memory errors more
 157      difficult, especially when using third-party tools like valgrind or
 158      debug versions of <code class="function">new</code>.
 159    </p><p>
 160      There are various ways to solve this problem. One would be to use
 161      a custom allocator that just called operators
 162      <code class="function">new</code> and <code class="function">delete</code>
 163      directly, for every allocation. (See the default allocator,
 164      <code class="filename">include/ext/new_allocator.h</code>, for instance.)
 165      However, that option may involve changing source code to use
 166      a non-default allocator. Another option is to force the
 167      default allocator to remove caching and pools, and to directly
 168      allocate with every call of <code class="function">allocate</code> and
 169      directly deallocate with every call of
 170      <code class="function">deallocate</code>, regardless of efficiency. As it
 171      turns out, this last option is also available.
 172    </p><p>
 173      To globally disable memory caching within the library for some of
 174      the optional non-default allocators, merely set
 175      <code class="constant">GLIBCXX_FORCE_NEW</code> (with any value) in the
 176      system's environment before running the program. If your program
 177      crashes with <code class="constant">GLIBCXX_FORCE_NEW</code> in the
 178      environment, it likely means that you linked against objects
 179      built against the older library (objects which might still using the
 180      cached allocations...).
 181   </p></div></div><div class="section"><div class="titlepage"><div><div><h4 class="title"><a id="allocator.using"></a>Using a Specific Allocator</h4></div></div></div><p>
 182      You can specify different memory management schemes on a
 183      per-container basis, by overriding the default
 184      <span class="type">Allocator</span> template parameter.  For example, an easy
 185       (but non-portable) method of specifying that only <code class="function">malloc</code> or <code class="function">free</code>
 186       should be used instead of the default node allocator is:
 187    </p><pre class="programlisting">
 188     std::list &lt;int, __gnu_cxx::malloc_allocator&lt;int&gt; &gt;  malloc_list;</pre><p>
 189       Likewise, a debugging form of whichever allocator is currently in use:
 190     </p><pre class="programlisting">
 191     std::deque &lt;int, __gnu_cxx::debug_allocator&lt;std::allocator&lt;int&gt; &gt; &gt;  debug_deque;
 192       </pre></div><div class="section"><div class="titlepage"><div><div><h4 class="title"><a id="allocator.custom"></a>Custom Allocators</h4></div></div></div><p>
 193     Writing a portable C++ allocator would dictate that the interface
 194     would look much like the one specified for
 195     <code class="classname">allocator</code>. Additional member functions, but
 196     not subtractions, would be permissible.
 197   </p><p>
 198      Probably the best place to start would be to copy one of the
 199    extension allocators: say a simple one like
 200    <code class="classname">new_allocator</code>.
 201    </p></div><div class="section"><div class="titlepage"><div><div><h4 class="title"><a id="allocator.ext"></a>Extension Allocators</h4></div></div></div><p>
 202     Several other allocators are provided as part of this
 203     implementation.  The location of the extension allocators and their
 204     names have changed, but in all cases, functionality is
 205     equivalent. Starting with gcc-3.4, all extension allocators are
 206     standard style. Before this point, SGI style was the norm. Because of
 207     this, the number of template arguments also changed. Here's a simple
 208     chart to track the changes.
 209   </p><p>
 210     More details on each of these extension allocators follows.
 211   </p><div class="orderedlist"><ol class="orderedlist" type="1"><li class="listitem"><p>
 212        <code class="classname">new_allocator</code>
 213        </p><p>
 214          Simply wraps <code class="function">::operator new</code>
 215          and <code class="function">::operator delete</code>.
 216        </p></li><li class="listitem"><p>
 217        <code class="classname">malloc_allocator</code>
 218        </p><p>
 219          Simply wraps <code class="function">malloc</code> and
 220          <code class="function">free</code>. There is also a hook for an
 221          out-of-memory handler (for
 222          <code class="function">new</code>/<code class="function">delete</code> this is
 223          taken care of elsewhere).
 224        </p></li><li class="listitem"><p>
 225        <code class="classname">array_allocator</code>
 226        </p><p>
 227          Allows allocations of known and fixed sizes using existing
 228          global or external storage allocated via construction of
 229          <code class="classname">std::tr1::array</code> objects. By using this
 230          allocator, fixed size containers (including
 231          <code class="classname">std::string</code>) can be used without
 232          instances calling <code class="function">::operator new</code> and
 233          <code class="function">::operator delete</code>. This capability
 234          allows the use of STL abstractions without runtime
 235          complications or overhead, even in situations such as program
 236          startup. For usage examples, please consult the testsuite.
 237        </p></li><li class="listitem"><p>
 238        <code class="classname">debug_allocator</code>
 239        </p><p>
 240          A wrapper around an arbitrary allocator A.  It passes on
 241          slightly increased size requests to A, and uses the extra
 242          memory to store size information.  When a pointer is passed
 243          to <code class="function">deallocate()</code>, the stored size is
 244          checked, and <code class="function">assert()</code> is used to
 245          guarantee they match.
 246        </p></li><li class="listitem"><p>
 247         <code class="classname">throw_allocator</code>
 248         </p><p>
 249           Includes memory tracking and marking abilities as well as hooks for
 250           throwing exceptions at configurable intervals (including random,
 251           all, none).
 252         </p></li><li class="listitem"><p>
 253        <code class="classname">__pool_alloc</code>
 254        </p><p>
 255          A high-performance, single pool allocator.  The reusable
 256          memory is shared among identical instantiations of this type.
 257          It calls through <code class="function">::operator new</code> to
 258          obtain new memory when its lists run out.  If a client
 259          container requests a block larger than a certain threshold
 260          size, then the pool is bypassed, and the allocate/deallocate
 261          request is passed to <code class="function">::operator new</code>
 262          directly.
 263        </p><p>
 264          Older versions of this class take a boolean template
 265          parameter, called <code class="varname">thr</code>, and an integer template
 266          parameter, called <code class="varname">inst</code>.
 267        </p><p>
 268          The <code class="varname">inst</code> number is used to track additional memory
 269       pools.  The point of the number is to allow multiple
 270       instantiations of the classes without changing the semantics at
 271       all.  All three of
 272        </p><pre class="programlisting">
 273     typedef  __pool_alloc&lt;true,0&gt;    normal;
 274     typedef  __pool_alloc&lt;true,1&gt;    private;
 275     typedef  __pool_alloc&lt;true,42&gt;   also_private;
 276    </pre><p>
 277      behave exactly the same way.  However, the memory pool for each type
 278       (and remember that different instantiations result in different types)
 279       remains separate.
 280    </p><p>
 281      The library uses <span class="emphasis"><em>0</em></span> in all its instantiations.  If you
 282       wish to keep separate free lists for a particular purpose, use a
 283       different number.
 284    </p><p>The <code class="varname">thr</code> boolean determines whether the
 285    pool should be manipulated atomically or not.  When
 286    <code class="varname">thr</code> = <code class="constant">true</code>, the allocator
 287    is thread-safe, while <code class="varname">thr</code> =
 288    <code class="constant">false</code>, is slightly faster but unsafe for
 289    multiple threads.
 290    </p><p>
 291      For thread-enabled configurations, the pool is locked with a
 292      single big lock. In some situations, this implementation detail
 293      may result in severe performance degradation.
 294    </p><p>
 295      (Note that the GCC thread abstraction layer allows us to provide
 296      safe zero-overhead stubs for the threading routines, if threads
 297      were disabled at configuration time.)
 298    </p></li><li class="listitem"><p>
 299        <code class="classname">__mt_alloc</code>
 300        </p><p>
 301          A high-performance fixed-size allocator with
 302          exponentially-increasing allocations. It has its own
 303          <a class="link" href="mt_allocator.html" title="Chapter 20. The mt_allocator">chapter</a>
 304          in the documentation.
 305        </p></li><li class="listitem"><p>
 306        <code class="classname">bitmap_allocator</code>
 307        </p><p>
 308          A high-performance allocator that uses a bit-map to keep track
 309          of the used and unused memory locations. It has its own
 310          <a class="link" href="bitmap_allocator.html" title="Chapter 21. The bitmap_allocator">chapter</a>
 311          in the documentation.
 312        </p></li></ol></div></div><div class="bibliography"><div class="titlepage"><div><div><h4 class="title"><a id="allocator.biblio"></a>Bibliography</h4></div></div></div><div class="biblioentry"><a id="idm270864875520"></a><p><span class="citetitle"><em class="citetitle">
 313     ISO/IEC 14882:1998 Programming languages - C++
 314     </em>. </span>
 315       isoc++_1998
 316     <span class="pagenums">20.4 Memory. </span></p></div><div class="biblioentry"><a id="idm270864873680"></a><p><span class="title"><em>
 317         <a class="link" href="http://www.drdobbs.com/the-standard-librarian-what-are-allocato/184403759" target="_top">
 318       The Standard Librarian: What Are Allocators Good For?
 319         </a>
 320       </em>. </span><span class="author"><span class="firstname">Matt</span> <span class="surname">Austern</span>. </span><span class="publisher"><span class="publishername">
 321         C/C++ Users Journal
 322       . </span></span></p></div><div class="biblioentry"><a id="idm270864869904"></a><p><span class="title"><em>
 323         <a class="link" href="http://www.hoard.org/" target="_top">
 324       The Hoard Memory Allocator
 325         </a>
 326       </em>. </span><span class="author"><span class="firstname">Emery</span> <span class="surname">Berger</span>. </span></p></div><div class="biblioentry"><a id="idm270864867136"></a><p><span class="title"><em>
 327         <a class="link" href="http://people.cs.umass.edu/~emery/pubs/berger-oopsla2002.pdf" target="_top">
 328       Reconsidering Custom Memory Allocation
 329         </a>
 330       </em>. </span><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></p></div><div class="biblioentry"><a id="idm270864860976"></a><p><span class="title"><em>
 331         <a class="link" href="http://www.angelikalanger.com/Articles/C++Report/Allocators/Allocators.html" target="_top">
 332       Allocator Types
 333         </a>
 334       </em>. </span><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">
 335         C/C++ Users Journal
 336       . </span></span></p></div><div class="biblioentry"><a id="idm270864856240"></a><p><span class="citetitle"><em class="citetitle">The C++ Programming Language</em>. </span><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">
 337         Addison Wesley
 338       . </span></span></p></div><div class="biblioentry"><a id="idm270864851808"></a><p><span class="citetitle"><em class="citetitle">Yalloc: A Recycling C++ Allocator</em>. </span><span class="author"><span class="firstname">Felix</span> <span class="surname">Yen</span>. </span></p></div></div></div><div class="section"><div class="titlepage"><div><div><h3 class="title"><a id="std.util.memory.auto_ptr"></a>auto_ptr</h3></div></div></div><div class="section"><div class="titlepage"><div><div><h4 class="title"><a id="auto_ptr.limitations"></a>Limitations</h4></div></div></div><p>Explaining all of the fun and delicious things that can
 339    happen with misuse of the <code class="classname">auto_ptr</code> class
 340    template (called <acronym class="acronym">AP</acronym> here) would take some
 341    time. Suffice it to say that the use of <acronym class="acronym">AP</acronym>
 342    safely in the presence of copying has some subtleties.
 343    </p><p>
 344      The AP class is a really
 345       nifty idea for a smart pointer, but it is one of the dumbest of
 346       all the smart pointers -- and that's fine.
 347    </p><p>
 348      AP is not meant to be a supersmart solution to all resource
 349       leaks everywhere.  Neither is it meant to be an effective form
 350       of garbage collection (although it can help, a little bit).
 351       And it can <span class="emphasis"><em>not</em></span>be used for arrays!
 352    </p><p>
 353      <acronym class="acronym">AP</acronym> is meant to prevent nasty leaks in the
 354      presence of exceptions.  That's <span class="emphasis"><em>all</em></span>.  This
 355      code is AP-friendly:
 356    </p><pre class="programlisting">
 357     // Not a recommend naming scheme, but good for web-based FAQs.
 358     typedef std::auto_ptr&lt;MyClass&gt;  APMC;
 359
 360     extern function_taking_MyClass_pointer (MyClass*);
 361     extern some_throwable_function ();
 362
 363     void func (int data)
 364     {
 365         APMC  ap (new MyClass(data));
 366
 367         some_throwable_function();   // this will throw an exception
 368
 369         function_taking_MyClass_pointer (ap.get());
 370     }
 371    </pre><p>When an exception gets thrown, the instance of MyClass that's
 372       been created on the heap will be <code class="function">delete</code>'d as the stack is
 373       unwound past <code class="function">func()</code>.
 374    </p><p>Changing that code as follows is not <acronym class="acronym">AP</acronym>-friendly:
 375    </p><pre class="programlisting">
 376         APMC  ap (new MyClass[22]);
 377    </pre><p>You will get the same problems as you would without the use
 378       of <acronym class="acronym">AP</acronym>:
 379    </p><pre class="programlisting">
 380         char*  array = new char[10];       // array new...
 381         ...
 382         delete array;                      // ...but single-object delete
 383    </pre><p>
 384      AP cannot tell whether the pointer you've passed at creation points
 385       to one or many things.  If it points to many things, you are about
 386       to die.  AP is trivial to write, however, so you could write your
 387       own <code class="code">auto_array_ptr</code> for that situation (in fact, this has
 388       been done many times; check the mailing lists, Usenet, Boost, etc).
 389    </p></div><div class="section"><div class="titlepage"><div><div><h4 class="title"><a id="auto_ptr.using"></a>Use in Containers</h4></div></div></div><p>
 390   </p><p>All of the <a class="link" href="containers.html" title="Chapter 9.  Containers">containers</a>
 391       described in the standard library require their contained types
 392       to have, among other things, a copy constructor like this:
 393   </p><pre class="programlisting">
 394     struct My_Type
 395     {
 396         My_Type (My_Type const&amp;);
 397     };
 398    </pre><p>
 399      Note the const keyword; the object being copied shouldn't change.
 400      The template class <code class="code">auto_ptr</code> (called AP here) does not
 401      meet this requirement.  Creating a new AP by copying an existing
 402      one transfers ownership of the pointed-to object, which means that
 403      the AP being copied must change, which in turn means that the
 404      copy ctors of AP do not take const objects.
 405    </p><p>
 406      The resulting rule is simple: <span class="emphasis"><em>Never ever use a
 407      container of auto_ptr objects</em></span>. The standard says that
 408      <span class="quote">“<span class="quote">undefined</span>”</span> behavior is the result, but it is
 409      guaranteed to be messy.
 410    </p><p>
 411      To prevent you from doing this to yourself, the
 412       <a class="link" href="ext_compile_checks.html" title="Chapter 16. Compile Time Checks">concept checks</a> built
 413       in to this implementation will issue an error if you try to
 414       compile code like this:
 415    </p><pre class="programlisting">
 416     #include &lt;vector&gt;
 417     #include &lt;memory&gt;
 418
 419     void f()
 420     {
 421         std::vector&lt; std::auto_ptr&lt;int&gt; &gt;   vec_ap_int;
 422     }
 423    </pre><p>
 424 Should you try this with the checks enabled, you will see an error.
 425    </p></div></div><div class="section"><div class="titlepage"><div><div><h3 class="title"><a id="std.util.memory.shared_ptr"></a>shared_ptr</h3></div></div></div><p>
 426 The shared_ptr class template stores a pointer, usually obtained via new,
 427 and implements shared ownership semantics.
 428 </p><div class="section"><div class="titlepage"><div><div><h4 class="title"><a id="shared_ptr.req"></a>Requirements</h4></div></div></div><p>
 429   </p><p>
 430     The standard deliberately doesn't require a reference-counted
 431     implementation, allowing other techniques such as a
 432     circular-linked-list.
 433   </p><p>
 434   </p></div><div class="section"><div class="titlepage"><div><div><h4 class="title"><a id="shared_ptr.design_issues"></a>Design Issues</h4></div></div></div><p>
 435 The <code class="classname">shared_ptr</code> code is kindly donated to GCC by the Boost
 436 project and the original authors of the code. The basic design and
 437 algorithms are from Boost, the notes below describe details specific to
 438 the GCC implementation. Names have been uglified in this implementation,
 439 but the design should be recognisable to anyone familiar with the Boost
 440 1.32 shared_ptr.
 441   </p><p>
 442 The basic design is an abstract base class, <code class="code">_Sp_counted_base</code> that
 443 does the reference-counting and calls virtual functions when the count
 444 drops to zero.
 445 Derived classes override those functions to destroy resources in a context
 446 where the correct dynamic type is known. This is an application of the
 447 technique known as type erasure.
 448   </p></div><div class="section"><div class="titlepage"><div><div><h4 class="title"><a id="shared_ptr.impl"></a>Implementation</h4></div></div></div><div class="section"><div class="titlepage"><div><div><h5 class="title"><a id="idm270864810704"></a>Class Hierarchy</h5></div></div></div><p>
 449 A <code class="classname">shared_ptr&lt;T&gt;</code> contains a pointer of
 450 type <span class="type">T*</span> and an object of type
 451 <code class="classname">__shared_count</code>. The shared_count contains a
 452 pointer of type <span class="type">_Sp_counted_base*</span> which points to the
 453 object that maintains the reference-counts and destroys the managed
 454 resource.
 455     </p><div class="variablelist"><dl class="variablelist"><dt><span class="term"><code class="classname">_Sp_counted_base&lt;Lp&gt;</code></span></dt><dd><p>
 456 The base of the hierarchy is parameterized on the lock policy (see below.)
 457 _Sp_counted_base doesn't depend on the type of pointer being managed,
 458 it only maintains the reference counts and calls virtual functions when
 459 the counts drop to zero. The managed object is destroyed when the last
 460 strong reference is dropped, but the _Sp_counted_base itself must exist
 461 until the last weak reference is dropped.
 462     </p></dd><dt><span class="term"><code class="classname">_Sp_counted_base_impl&lt;Ptr, Deleter, Lp&gt;</code></span></dt><dd><p>
 463 Inherits from _Sp_counted_base and stores a pointer of type <code class="code">Ptr</code>
 464 and a deleter of type <code class="code">Deleter</code>.  <code class="classname">_Sp_deleter</code> is
 465 used when the user doesn't supply a custom deleter. Unlike Boost's, this
 466 default deleter is not "checked" because GCC already issues a warning if
 467 <code class="function">delete</code> is used with an incomplete type.
 468 This is the only derived type used by <code class="classname">tr1::shared_ptr&lt;Ptr&gt;</code>
 469 and it is never used by <code class="classname">std::shared_ptr</code>, which uses one of
 470 the following types, depending on how the shared_ptr is constructed.
 471     </p></dd><dt><span class="term"><code class="classname">_Sp_counted_ptr&lt;Ptr, Lp&gt;</code></span></dt><dd><p>
 472 Inherits from _Sp_counted_base and stores a pointer of type <span class="type">Ptr</span>,
 473 which is passed to <code class="function">delete</code> when the last reference is dropped.
 474 This is the simplest form and is used when there is no custom deleter or
 475 allocator.
 476     </p></dd><dt><span class="term"><code class="classname">_Sp_counted_deleter&lt;Ptr, Deleter, Alloc&gt;</code></span></dt><dd><p>
 477 Inherits from _Sp_counted_ptr and adds support for custom deleter and
 478 allocator. Empty Base Optimization is used for the allocator. This class
 479 is used even when the user only provides a custom deleter, in which case
 480 <code class="classname">allocator</code> is used as the allocator.
 481     </p></dd><dt><span class="term"><code class="classname">_Sp_counted_ptr_inplace&lt;Tp, Alloc, Lp&gt;</code></span></dt><dd><p>
 482 Used by <code class="code">allocate_shared</code> and <code class="code">make_shared</code>.
 483 Contains aligned storage to hold an object of type <span class="type">Tp</span>,
 484 which is constructed in-place with placement <code class="function">new</code>.
 485 Has a variadic template constructor allowing any number of arguments to
 486 be forwarded to <span class="type">Tp</span>'s constructor.
 487 Unlike the other <code class="classname">_Sp_counted_*</code> classes, this one is parameterized on the
 488 type of object, not the type of pointer; this is purely a convenience
 489 that simplifies the implementation slightly.
 490     </p></dd></dl></div><p>
 491 C++11-only features are: rvalue-ref/move support, allocator support,
 492 aliasing constructor, make_shared &amp; allocate_shared. Additionally,
 493 the constructors taking <code class="classname">auto_ptr</code> parameters are
 494 deprecated in C++11 mode.
 495     </p></div><div class="section"><div class="titlepage"><div><div><h5 class="title"><a id="idm270864788176"></a>Thread Safety</h5></div></div></div><p>
 496 The
 497 <a class="link" href="http://www.boost.org/libs/smart_ptr/shared_ptr.htm#ThreadSafety" target="_top">Thread
 498 Safety</a> section of the Boost shared_ptr documentation says "shared_ptr
 499 objects offer the same level of thread safety as built-in types."
 500 The implementation must ensure that concurrent updates to separate shared_ptr
 501 instances are correct even when those instances share a reference count e.g.
 502 </p><pre class="programlisting">
 503 shared_ptr&lt;A&gt; a(new A);
 504 shared_ptr&lt;A&gt; b(a);
 505
 506 // Thread 1     // Thread 2
 507    a.reset();      b.reset();
 508 </pre><p>
 509 The dynamically-allocated object must be destroyed by exactly one of the
 510 threads. Weak references make things even more interesting.
 511 The shared state used to implement shared_ptr must be transparent to the
 512 user and invariants must be preserved at all times.
 513 The key pieces of shared state are the strong and weak reference counts.
 514 Updates to these need to be atomic and visible to all threads to ensure
 515 correct cleanup of the managed resource (which is, after all, shared_ptr's
 516 job!)
 517 On multi-processor systems memory synchronisation may be needed so that
 518 reference-count updates and the destruction of the managed resource are
 519 race-free.
 520 </p><p>
 521 The function <code class="function">_Sp_counted_base::_M_add_ref_lock()</code>, called when
 522 obtaining a shared_ptr from a weak_ptr, has to test if the managed
 523 resource still exists and either increment the reference count or throw
 524 <code class="classname">bad_weak_ptr</code>.
 525 In a multi-threaded program there is a potential race condition if the last
 526 reference is dropped (and the managed resource destroyed) between testing
 527 the reference count and incrementing it, which could result in a shared_ptr
 528 pointing to invalid memory.
 529 </p><p>
 530 The Boost shared_ptr (as used in GCC) features a clever lock-free
 531 algorithm to avoid the race condition, but this relies on the
 532 processor supporting an atomic <span class="emphasis"><em>Compare-And-Swap</em></span>
 533 instruction. For other platforms there are fall-backs using mutex
 534 locks.  Boost (as of version 1.35) includes several different
 535 implementations and the preprocessor selects one based on the
 536 compiler, standard library, platform etc. For the version of
 537 shared_ptr in libstdc++ the compiler and library are fixed, which
 538 makes things much simpler: we have an atomic CAS or we don't, see Lock
 539 Policy below for details.
 540 </p></div><div class="section"><div class="titlepage"><div><div><h5 class="title"><a id="idm270864780880"></a>Selecting Lock Policy</h5></div></div></div><p>
 541     </p><p>
 542 There is a single <code class="classname">_Sp_counted_base</code> class,
 543 which is a template parameterized on the enum
 544 <span class="type">__gnu_cxx::_Lock_policy</span>.  The entire family of classes is
 545 parameterized on the lock policy, right up to
 546 <code class="classname">__shared_ptr</code>, <code class="classname">__weak_ptr</code> and
 547 <code class="classname">__enable_shared_from_this</code>. The actual
 548 <code class="classname">std::shared_ptr</code> class inherits from
 549 <code class="classname">__shared_ptr</code> with the lock policy parameter
 550 selected automatically based on the thread model and platform that
 551 libstdc++ is configured for, so that the best available template
 552 specialization will be used. This design is necessary because it would
 553 not be conforming for <code class="classname">shared_ptr</code> to have an
 554 extra template parameter, even if it had a default value.  The
 555 available policies are:
 556     </p><div class="orderedlist"><ol class="orderedlist" type="1"><li class="listitem"><p>
 557        <code class="constant">_S_Atomic</code>
 558        </p><p>
 559 Selected when GCC supports a builtin atomic compare-and-swap operation
 560 on the target processor (see <a class="link" href="http://gcc.gnu.org/onlinedocs/gcc/_005f_005fatomic-Builtins.html" target="_top">Atomic
 561 Builtins</a>.)  The reference counts are maintained using a lock-free
 562 algorithm and GCC's atomic builtins, which provide the required memory
 563 synchronisation.
 564        </p></li><li class="listitem"><p>
 565        <code class="constant">_S_Mutex</code>
 566        </p><p>
 567 The _Sp_counted_base specialization for this policy contains a mutex,
 568 which is locked in add_ref_lock(). This policy is used when GCC's atomic
 569 builtins aren't available so explicit memory barriers are needed in places.
 570        </p></li><li class="listitem"><p>
 571        <code class="constant">_S_Single</code>
 572        </p><p>
 573 This policy uses a non-reentrant add_ref_lock() with no locking. It is
 574 used when libstdc++ is built without <code class="literal">--enable-threads</code>.
 575        </p></li></ol></div><p>
 576        For all three policies, reference count increments and
 577        decrements are done via the functions in
 578        <code class="filename">ext/atomicity.h</code>, which detect if the program
 579        is multi-threaded.  If only one thread of execution exists in
 580        the program then less expensive non-atomic operations are used.
 581      </p></div><div class="section"><div class="titlepage"><div><div><h5 class="title"><a id="idm270864766240"></a>Related functions and classes</h5></div></div></div><div class="variablelist"><dl class="variablelist"><dt><span class="term"><code class="code">dynamic_pointer_cast</code>, <code class="code">static_pointer_cast</code>,
 582 <code class="code">const_pointer_cast</code></span></dt><dd><p>
 583 As noted in N2351, these functions can be implemented non-intrusively using
 584 the alias constructor.  However the aliasing constructor is only available
 585 in C++11 mode, so in TR1 mode these casts rely on three non-standard
 586 constructors in shared_ptr and __shared_ptr.
 587 In C++11 mode these constructors and the related tag types are not needed.
 588     </p></dd><dt><span class="term"><code class="code">enable_shared_from_this</code></span></dt><dd><p>
 589 The clever overload to detect a base class of type
 590 <code class="code">enable_shared_from_this</code> comes straight from Boost.
 591 There is an extra overload for <code class="code">__enable_shared_from_this</code> to
 592 work smoothly with <code class="code">__shared_ptr&lt;Tp, Lp&gt;</code> using any lock
 593 policy.
 594     </p></dd><dt><span class="term"><code class="code">make_shared</code>, <code class="code">allocate_shared</code></span></dt><dd><p>
 595 <code class="code">make_shared</code> simply forwards to <code class="code">allocate_shared</code>
 596 with <code class="code">std::allocator</code> as the allocator.
 597 Although these functions can be implemented non-intrusively using the
 598 alias constructor, if they have access to the implementation then it is
 599 possible to save storage and reduce the number of heap allocations. The
 600 newly constructed object and the _Sp_counted_* can be allocated in a single
 601 block and the standard says implementations are "encouraged, but not required,"
 602 to do so. This implementation provides additional non-standard constructors
 603 (selected with the type <code class="code">_Sp_make_shared_tag</code>) which create an
 604 object of type <code class="code">_Sp_counted_ptr_inplace</code> to hold the new object.
 605 The returned <code class="code">shared_ptr&lt;A&gt;</code> needs to know the address of the
 606 new <code class="code">A</code> object embedded in the <code class="code">_Sp_counted_ptr_inplace</code>,
 607 but it has no way to access it.
 608 This implementation uses a "covert channel" to return the address of the
 609 embedded object when <code class="code">get_deleter&lt;_Sp_make_shared_tag&gt;()</code>
 610 is called.  Users should not try to use this.
 611 As well as the extra constructors, this implementation also needs some
 612 members of _Sp_counted_deleter to be protected where they could otherwise
 613 be private.
 614     </p></dd></dl></div></div></div><div class="section"><div class="titlepage"><div><div><h4 class="title"><a id="shared_ptr.using"></a>Use</h4></div></div></div><div class="section"><div class="titlepage"><div><div><h5 class="title"><a id="idm270864749888"></a>Examples</h5></div></div></div><p>
 615       Examples of use can be found in the testsuite, under
 616       <code class="filename">testsuite/tr1/2_general_utilities/shared_ptr</code>,
 617       <code class="filename">testsuite/20_util/shared_ptr</code>
 618       and
 619       <code class="filename">testsuite/20_util/weak_ptr</code>.
 620     </p></div><div class="section"><div class="titlepage"><div><div><h5 class="title"><a id="idm270864746304"></a>Unresolved Issues</h5></div></div></div><p>
 621       The <span class="emphasis"><em><code class="classname">shared_ptr</code> atomic access</em></span>
 622       clause in the C++11 standard is not implemented in GCC.
 623     </p><p>
 624       The <span class="type">_S_single</span> policy uses atomics when used in MT
 625       code, because it uses the same dispatcher functions that check
 626       <code class="function">__gthread_active_p()</code>. This could be
 627       addressed by providing template specialisations for some members
 628       of <code class="classname">_Sp_counted_base&lt;_S_single&gt;</code>.
 629     </p><p>
 630       Unlike Boost, this implementation does not use separate classes
 631       for the pointer+deleter and pointer+deleter+allocator cases in
 632       C++11 mode, combining both into _Sp_counted_deleter and using
 633       <code class="classname">allocator</code> when the user doesn't specify
 634       an allocator.  If it was found to be beneficial an additional
 635       class could easily be added.  With the current implementation,
 636       the _Sp_counted_deleter and __shared_count constructors taking a
 637       custom deleter but no allocator are technically redundant and
 638       could be removed, changing callers to always specify an
 639       allocator. If a separate pointer+deleter class was added the
 640       __shared_count constructor would be needed, so it has been kept
 641       for now.
 642     </p><p>
 643       The hack used to get the address of the managed object from
 644       <code class="function">_Sp_counted_ptr_inplace::_M_get_deleter()</code>
 645       is accessible to users. This could be prevented if
 646       <code class="function">get_deleter&lt;_Sp_make_shared_tag&gt;()</code>
 647       always returned NULL, since the hack only needs to work at a
 648       lower level, not in the public API. This wouldn't be difficult,
 649       but hasn't been done since there is no danger of accidental
 650       misuse: users already know they are relying on unsupported
 651       features if they refer to implementation details such as
 652       _Sp_make_shared_tag.
 653     </p><p>
 654       tr1::_Sp_deleter could be a private member of tr1::__shared_count but it
 655       would alter the ABI.
 656     </p></div></div><div class="section"><div class="titlepage"><div><div><h4 class="title"><a id="shared_ptr.ack"></a>Acknowledgments</h4></div></div></div><p>
 657     The original authors of the Boost shared_ptr, which is really nice
 658     code to work with, Peter Dimov in particular for his help and
 659     invaluable advice on thread safety.  Phillip Jordan and Paolo
 660     Carlini for the lock policy implementation.
 661   </p></div><div class="bibliography"><div class="titlepage"><div><div><h4 class="title"><a id="shared_ptr.biblio"></a>Bibliography</h4></div></div></div><div class="biblioentry"><a id="idm270864735008"></a><p><span class="title"><em>
 662         <a class="link" href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2007/n2351.htm" target="_top">
 663       Improving shared_ptr for C++0x, Revision 2
 664         </a>
 665       </em>. </span><span class="subtitle">
 666       N2351
 667     . </span></p></div><div class="biblioentry"><a id="idm270864732720"></a><p><span class="title"><em>
 668         <a class="link" href="http://open-std.org/jtc1/sc22/wg21/docs/papers/2007/n2456.html" target="_top">
 669       C++ Standard Library Active Issues List
 670         </a>
 671       </em>. </span><span class="subtitle">
 672       N2456
 673     . </span></p></div><div class="biblioentry"><a id="idm270864730432"></a><p><span class="title"><em>
 674         <a class="link" href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2007/n2461.pdf" target="_top">
 675       Working Draft, Standard for Programming Language C++
 676         </a>
 677       </em>. </span><span class="subtitle">
 678       N2461
 679     . </span></p></div><div class="biblioentry"><a id="idm270864728128"></a><p><span class="title"><em>
 680         <a class="link" href="http://boost.org/libs/smart_ptr/shared_ptr.htm" target="_top">
 681       Boost C++ Libraries documentation, shared_ptr
 682         </a>
 683       </em>. </span><span class="subtitle">
 684       N2461
 685     . </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="traits.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">Pairs </td><td width="20%" align="center"><a accesskey="h" href="../index.html">Home</a></td><td width="40%" align="right" valign="top"> Traits</td></tr></table></div></body></html>