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<html xmlns="http://www.w3.org/1999/xhtml"><head><title>Memory</title><meta name="generator" content="DocBook XSL-NS Stylesheets V1.76.1"/><meta name="keywords" content=" ISO C++ , library "/><meta name="keywords" content=" ISO C++ , runtime , library "/><link rel="home" href="../index.html" title="The GNU C++ Library"/><link rel="up" href="utilities.html" title="Chapter 6. Utilities"/><link rel="prev" href="pairs.html" title="Pairs"/><link rel="next" href="traits.html" title="Traits"/></head><body><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="3" align="center">Memory</th></tr><tr><td align="left"><a accesskey="p" href="pairs.html">Prev</a> </td><th width="60%" align="center">Chapter 6.
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Utilities
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</th><td align="right"> <a accesskey="n" href="traits.html">Next</a></td></tr></table><hr/></div><div class="section" title="Memory"><div class="titlepage"><div><div><h2 class="title"><a id="std.util.memory"/>Memory</h2></div></div></div><p>
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Memory contains three general areas. First, function and operator
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calls via <code class="function">new</code> and <code class="function">delete</code>
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operator or member function calls. Second, allocation via
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<code class="classname">allocator</code>. And finally, smart pointer and
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intelligent pointer abstractions.
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</p><div class="section" title="Allocators"><div class="titlepage"><div><div><h3 class="title"><a id="std.util.memory.allocator"/>Allocators</h3></div></div></div><p>
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Memory management for Standard Library entities is encapsulated in a
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class template called <code class="classname">allocator</code>. The
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<code class="classname">allocator</code> abstraction is used throughout the
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library in <code class="classname">string</code>, container classes,
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algorithms, and parts of iostreams. This class, and base classes of
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it, are the superset of available free store (<span class="quote">“<span class="quote">heap</span>”</span>)
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management classes.
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</p><div class="section" title="Requirements"><div class="titlepage"><div><div><h4 class="title"><a id="allocator.req"/>Requirements</h4></div></div></div><p>
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The C++ standard only gives a few directives in this area:
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</p><div class="itemizedlist"><ul class="itemizedlist"><li class="listitem"><p>
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When you add elements to a container, and the container must
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allocate more memory to hold them, the container makes the
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request via its <span class="type">Allocator</span> template
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parameter, which is usually aliased to
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<span class="type">allocator_type</span>. This includes adding chars
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to the string class, which acts as a regular STL container in
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this respect.
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</p></li><li class="listitem"><p>
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The default <span class="type">Allocator</span> argument of every
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container-of-T is <code class="classname">allocator<T></code>.
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</p></li><li class="listitem"><p>
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The interface of the <code class="classname">allocator<T></code> class is
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extremely simple. It has about 20 public declarations (nested
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typedefs, member functions, etc), but the two which concern us most
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are:
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</p><pre class="programlisting">
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T* allocate (size_type n, const void* hint = 0);
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void deallocate (T* p, size_type n);
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</pre><p>
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The <code class="varname">n</code> arguments in both those
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functions is a <span class="emphasis"><em>count</em></span> of the number of
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<span class="type">T</span>'s to allocate space for, <span class="emphasis"><em>not their
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total size</em></span>.
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(This is a simplification; the real signatures use nested typedefs.)
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</p></li><li class="listitem"><p>
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The storage is obtained by calling <code class="function">::operator
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new</code>, but it is unspecified when or how
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often this function is called. The use of the
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<code class="varname">hint</code> is unspecified, but intended as an
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aid to locality if an implementation so
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desires. <code class="constant">[20.4.1.1]/6</code>
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</p></li></ul></div><p>
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Complete details can be found in the C++ standard, look in
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<code class="constant">[20.4 Memory]</code>.
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</p></div><div class="section" title="Design Issues"><div class="titlepage"><div><div><h4 class="title"><a id="allocator.design_issues"/>Design Issues</h4></div></div></div><p>
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The easiest way of fulfilling the requirements is to call
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<code class="function">operator new</code> each time a container needs
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memory, and to call <code class="function">operator delete</code> each time
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the container releases memory. This method may be <a class="link" href="http://gcc.gnu.org/ml/libstdc++/2001-05/msg00105.html">slower</a>
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than caching the allocations and re-using previously-allocated
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memory, but has the advantage of working correctly across a wide
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variety of hardware and operating systems, including large
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clusters. The <code class="classname">__gnu_cxx::new_allocator</code>
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implements the simple operator new and operator delete semantics,
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while <code class="classname">__gnu_cxx::malloc_allocator</code>
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implements much the same thing, only with the C language functions
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<code class="function">std::malloc</code> and <code class="function">free</code>.
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</p><p>
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Another approach is to use intelligence within the allocator
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class to cache allocations. This extra machinery can take a variety
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of forms: a bitmap index, an index into an exponentially increasing
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power-of-two-sized buckets, or simpler fixed-size pooling cache.
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The cache is shared among all the containers in the program: when
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your program's <code class="classname">std::vector<int></code> gets
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cut in half and frees a bunch of its storage, that memory can be
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reused by the private
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<code class="classname">std::list<WonkyWidget></code> brought in from
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a KDE library that you linked against. And operators
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<code class="function">new</code> and <code class="function">delete</code> are not
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always called to pass the memory on, either, which is a speed
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bonus. Examples of allocators that use these techniques are
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<code class="classname">__gnu_cxx::bitmap_allocator</code>,
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<code class="classname">__gnu_cxx::pool_allocator</code>, and
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<code class="classname">__gnu_cxx::__mt_alloc</code>.
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</p><p>
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Depending on the implementation techniques used, the underlying
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operating system, and compilation environment, scaling caching
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allocators can be tricky. In particular, order-of-destruction and
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order-of-creation for memory pools may be difficult to pin down
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with certainty, which may create problems when used with plugins
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or loading and unloading shared objects in memory. As such, using
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caching allocators on systems that do not support
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<code class="function">abi::__cxa_atexit</code> is not recommended.
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</p></div><div class="section" title="Implementation"><div class="titlepage"><div><div><h4 class="title"><a id="allocator.impl"/>Implementation</h4></div></div></div><div class="section" title="Interface Design"><div class="titlepage"><div><div><h5 class="title"><a id="id485345"/>Interface Design</h5></div></div></div><p>
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The only allocator interface that
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is supported is the standard C++ interface. As such, all STL
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containers have been adjusted, and all external allocators have
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been modified to support this change.
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</p><p>
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The class <code class="classname">allocator</code> just has typedef,
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constructor, and rebind members. It inherits from one of the
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high-speed extension allocators, covered below. Thus, all
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allocation and deallocation depends on the base class.
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</p><p>
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The base class that <code class="classname">allocator</code> is derived from
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may not be user-configurable.
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</p></div><div class="section" title="Selecting Default Allocation Policy"><div class="titlepage"><div><div><h5 class="title"><a id="id485374"/>Selecting Default Allocation Policy</h5></div></div></div><p>
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It's difficult to pick an allocation strategy that will provide
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maximum utility, without excessively penalizing some behavior. In
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fact, it's difficult just deciding which typical actions to measure
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for speed.
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</p><p>
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Three synthetic benchmarks have been created that provide data
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that is used to compare different C++ allocators. These tests are:
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</p><div class="orderedlist"><ol class="orderedlist"><li class="listitem"><p>
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Insertion.
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</p><p>
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Over multiple iterations, various STL container
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objects have elements inserted to some maximum amount. A variety
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of allocators are tested.
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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">sequence</a>
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and <a class="link" href="http://gcc.gnu.org/viewcvs/trunk/libstdc%2B%2B-v3/testsuite/performance/23_containers/insert/associative.cc?view=markup">associative</a>
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containers.
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</p></li><li class="listitem"><p>
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Insertion and erasure in a multi-threaded environment.
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</p><p>
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This test shows the ability of the allocator to reclaim memory
|
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on a per-thread basis, as well as measuring thread contention
|
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for memory resources.
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Test source
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<a class="link" href="http://gcc.gnu.org/viewcvs/trunk/libstdc%2B%2B-v3/testsuite/performance/23_containers/insert_erase/associative.cc?view=markup">here</a>.
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</p></li><li class="listitem"><p>
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A threaded producer/consumer model.
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</p><p>
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Test source for
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<a class="link" href="http://gcc.gnu.org/viewcvs/trunk/libstdc++-v3/testsuite/performance/23_containers/producer_consumer/sequence.cc?view=markup">sequence</a>
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and
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<a class="link" href="http://gcc.gnu.org/viewcvs/trunk/libstdc++-v3/testsuite/performance/23_containers/producer_consumer/associative.cc?view=markup">associative</a>
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containers.
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</p></li></ol></div><p>
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The current default choice for
|
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<code class="classname">allocator</code> is
|
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<code class="classname">__gnu_cxx::new_allocator</code>.
|
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</p></div><div class="section" title="Disabling Memory Caching"><div class="titlepage"><div><div><h5 class="title"><a id="id485485"/>Disabling Memory Caching</h5></div></div></div><p>
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In use, <code class="classname">allocator</code> may allocate and
|
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deallocate using implementation-specified strategies and
|
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heuristics. Because of this, every call to an allocator object's
|
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<code class="function">allocate</code> member function may not actually
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call the global operator new. This situation is also duplicated
|
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for calls to the <code class="function">deallocate</code> member
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function.
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</p><p>
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This can be confusing.
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</p><p>
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In particular, this can make debugging memory errors more
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difficult, especially when using third party tools like valgrind or
|
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|
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debug versions of <code class="function">new</code>.
|
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</p><p>
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There are various ways to solve this problem. One would be to use
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a custom allocator that just called operators
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<code class="function">new</code> and <code class="function">delete</code>
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directly, for every allocation. (See
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<code class="filename">include/ext/new_allocator.h</code>, for instance.)
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However, that option would involve changing source code to use
|
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a non-default allocator. Another option is to force the
|
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default allocator to remove caching and pools, and to directly
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allocate with every call of <code class="function">allocate</code> and
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directly deallocate with every call of
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<code class="function">deallocate</code>, regardless of efficiency. As it
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turns out, this last option is also available.
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</p><p>
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To globally disable memory caching within the library for the
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default allocator, merely set
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<code class="constant">GLIBCXX_FORCE_NEW</code> (with any value) in the
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system's environment before running the program. If your program
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crashes with <code class="constant">GLIBCXX_FORCE_NEW</code> in the
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environment, it likely means that you linked against objects
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built against the older library (objects which might still using the
|
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cached allocations...).
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</p></div></div><div class="section" title="Using a Specific Allocator"><div class="titlepage"><div><div><h4 class="title"><a id="allocator.using"/>Using a Specific Allocator</h4></div></div></div><p>
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You can specify different memory management schemes on a
|
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per-container basis, by overriding the default
|
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<span class="type">Allocator</span> template parameter. For example, an easy
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(but non-portable) method of specifying that only <code class="function">malloc</code> or <code class="function">free</code>
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should be used instead of the default node allocator is:
|
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</p><pre class="programlisting">
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std::list <int, __gnu_cxx::malloc_allocator<int> > malloc_list;</pre><p>
|
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|
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Likewise, a debugging form of whichever allocator is currently in use:
|
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</p><pre class="programlisting">
|
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std::deque <int, __gnu_cxx::debug_allocator<std::allocator<int> > > debug_deque;
|
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</pre></div><div class="section" title="Custom Allocators"><div class="titlepage"><div><div><h4 class="title"><a id="allocator.custom"/>Custom Allocators</h4></div></div></div><p>
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Writing a portable C++ allocator would dictate that the interface
|
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would look much like the one specified for
|
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<code class="classname">allocator</code>. Additional member functions, but
|
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not subtractions, would be permissible.
|
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</p><p>
|
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Probably the best place to start would be to copy one of the
|
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extension allocators: say a simple one like
|
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<code class="classname">new_allocator</code>.
|
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</p></div><div class="section" title="Extension Allocators"><div class="titlepage"><div><div><h4 class="title"><a id="allocator.ext"/>Extension Allocators</h4></div></div></div><p>
|
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Several other allocators are provided as part of this
|
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implementation. The location of the extension allocators and their
|
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names have changed, but in all cases, functionality is
|
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equivalent. Starting with gcc-3.4, all extension allocators are
|
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standard style. Before this point, SGI style was the norm. Because of
|
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this, the number of template arguments also changed. Here's a simple
|
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chart to track the changes.
|
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</p><p>
|
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More details on each of these extension allocators follows.
|
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</p><div class="orderedlist"><ol class="orderedlist"><li class="listitem"><p>
|
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<code class="classname">new_allocator</code>
|
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</p><p>
|
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Simply wraps <code class="function">::operator new</code>
|
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and <code class="function">::operator delete</code>.
|
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|
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</p></li><li class="listitem"><p>
|
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|
|
<code class="classname">malloc_allocator</code>
|
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|
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</p><p>
|
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Simply wraps <code class="function">malloc</code> and
|
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<code class="function">free</code>. There is also a hook for an
|
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out-of-memory handler (for
|
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<code class="function">new</code>/<code class="function">delete</code> this is
|
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taken care of elsewhere).
|
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|
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</p></li><li class="listitem"><p>
|
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|
|
<code class="classname">array_allocator</code>
|
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|
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</p><p>
|
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|
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Allows allocations of known and fixed sizes using existing
|
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|
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global or external storage allocated via construction of
|
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<code class="classname">std::tr1::array</code> objects. By using this
|
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allocator, fixed size containers (including
|
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<code class="classname">std::string</code>) can be used without
|
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instances calling <code class="function">::operator new</code> and
|
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<code class="function">::operator delete</code>. This capability
|
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allows the use of STL abstractions without runtime
|
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complications or overhead, even in situations such as program
|
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startup. For usage examples, please consult the testsuite.
|
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</p></li><li class="listitem"><p>
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<code class="classname">debug_allocator</code>
|
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</p><p>
|
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A wrapper around an arbitrary allocator A. It passes on
|
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slightly increased size requests to A, and uses the extra
|
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memory to store size information. When a pointer is passed
|
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to <code class="function">deallocate()</code>, the stored size is
|
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checked, and <code class="function">assert()</code> is used to
|
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guarantee they match.
|
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|
|
</p></li><li class="listitem"><p>
|
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|
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<code class="classname">throw_allocator</code>
|
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|
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</p><p>
|
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|
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Includes memory tracking and marking abilities as well as hooks for
|
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throwing exceptions at configurable intervals (including random,
|
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all, none).
|
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|
|
</p></li><li class="listitem"><p>
|
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|
|
<code class="classname">__pool_alloc</code>
|
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|
|
</p><p>
|
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|
|
A high-performance, single pool allocator. The reusable
|
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|
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memory is shared among identical instantiations of this type.
|
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|
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It calls through <code class="function">::operator new</code> to
|
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|
|
obtain new memory when its lists run out. If a client
|
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container requests a block larger than a certain threshold
|
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|
|
size, then the pool is bypassed, and the allocate/deallocate
|
262 |
|
|
request is passed to <code class="function">::operator new</code>
|
263 |
|
|
directly.
|
264 |
|
|
</p><p>
|
265 |
|
|
Older versions of this class take a boolean template
|
266 |
|
|
parameter, called <code class="varname">thr</code>, and an integer template
|
267 |
|
|
parameter, called <code class="varname">inst</code>.
|
268 |
|
|
</p><p>
|
269 |
|
|
The <code class="varname">inst</code> number is used to track additional memory
|
270 |
|
|
pools. The point of the number is to allow multiple
|
271 |
|
|
instantiations of the classes without changing the semantics at
|
272 |
|
|
all. All three of
|
273 |
|
|
</p><pre class="programlisting">
|
274 |
|
|
typedef __pool_alloc<true,0> normal;
|
275 |
|
|
typedef __pool_alloc<true,1> private;
|
276 |
|
|
typedef __pool_alloc<true,42> also_private;
|
277 |
|
|
</pre><p>
|
278 |
|
|
behave exactly the same way. However, the memory pool for each type
|
279 |
|
|
(and remember that different instantiations result in different types)
|
280 |
|
|
remains separate.
|
281 |
|
|
</p><p>
|
282 |
|
|
The library uses <span class="emphasis"><em>0</em></span> in all its instantiations. If you
|
283 |
|
|
wish to keep separate free lists for a particular purpose, use a
|
284 |
|
|
different number.
|
285 |
|
|
</p><p>The <code class="varname">thr</code> boolean determines whether the
|
286 |
|
|
pool should be manipulated atomically or not. When
|
287 |
|
|
<code class="varname">thr</code> = <code class="constant">true</code>, the allocator
|
288 |
|
|
is thread-safe, while <code class="varname">thr</code> =
|
289 |
|
|
<code class="constant">false</code>, is slightly faster but unsafe for
|
290 |
|
|
multiple threads.
|
291 |
|
|
</p><p>
|
292 |
|
|
For thread-enabled configurations, the pool is locked with a
|
293 |
|
|
single big lock. In some situations, this implementation detail
|
294 |
|
|
may result in severe performance degradation.
|
295 |
|
|
</p><p>
|
296 |
|
|
(Note that the GCC thread abstraction layer allows us to provide
|
297 |
|
|
safe zero-overhead stubs for the threading routines, if threads
|
298 |
|
|
were disabled at configuration time.)
|
299 |
|
|
</p></li><li class="listitem"><p>
|
300 |
|
|
<code class="classname">__mt_alloc</code>
|
301 |
|
|
</p><p>
|
302 |
|
|
A high-performance fixed-size allocator with
|
303 |
|
|
exponentially-increasing allocations. It has its own
|
304 |
|
|
documentation, found <a class="link" href="mt_allocator.html" title="Chapter 20. The mt_allocator">here</a>.
|
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 |
|
|
documentation, found <a class="link" href="bitmap_allocator.html" title="Chapter 21. The bitmap_allocator">here</a>.
|
311 |
|
|
</p></li></ol></div></div><div class="bibliography" title="Bibliography"><div class="titlepage"><div><div><h4 class="title"><a id="allocator.biblio"/>Bibliography</h4></div></div></div><div class="biblioentry"><a id="id485936"/><p><span class="citetitle"><em class="citetitle">
|
312 |
|
|
ISO/IEC 14882:1998 Programming languages - C++
|
313 |
|
|
</em>. </span>
|
314 |
|
|
isoc++_1998
|
315 |
|
|
<span class="pagenums">20.4 Memory. </span></p></div><div class="biblioentry" title="The Standard Librarian: What Are Allocators Good For?"><a id="id485951"/><p><span class="title"><em>
|
316 |
|
|
<a class="link" href="http://www.drdobbs.com/cpp/184403759">
|
317 |
|
|
The Standard Librarian: What Are Allocators Good For?
|
318 |
|
|
</a>
|
319 |
|
|
</em>. </span><span class="author"><span class="firstname">Matt</span> <span class="surname">Austern</span>. </span><span class="publisher"><span class="publishername">
|
320 |
|
|
C/C++ Users Journal
|
321 |
|
|
. </span></span></p></div><div class="biblioentry" title="The Hoard Memory Allocator"><a id="id485982"/><p><span class="title"><em>
|
322 |
|
|
<a class="link" href="http://www.cs.umass.edu/~emery/hoard">
|
323 |
|
|
The Hoard Memory Allocator
|
324 |
|
|
</a>
|
325 |
|
|
</em>. </span><span class="author"><span class="firstname">Emery</span> <span class="surname">Berger</span>. </span></p></div><div class="biblioentry" title="Reconsidering Custom Memory Allocation"><a id="id486005"/><p><span class="title"><em>
|
326 |
|
|
<a class="link" href="http://www.cs.umass.edu/~emery/pubs/berger-oopsla2002.pdf">
|
327 |
|
|
Reconsidering Custom Memory Allocation
|
328 |
|
|
</a>
|
329 |
|
|
</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" title="Allocator Types"><a id="id486057"/><p><span class="title"><em>
|
330 |
|
|
<a class="link" href="http://www.angelikalanger.com/Articles/C++Report/Allocators/Allocators.html">
|
331 |
|
|
Allocator Types
|
332 |
|
|
</a>
|
333 |
|
|
</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">
|
334 |
|
|
C/C++ Users Journal
|
335 |
|
|
. </span></span></p></div><div class="biblioentry"><a id="id486096"/><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">
|
336 |
|
|
Addison Wesley
|
337 |
|
|
. </span></span></p></div><div class="biblioentry"><a id="id486133"/><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" title="auto_ptr"><div class="titlepage"><div><div><h3 class="title"><a id="std.util.memory.auto_ptr"/>auto_ptr</h3></div></div></div><div class="section" title="Limitations"><div class="titlepage"><div><div><h4 class="title"><a id="auto_ptr.limitations"/>Limitations</h4></div></div></div><p>Explaining all of the fun and delicious things that can
|
338 |
|
|
happen with misuse of the <code class="classname">auto_ptr</code> class
|
339 |
|
|
template (called <acronym class="acronym">AP</acronym> here) would take some
|
340 |
|
|
time. Suffice it to say that the use of <acronym class="acronym">AP</acronym>
|
341 |
|
|
safely in the presence of copying has some subtleties.
|
342 |
|
|
</p><p>
|
343 |
|
|
The AP class is a really
|
344 |
|
|
nifty idea for a smart pointer, but it is one of the dumbest of
|
345 |
|
|
all the smart pointers -- and that's fine.
|
346 |
|
|
</p><p>
|
347 |
|
|
AP is not meant to be a supersmart solution to all resource
|
348 |
|
|
leaks everywhere. Neither is it meant to be an effective form
|
349 |
|
|
of garbage collection (although it can help, a little bit).
|
350 |
|
|
And it can <span class="emphasis"><em>not</em></span>be used for arrays!
|
351 |
|
|
</p><p>
|
352 |
|
|
<acronym class="acronym">AP</acronym> is meant to prevent nasty leaks in the
|
353 |
|
|
presence of exceptions. That's <span class="emphasis"><em>all</em></span>. This
|
354 |
|
|
code is AP-friendly:
|
355 |
|
|
</p><pre class="programlisting">
|
356 |
|
|
// Not a recommend naming scheme, but good for web-based FAQs.
|
357 |
|
|
typedef std::auto_ptr<MyClass> APMC;
|
358 |
|
|
|
359 |
|
|
extern function_taking_MyClass_pointer (MyClass*);
|
360 |
|
|
extern some_throwable_function ();
|
361 |
|
|
|
362 |
|
|
void func (int data)
|
363 |
|
|
{
|
364 |
|
|
APMC ap (new MyClass(data));
|
365 |
|
|
|
366 |
|
|
some_throwable_function(); // this will throw an exception
|
367 |
|
|
|
368 |
|
|
function_taking_MyClass_pointer (ap.get());
|
369 |
|
|
}
|
370 |
|
|
</pre><p>When an exception gets thrown, the instance of MyClass that's
|
371 |
|
|
been created on the heap will be <code class="function">delete</code>'d as the stack is
|
372 |
|
|
unwound past <code class="function">func()</code>.
|
373 |
|
|
</p><p>Changing that code as follows is not <acronym class="acronym">AP</acronym>-friendly:
|
374 |
|
|
</p><pre class="programlisting">
|
375 |
|
|
APMC ap (new MyClass[22]);
|
376 |
|
|
</pre><p>You will get the same problems as you would without the use
|
377 |
|
|
of <acronym class="acronym">AP</acronym>:
|
378 |
|
|
</p><pre class="programlisting">
|
379 |
|
|
char* array = new char[10]; // array new...
|
380 |
|
|
...
|
381 |
|
|
delete array; // ...but single-object delete
|
382 |
|
|
</pre><p>
|
383 |
|
|
AP cannot tell whether the pointer you've passed at creation points
|
384 |
|
|
to one or many things. If it points to many things, you are about
|
385 |
|
|
to die. AP is trivial to write, however, so you could write your
|
386 |
|
|
own <code class="code">auto_array_ptr</code> for that situation (in fact, this has
|
387 |
|
|
been done many times; check the mailing lists, Usenet, Boost, etc).
|
388 |
|
|
</p></div><div class="section" title="Use in Containers"><div class="titlepage"><div><div><h4 class="title"><a id="auto_ptr.using"/>Use in Containers</h4></div></div></div><p>
|
389 |
|
|
</p><p>All of the <a class="link" href="containers.html" title="Chapter 9. Containers">containers</a>
|
390 |
|
|
described in the standard library require their contained types
|
391 |
|
|
to have, among other things, a copy constructor like this:
|
392 |
|
|
</p><pre class="programlisting">
|
393 |
|
|
struct My_Type
|
394 |
|
|
{
|
395 |
|
|
My_Type (My_Type const&);
|
396 |
|
|
};
|
397 |
|
|
</pre><p>
|
398 |
|
|
Note the const keyword; the object being copied shouldn't change.
|
399 |
|
|
The template class <code class="code">auto_ptr</code> (called AP here) does not
|
400 |
|
|
meet this requirement. Creating a new AP by copying an existing
|
401 |
|
|
one transfers ownership of the pointed-to object, which means that
|
402 |
|
|
the AP being copied must change, which in turn means that the
|
403 |
|
|
copy ctors of AP do not take const objects.
|
404 |
|
|
</p><p>
|
405 |
|
|
The resulting rule is simple: <span class="emphasis"><em>Never ever use a
|
406 |
|
|
container of auto_ptr objects</em></span>. The standard says that
|
407 |
|
|
<span class="quote">“<span class="quote">undefined</span>”</span> behavior is the result, but it is
|
408 |
|
|
guaranteed to be messy.
|
409 |
|
|
</p><p>
|
410 |
|
|
To prevent you from doing this to yourself, the
|
411 |
|
|
<a class="link" href="ext_compile_checks.html" title="Chapter 16. Compile Time Checks">concept checks</a> built
|
412 |
|
|
in to this implementation will issue an error if you try to
|
413 |
|
|
compile code like this:
|
414 |
|
|
</p><pre class="programlisting">
|
415 |
|
|
#include <vector>
|
416 |
|
|
#include <memory>
|
417 |
|
|
|
418 |
|
|
void f()
|
419 |
|
|
{
|
420 |
|
|
std::vector< std::auto_ptr<int> > vec_ap_int;
|
421 |
|
|
}
|
422 |
|
|
</pre><p>
|
423 |
|
|
Should you try this with the checks enabled, you will see an error.
|
424 |
|
|
</p></div></div><div class="section" title="shared_ptr"><div class="titlepage"><div><div><h3 class="title"><a id="std.util.memory.shared_ptr"/>shared_ptr</h3></div></div></div><p>
|
425 |
|
|
The shared_ptr class template stores a pointer, usually obtained via new,
|
426 |
|
|
and implements shared ownership semantics.
|
427 |
|
|
</p><div class="section" title="Requirements"><div class="titlepage"><div><div><h4 class="title"><a id="shared_ptr.req"/>Requirements</h4></div></div></div><p>
|
428 |
|
|
</p><p>
|
429 |
|
|
The standard deliberately doesn't require a reference-counted
|
430 |
|
|
implementation, allowing other techniques such as a
|
431 |
|
|
circular-linked-list.
|
432 |
|
|
</p><p>
|
433 |
|
|
</p></div><div class="section" title="Design Issues"><div class="titlepage"><div><div><h4 class="title"><a id="shared_ptr.design_issues"/>Design Issues</h4></div></div></div><p>
|
434 |
|
|
The <code class="classname">shared_ptr</code> code is kindly donated to GCC by the Boost
|
435 |
|
|
project and the original authors of the code. The basic design and
|
436 |
|
|
algorithms are from Boost, the notes below describe details specific to
|
437 |
|
|
the GCC implementation. Names have been uglified in this implementation,
|
438 |
|
|
but the design should be recognisable to anyone familiar with the Boost
|
439 |
|
|
1.32 shared_ptr.
|
440 |
|
|
</p><p>
|
441 |
|
|
The basic design is an abstract base class, <code class="code">_Sp_counted_base</code> that
|
442 |
|
|
does the reference-counting and calls virtual functions when the count
|
443 |
|
|
drops to zero.
|
444 |
|
|
Derived classes override those functions to destroy resources in a context
|
445 |
|
|
where the correct dynamic type is known. This is an application of the
|
446 |
|
|
technique known as type erasure.
|
447 |
|
|
</p></div><div class="section" title="Implementation"><div class="titlepage"><div><div><h4 class="title"><a id="shared_ptr.impl"/>Implementation</h4></div></div></div><div class="section" title="Class Hierarchy"><div class="titlepage"><div><div><h5 class="title"><a id="id486484"/>Class Hierarchy</h5></div></div></div><p>
|
448 |
|
|
A <code class="classname">shared_ptr<T></code> contains a pointer of
|
449 |
|
|
type <span class="type">T*</span> and an object of type
|
450 |
|
|
<code class="classname">__shared_count</code>. The shared_count contains a
|
451 |
|
|
pointer of type <span class="type">_Sp_counted_base*</span> which points to the
|
452 |
|
|
object that maintains the reference-counts and destroys the managed
|
453 |
|
|
resource.
|
454 |
|
|
</p><div class="variablelist"><dl><dt><span class="term"><code class="classname">_Sp_counted_base<Lp></code></span></dt><dd><p>
|
455 |
|
|
The base of the hierarchy is parameterized on the lock policy (see below.)
|
456 |
|
|
_Sp_counted_base doesn't depend on the type of pointer being managed,
|
457 |
|
|
it only maintains the reference counts and calls virtual functions when
|
458 |
|
|
the counts drop to zero. The managed object is destroyed when the last
|
459 |
|
|
strong reference is dropped, but the _Sp_counted_base itself must exist
|
460 |
|
|
until the last weak reference is dropped.
|
461 |
|
|
</p></dd><dt><span class="term"><code class="classname">_Sp_counted_base_impl<Ptr, Deleter, Lp></code></span></dt><dd><p>
|
462 |
|
|
Inherits from _Sp_counted_base and stores a pointer of type <code class="code">Ptr</code>
|
463 |
|
|
and a deleter of type <code class="code">Deleter</code>. <code class="classname">_Sp_deleter</code> is
|
464 |
|
|
used when the user doesn't supply a custom deleter. Unlike Boost's, this
|
465 |
|
|
default deleter is not "checked" because GCC already issues a warning if
|
466 |
|
|
<code class="function">delete</code> is used with an incomplete type.
|
467 |
|
|
This is the only derived type used by <code class="classname">tr1::shared_ptr<Ptr></code>
|
468 |
|
|
and it is never used by <code class="classname">std::shared_ptr</code>, which uses one of
|
469 |
|
|
the following types, depending on how the shared_ptr is constructed.
|
470 |
|
|
</p></dd><dt><span class="term"><code class="classname">_Sp_counted_ptr<Ptr, Lp></code></span></dt><dd><p>
|
471 |
|
|
Inherits from _Sp_counted_base and stores a pointer of type <span class="type">Ptr</span>,
|
472 |
|
|
which is passed to <code class="function">delete</code> when the last reference is dropped.
|
473 |
|
|
This is the simplest form and is used when there is no custom deleter or
|
474 |
|
|
allocator.
|
475 |
|
|
</p></dd><dt><span class="term"><code class="classname">_Sp_counted_deleter<Ptr, Deleter, Alloc></code></span></dt><dd><p>
|
476 |
|
|
Inherits from _Sp_counted_ptr and adds support for custom deleter and
|
477 |
|
|
allocator. Empty Base Optimization is used for the allocator. This class
|
478 |
|
|
is used even when the user only provides a custom deleter, in which case
|
479 |
|
|
<code class="classname">allocator</code> is used as the allocator.
|
480 |
|
|
</p></dd><dt><span class="term"><code class="classname">_Sp_counted_ptr_inplace<Tp, Alloc, Lp></code></span></dt><dd><p>
|
481 |
|
|
Used by <code class="code">allocate_shared</code> and <code class="code">make_shared</code>.
|
482 |
|
|
Contains aligned storage to hold an object of type <span class="type">Tp</span>,
|
483 |
|
|
which is constructed in-place with placement <code class="function">new</code>.
|
484 |
|
|
Has a variadic template constructor allowing any number of arguments to
|
485 |
|
|
be forwarded to <span class="type">Tp</span>'s constructor.
|
486 |
|
|
Unlike the other <code class="classname">_Sp_counted_*</code> classes, this one is parameterized on the
|
487 |
|
|
type of object, not the type of pointer; this is purely a convenience
|
488 |
|
|
that simplifies the implementation slightly.
|
489 |
|
|
</p></dd></dl></div><p>
|
490 |
|
|
C++11-only features are: rvalue-ref/move support, allocator support,
|
491 |
|
|
aliasing constructor, make_shared & allocate_shared. Additionally,
|
492 |
|
|
the constructors taking <code class="classname">auto_ptr</code> parameters are
|
493 |
|
|
deprecated in C++11 mode.
|
494 |
|
|
</p></div><div class="section" title="Thread Safety"><div class="titlepage"><div><div><h5 class="title"><a id="id486672"/>Thread Safety</h5></div></div></div><p>
|
495 |
|
|
The
|
496 |
|
|
<a class="link" href="http://boost.org/libs/smart_ptr/shared_ptr.htm#ThreadSafety">Thread
|
497 |
|
|
Safety</a> section of the Boost shared_ptr documentation says "shared_ptr
|
498 |
|
|
objects offer the same level of thread safety as built-in types."
|
499 |
|
|
The implementation must ensure that concurrent updates to separate shared_ptr
|
500 |
|
|
instances are correct even when those instances share a reference count e.g.
|
501 |
|
|
</p><pre class="programlisting">
|
502 |
|
|
shared_ptr<A> a(new A);
|
503 |
|
|
shared_ptr<A> b(a);
|
504 |
|
|
|
505 |
|
|
// Thread 1 // Thread 2
|
506 |
|
|
a.reset(); b.reset();
|
507 |
|
|
</pre><p>
|
508 |
|
|
The dynamically-allocated object must be destroyed by exactly one of the
|
509 |
|
|
threads. Weak references make things even more interesting.
|
510 |
|
|
The shared state used to implement shared_ptr must be transparent to the
|
511 |
|
|
user and invariants must be preserved at all times.
|
512 |
|
|
The key pieces of shared state are the strong and weak reference counts.
|
513 |
|
|
Updates to these need to be atomic and visible to all threads to ensure
|
514 |
|
|
correct cleanup of the managed resource (which is, after all, shared_ptr's
|
515 |
|
|
job!)
|
516 |
|
|
On multi-processor systems memory synchronisation may be needed so that
|
517 |
|
|
reference-count updates and the destruction of the managed resource are
|
518 |
|
|
race-free.
|
519 |
|
|
</p><p>
|
520 |
|
|
The function <code class="function">_Sp_counted_base::_M_add_ref_lock()</code>, called when
|
521 |
|
|
obtaining a shared_ptr from a weak_ptr, has to test if the managed
|
522 |
|
|
resource still exists and either increment the reference count or throw
|
523 |
|
|
<code class="classname">bad_weak_ptr</code>.
|
524 |
|
|
In a multi-threaded program there is a potential race condition if the last
|
525 |
|
|
reference is dropped (and the managed resource destroyed) between testing
|
526 |
|
|
the reference count and incrementing it, which could result in a shared_ptr
|
527 |
|
|
pointing to invalid memory.
|
528 |
|
|
</p><p>
|
529 |
|
|
The Boost shared_ptr (as used in GCC) features a clever lock-free
|
530 |
|
|
algorithm to avoid the race condition, but this relies on the
|
531 |
|
|
processor supporting an atomic <span class="emphasis"><em>Compare-And-Swap</em></span>
|
532 |
|
|
instruction. For other platforms there are fall-backs using mutex
|
533 |
|
|
locks. Boost (as of version 1.35) includes several different
|
534 |
|
|
implementations and the preprocessor selects one based on the
|
535 |
|
|
compiler, standard library, platform etc. For the version of
|
536 |
|
|
shared_ptr in libstdc++ the compiler and library are fixed, which
|
537 |
|
|
makes things much simpler: we have an atomic CAS or we don't, see Lock
|
538 |
|
|
Policy below for details.
|
539 |
|
|
</p></div><div class="section" title="Selecting Lock Policy"><div class="titlepage"><div><div><h5 class="title"><a id="id486733"/>Selecting Lock Policy</h5></div></div></div><p>
|
540 |
|
|
</p><p>
|
541 |
|
|
There is a single <code class="classname">_Sp_counted_base</code> class,
|
542 |
|
|
which is a template parameterized on the enum
|
543 |
|
|
<span class="type">__gnu_cxx::_Lock_policy</span>. The entire family of classes is
|
544 |
|
|
parameterized on the lock policy, right up to
|
545 |
|
|
<code class="classname">__shared_ptr</code>, <code class="classname">__weak_ptr</code> and
|
546 |
|
|
<code class="classname">__enable_shared_from_this</code>. The actual
|
547 |
|
|
<code class="classname">std::shared_ptr</code> class inherits from
|
548 |
|
|
<code class="classname">__shared_ptr</code> with the lock policy parameter
|
549 |
|
|
selected automatically based on the thread model and platform that
|
550 |
|
|
libstdc++ is configured for, so that the best available template
|
551 |
|
|
specialization will be used. This design is necessary because it would
|
552 |
|
|
not be conforming for <code class="classname">shared_ptr</code> to have an
|
553 |
|
|
extra template parameter, even if it had a default value. The
|
554 |
|
|
available policies are:
|
555 |
|
|
</p><div class="orderedlist"><ol class="orderedlist"><li class="listitem"><p>
|
556 |
|
|
<code class="constant">_S_Atomic</code>
|
557 |
|
|
</p><p>
|
558 |
|
|
Selected when GCC supports a builtin atomic compare-and-swap operation
|
559 |
|
|
on the target processor (see <a class="link" href="http://gcc.gnu.org/onlinedocs/gcc/Atomic-Builtins.html">Atomic
|
560 |
|
|
Builtins</a>.) The reference counts are maintained using a lock-free
|
561 |
|
|
algorithm and GCC's atomic builtins, which provide the required memory
|
562 |
|
|
synchronisation.
|
563 |
|
|
</p></li><li class="listitem"><p>
|
564 |
|
|
<code class="constant">_S_Mutex</code>
|
565 |
|
|
</p><p>
|
566 |
|
|
The _Sp_counted_base specialization for this policy contains a mutex,
|
567 |
|
|
which is locked in add_ref_lock(). This policy is used when GCC's atomic
|
568 |
|
|
builtins aren't available so explicit memory barriers are needed in places.
|
569 |
|
|
</p></li><li class="listitem"><p>
|
570 |
|
|
<code class="constant">_S_Single</code>
|
571 |
|
|
</p><p>
|
572 |
|
|
This policy uses a non-reentrant add_ref_lock() with no locking. It is
|
573 |
|
|
used when libstdc++ is built without <code class="literal">--enable-threads</code>.
|
574 |
|
|
</p></li></ol></div><p>
|
575 |
|
|
For all three policies, reference count increments and
|
576 |
|
|
decrements are done via the functions in
|
577 |
|
|
<code class="filename">ext/atomicity.h</code>, which detect if the program
|
578 |
|
|
is multi-threaded. If only one thread of execution exists in
|
579 |
|
|
the program then less expensive non-atomic operations are used.
|
580 |
|
|
</p></div><div class="section" title="Related functions and classes"><div class="titlepage"><div><div><h5 class="title"><a id="id486854"/>Related functions and classes</h5></div></div></div><div class="variablelist"><dl><dt><span class="term"><code class="code">dynamic_pointer_cast</code>, <code class="code">static_pointer_cast</code>,
|
581 |
|
|
<code class="code">const_pointer_cast</code></span></dt><dd><p>
|
582 |
|
|
As noted in N2351, these functions can be implemented non-intrusively using
|
583 |
|
|
the alias constructor. However the aliasing constructor is only available
|
584 |
|
|
in C++11 mode, so in TR1 mode these casts rely on three non-standard
|
585 |
|
|
constructors in shared_ptr and __shared_ptr.
|
586 |
|
|
In C++11 mode these constructors and the related tag types are not needed.
|
587 |
|
|
</p></dd><dt><span class="term"><code class="code">enable_shared_from_this</code></span></dt><dd><p>
|
588 |
|
|
The clever overload to detect a base class of type
|
589 |
|
|
<code class="code">enable_shared_from_this</code> comes straight from Boost.
|
590 |
|
|
There is an extra overload for <code class="code">__enable_shared_from_this</code> to
|
591 |
|
|
work smoothly with <code class="code">__shared_ptr<Tp, Lp></code> using any lock
|
592 |
|
|
policy.
|
593 |
|
|
</p></dd><dt><span class="term"><code class="code">make_shared</code>, <code class="code">allocate_shared</code></span></dt><dd><p>
|
594 |
|
|
<code class="code">make_shared</code> simply forwards to <code class="code">allocate_shared</code>
|
595 |
|
|
with <code class="code">std::allocator</code> as the allocator.
|
596 |
|
|
Although these functions can be implemented non-intrusively using the
|
597 |
|
|
alias constructor, if they have access to the implementation then it is
|
598 |
|
|
possible to save storage and reduce the number of heap allocations. The
|
599 |
|
|
newly constructed object and the _Sp_counted_* can be allocated in a single
|
600 |
|
|
block and the standard says implementations are "encouraged, but not required,"
|
601 |
|
|
to do so. This implementation provides additional non-standard constructors
|
602 |
|
|
(selected with the type <code class="code">_Sp_make_shared_tag</code>) which create an
|
603 |
|
|
object of type <code class="code">_Sp_counted_ptr_inplace</code> to hold the new object.
|
604 |
|
|
The returned <code class="code">shared_ptr<A></code> needs to know the address of the
|
605 |
|
|
new <code class="code">A</code> object embedded in the <code class="code">_Sp_counted_ptr_inplace</code>,
|
606 |
|
|
but it has no way to access it.
|
607 |
|
|
This implementation uses a "covert channel" to return the address of the
|
608 |
|
|
embedded object when <code class="code">get_deleter<_Sp_make_shared_tag>()</code>
|
609 |
|
|
is called. Users should not try to use this.
|
610 |
|
|
As well as the extra constructors, this implementation also needs some
|
611 |
|
|
members of _Sp_counted_deleter to be protected where they could otherwise
|
612 |
|
|
be private.
|
613 |
|
|
</p></dd></dl></div></div></div><div class="section" title="Use"><div class="titlepage"><div><div><h4 class="title"><a id="shared_ptr.using"/>Use</h4></div></div></div><div class="section" title="Examples"><div class="titlepage"><div><div><h5 class="title"><a id="id499306"/>Examples</h5></div></div></div><p>
|
614 |
|
|
Examples of use can be found in the testsuite, under
|
615 |
|
|
<code class="filename">testsuite/tr1/2_general_utilities/shared_ptr</code>,
|
616 |
|
|
<code class="filename">testsuite/20_util/shared_ptr</code>
|
617 |
|
|
and
|
618 |
|
|
<code class="filename">testsuite/20_util/weak_ptr</code>.
|
619 |
|
|
</p></div><div class="section" title="Unresolved Issues"><div class="titlepage"><div><div><h5 class="title"><a id="id499336"/>Unresolved Issues</h5></div></div></div><p>
|
620 |
|
|
The <span class="emphasis"><em><code class="classname">shared_ptr</code> atomic access</em></span>
|
621 |
|
|
clause in the C++11 standard is not implemented in GCC.
|
622 |
|
|
</p><p>
|
623 |
|
|
The <span class="type">_S_single</span> policy uses atomics when used in MT
|
624 |
|
|
code, because it uses the same dispatcher functions that check
|
625 |
|
|
<code class="function">__gthread_active_p()</code>. This could be
|
626 |
|
|
addressed by providing template specialisations for some members
|
627 |
|
|
of <code class="classname">_Sp_counted_base<_S_single></code>.
|
628 |
|
|
</p><p>
|
629 |
|
|
Unlike Boost, this implementation does not use separate classes
|
630 |
|
|
for the pointer+deleter and pointer+deleter+allocator cases in
|
631 |
|
|
C++11 mode, combining both into _Sp_counted_deleter and using
|
632 |
|
|
<code class="classname">allocator</code> when the user doesn't specify
|
633 |
|
|
an allocator. If it was found to be beneficial an additional
|
634 |
|
|
class could easily be added. With the current implementation,
|
635 |
|
|
the _Sp_counted_deleter and __shared_count constructors taking a
|
636 |
|
|
custom deleter but no allocator are technically redundant and
|
637 |
|
|
could be removed, changing callers to always specify an
|
638 |
|
|
allocator. If a separate pointer+deleter class was added the
|
639 |
|
|
__shared_count constructor would be needed, so it has been kept
|
640 |
|
|
for now.
|
641 |
|
|
</p><p>
|
642 |
|
|
The hack used to get the address of the managed object from
|
643 |
|
|
<code class="function">_Sp_counted_ptr_inplace::_M_get_deleter()</code>
|
644 |
|
|
is accessible to users. This could be prevented if
|
645 |
|
|
<code class="function">get_deleter<_Sp_make_shared_tag>()</code>
|
646 |
|
|
always returned NULL, since the hack only needs to work at a
|
647 |
|
|
lower level, not in the public API. This wouldn't be difficult,
|
648 |
|
|
but hasn't been done since there is no danger of accidental
|
649 |
|
|
misuse: users already know they are relying on unsupported
|
650 |
|
|
features if they refer to implementation details such as
|
651 |
|
|
_Sp_make_shared_tag.
|
652 |
|
|
</p><p>
|
653 |
|
|
tr1::_Sp_deleter could be a private member of tr1::__shared_count but it
|
654 |
|
|
would alter the ABI.
|
655 |
|
|
</p></div></div><div class="section" title="Acknowledgments"><div class="titlepage"><div><div><h4 class="title"><a id="shared_ptr.ack"/>Acknowledgments</h4></div></div></div><p>
|
656 |
|
|
The original authors of the Boost shared_ptr, which is really nice
|
657 |
|
|
code to work with, Peter Dimov in particular for his help and
|
658 |
|
|
invaluable advice on thread safety. Phillip Jordan and Paolo
|
659 |
|
|
Carlini for the lock policy implementation.
|
660 |
|
|
</p></div><div class="bibliography" title="Bibliography"><div class="titlepage"><div><div><h4 class="title"><a id="shared_ptr.biblio"/>Bibliography</h4></div></div></div><div class="biblioentry" title="Improving shared_ptr for C++0x, Revision 2"><a id="id499429"/><p><span class="title"><em>
|
661 |
|
|
<a class="link" href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2007/n2351.htm">
|
662 |
|
|
Improving shared_ptr for C++0x, Revision 2
|
663 |
|
|
</a>
|
664 |
|
|
</em>. </span><span class="subtitle">
|
665 |
|
|
N2351
|
666 |
|
|
. </span></p></div><div class="biblioentry" title="C++ Standard Library Active Issues List"><a id="id499448"/><p><span class="title"><em>
|
667 |
|
|
<a class="link" href="http://open-std.org/jtc1/sc22/wg21/docs/papers/2007/n2456.html">
|
668 |
|
|
C++ Standard Library Active Issues List
|
669 |
|
|
</a>
|
670 |
|
|
</em>. </span><span class="subtitle">
|
671 |
|
|
N2456
|
672 |
|
|
. </span></p></div><div class="biblioentry" title="Working Draft, Standard for Programming Language C++"><a id="id499467"/><p><span class="title"><em>
|
673 |
|
|
<a class="link" href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2007/n2461.pdf">
|
674 |
|
|
Working Draft, Standard for Programming Language C++
|
675 |
|
|
</a>
|
676 |
|
|
</em>. </span><span class="subtitle">
|
677 |
|
|
N2461
|
678 |
|
|
. </span></p></div><div class="biblioentry" title="Boost C++ Libraries documentation, shared_ptr"><a id="id499486"/><p><span class="title"><em>
|
679 |
|
|
<a class="link" href="http://boost.org/libs/smart_ptr/shared_ptr.htm">
|
680 |
|
|
Boost C++ Libraries documentation, shared_ptr
|
681 |
|
|
</a>
|
682 |
|
|
</em>. </span><span class="subtitle">
|
683 |
|
|
N2461
|
684 |
|
|
. </span></p></div></div></div></div><div class="navfooter"><hr/><table width="100%" summary="Navigation footer"><tr><td align="left"><a accesskey="p" href="pairs.html">Prev</a> </td><td align="center"><a accesskey="u" href="utilities.html">Up</a></td><td align="right"> <a accesskey="n" href="traits.html">Next</a></td></tr><tr><td align="left" valign="top">Pairs </td><td align="center"><a accesskey="h" href="../index.html">Home</a></td><td align="right" valign="top"> Traits</td></tr></table></div></body></html>
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