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+<html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /><title>shared_ptr</title><meta name="generator" content="DocBook XSL Stylesheets V1.74.0" /><meta name="keywords" content=" ISO C++ , shared_ptr " /><meta name="keywords" content=" ISO C++ , library " /><link rel="home" href="../spine.html" title="The GNU C++ Library Documentation" /><link rel="up" href="memory.html" title="Chapter 11. Memory" /><link rel="prev" href="auto_ptr.html" title="auto_ptr" /><link rel="next" href="traits.html" title="Chapter 12. Traits" /></head><body><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="3" align="center">shared_ptr</th></tr><tr><td width="20%" align="left"><a accesskey="p" href="auto_ptr.html">Prev</a> </td><th width="60%" align="center">Chapter 11. Memory</th><td width="20%" align="right"> <a accesskey="n" href="traits.html">Next</a></td></tr></table><hr /></div><div class="sect1" lang="en" xml:lang="en"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a id="manual.util.memory.shared_ptr"></a>shared_ptr</h2></div></div></div><p>
+The shared_ptr class template stores a pointer, usually obtained via new,
+and implements shared ownership semantics.
+</p><div class="sect2" lang="en" xml:lang="en"><div class="titlepage"><div><div><h3 class="title"><a id="shared_ptr.req"></a>Requirements</h3></div></div></div><p>
+ </p><p>
+ The standard deliberately doesn't require a reference-counted
+ implementation, allowing other techniques such as a
+ circular-linked-list.
+ </p><p>
+ At the time of writing the C++0x working paper doesn't mention how
+ threads affect shared_ptr, but it is likely to follow the existing
+ practice set by <code class="classname">boost::shared_ptr</code>. The
+ shared_ptr in libstdc++ is derived from Boost's, so the same rules
+ apply.
+ </p><p>
+ </p></div><div class="sect2" lang="en" xml:lang="en"><div class="titlepage"><div><div><h3 class="title"><a id="shared_ptr.design_issues"></a>Design Issues</h3></div></div></div><p>
+The <code class="classname">shared_ptr</code> code is kindly donated to GCC by the Boost
+project and the original authors of the code. The basic design and
+algorithms are from Boost, the notes below describe details specific to
+the GCC implementation. Names have been uglified in this implementation,
+but the design should be recognisable to anyone familiar with the Boost
+1.32 shared_ptr.
+ </p><p>
+The basic design is an abstract base class, <code class="code">_Sp_counted_base</code> that
+does the reference-counting and calls virtual functions when the count
+drops to zero.
+Derived classes override those functions to destroy resources in a context
+where the correct dynamic type is known. This is an application of the
+technique known as type erasure.
+ </p></div><div class="sect2" lang="en" xml:lang="en"><div class="titlepage"><div><div><h3 class="title"><a id="shared_ptr.impl"></a>Implementation</h3></div></div></div><div class="sect3" lang="en" xml:lang="en"><div class="titlepage"><div><div><h4 class="title"><a id="id400915"></a>Class Hierarchy</h4></div></div></div><p>
+A <code class="classname">shared_ptr<T></code> contains a pointer of
+type <span class="type">T*</span> and an object of type
+<code class="classname">__shared_count</code>. The shared_count contains a
+pointer of type <span class="type">_Sp_counted_base*</span> which points to the
+object that maintains the reference-counts and destroys the managed
+resource.
+ </p><div class="variablelist"><dl><dt><span class="term"><code class="classname">_Sp_counted_base<Lp></code></span></dt><dd><p>
+The base of the hierarchy is parameterized on the lock policy alone.
+_Sp_counted_base doesn't depend on the type of pointer being managed,
+it only maintains the reference counts and calls virtual functions when
+the counts drop to zero. The managed object is destroyed when the last
+strong reference is dropped, but the _Sp_counted_base itself must exist
+until the last weak reference is dropped.
+ </p></dd><dt><span class="term"><code class="classname">_Sp_counted_base_impl<Ptr, Deleter, Lp></code></span></dt><dd><p>
+Inherits from _Sp_counted_base and stores a pointer of type <span class="type">Ptr</span>
+and a deleter of type <code class="code">Deleter</code>. <code class="code">_Sp_deleter</code> is
+used when the user doesn't supply a custom deleter. Unlike Boost's, this
+default deleter is not "checked" because GCC already issues a warning if
+<code class="function">delete</code> is used with an incomplete type.
+This is the only derived type used by <code class="classname">shared_ptr<Ptr></code>
+and it is never used by <code class="classname">shared_ptr</code>, which uses one of
+the following types, depending on how the shared_ptr is constructed.
+ </p></dd><dt><span class="term"><code class="classname">_Sp_counted_ptr<Ptr, Lp></code></span></dt><dd><p>
+Inherits from _Sp_counted_base and stores a pointer of type <span class="type">Ptr</span>,
+which is passed to <code class="function">delete</code> when the last reference is dropped.
+This is the simplest form and is used when there is no custom deleter or
+allocator.
+ </p></dd><dt><span class="term"><code class="classname">_Sp_counted_deleter<Ptr, Deleter, Alloc></code></span></dt><dd><p>
+Inherits from _Sp_counted_ptr and adds support for custom deleter and
+allocator. Empty Base Optimization is used for the allocator. This class
+is used even when the user only provides a custom deleter, in which case
+<code class="classname">allocator</code> is used as the allocator.
+ </p></dd><dt><span class="term"><code class="classname">_Sp_counted_ptr_inplace<Tp, Alloc, Lp></code></span></dt><dd><p>
+Used by <code class="code">allocate_shared</code> and <code class="code">make_shared</code>.
+Contains aligned storage to hold an object of type <span class="type">Tp</span>,
+which is constructed in-place with placement <code class="function">new</code>.
+Has a variadic template constructor allowing any number of arguments to
+be forwarded to <span class="type">Tp</span>'s constructor.
+Unlike the other <code class="classname">_Sp_counted_*</code> classes, this one is parameterized on the
+type of object, not the type of pointer; this is purely a convenience
+that simplifies the implementation slightly.
+ </p></dd></dl></div></div><div class="sect3" lang="en" xml:lang="en"><div class="titlepage"><div><div><h4 class="title"><a id="id407287"></a>Thread Safety</h4></div></div></div><p>
+The interface of <code class="classname">tr1::shared_ptr</code> was extended for C++0x
+with support for rvalue-references and the other features from
+N2351. As with other libstdc++ headers shared by TR1 and C++0x,
+boost_shared_ptr.h uses conditional compilation, based on the macros
+<code class="constant">_GLIBCXX_INCLUDE_AS_CXX0X</code> and
+<code class="constant">_GLIBCXX_INCLUDE_AS_TR1</code>, to enable and disable
+features.
+ </p><p>
+C++0x-only features are: rvalue-ref/move support, allocator support,
+aliasing constructor, make_shared & allocate_shared. Additionally,
+the constructors taking <code class="classname">auto_ptr</code> parameters are
+deprecated in C++0x mode.
+ </p><p>
+The
+<a class="ulink" href="http://boost.org/libs/smart_ptr/shared_ptr.htm#ThreadSafety" target="_top">Thread
+Safety</a> section of the Boost shared_ptr documentation says "shared_ptr
+objects offer the same level of thread safety as built-in types."
+The implementation must ensure that concurrent updates to separate shared_ptr
+instances are correct even when those instances share a reference count e.g.
+</p><pre class="programlisting">
+shared_ptr<A> a(new A);
+shared_ptr<A> b(a);
+
+// Thread 1 // Thread 2
+ a.reset(); b.reset();
+</pre><p>
+The dynamically-allocated object must be destroyed by exactly one of the
+threads. Weak references make things even more interesting.
+The shared state used to implement shared_ptr must be transparent to the
+user and invariants must be preserved at all times.
+The key pieces of shared state are the strong and weak reference counts.
+Updates to these need to be atomic and visible to all threads to ensure
+correct cleanup of the managed resource (which is, after all, shared_ptr's
+job!)
+On multi-processor systems memory synchronisation may be needed so that
+reference-count updates and the destruction of the managed resource are
+race-free.
+</p><p>
+The function <code class="function">_Sp_counted_base::_M_add_ref_lock()</code>, called when
+obtaining a shared_ptr from a weak_ptr, has to test if the managed
+resource still exists and either increment the reference count or throw
+<code class="classname">bad_weak_ptr</code>.
+In a multi-threaded program there is a potential race condition if the last
+reference is dropped (and the managed resource destroyed) between testing
+the reference count and incrementing it, which could result in a shared_ptr
+pointing to invalid memory.
+</p><p>
+The Boost shared_ptr (as used in GCC) features a clever lock-free
+algorithm to avoid the race condition, but this relies on the
+processor supporting an atomic <span class="emphasis"><em>Compare-And-Swap</em></span>
+instruction. For other platforms there are fall-backs using mutex
+locks. Boost (as of version 1.35) includes several different
+implementations and the preprocessor selects one based on the
+compiler, standard library, platform etc. For the version of
+shared_ptr in libstdc++ the compiler and library are fixed, which
+makes things much simpler: we have an atomic CAS or we don't, see Lock
+Policy below for details.
+</p></div><div class="sect3" lang="en" xml:lang="en"><div class="titlepage"><div><div><h4 class="title"><a id="id402403"></a>Selecting Lock Policy</h4></div></div></div><p>
+ </p><p>
+There is a single <code class="classname">_Sp_counted_base</code> class,
+which is a template parameterized on the enum
+<span class="type">__gnu_cxx::_Lock_policy</span>. The entire family of classes is
+parameterized on the lock policy, right up to
+<code class="classname">__shared_ptr</code>, <code class="classname">__weak_ptr</code> and
+<code class="classname">__enable_shared_from_this</code>. The actual
+<code class="classname">std::shared_ptr</code> class inherits from
+<code class="classname">__shared_ptr</code> with the lock policy parameter
+selected automatically based on the thread model and platform that
+libstdc++ is configured for, so that the best available template
+specialization will be used. This design is necessary because it would
+not be conforming for <code class="classname">shared_ptr</code> to have an
+extra template parameter, even if it had a default value. The
+available policies are:
+ </p><div class="orderedlist"><ol type="1"><li><p>
+ <span class="type">_S_Atomic</span>
+ </p><p>
+Selected when GCC supports a builtin atomic compare-and-swap operation
+on the target processor (see <a class="ulink" href="http://gcc.gnu.org/onlinedocs/gcc/Atomic-Builtins.html" target="_top">Atomic
+Builtins</a>.) The reference counts are maintained using a lock-free
+algorithm and GCC's atomic builtins, which provide the required memory
+synchronisation.
+ </p></li><li><p>
+ <span class="type">_S_Mutex</span>
+ </p><p>
+The _Sp_counted_base specialization for this policy contains a mutex,
+which is locked in add_ref_lock(). This policy is used when GCC's atomic
+builtins aren't available so explicit memory barriers are needed in places.
+ </p></li><li><p>
+ <span class="type">_S_Single</span>
+ </p><p>
+This policy uses a non-reentrant add_ref_lock() with no locking. It is
+used when libstdc++ is built without <code class="literal">--enable-threads</code>.
+ </p></li></ol></div><p>
+ For all three policies, reference count increments and
+ decrements are done via the functions in
+ <code class="filename">ext/atomicity.h</code>, which detect if the program
+ is multi-threaded. If only one thread of execution exists in
+ the program then less expensive non-atomic operations are used.
+ </p></div><div class="sect3" lang="en" xml:lang="en"><div class="titlepage"><div><div><h4 class="title"><a id="id406621"></a>Dual C++0x and TR1 Implementation</h4></div></div></div><p>
+The classes derived from <code class="classname">_Sp_counted_base</code> (see Class Hierarchy
+below) and <code class="classname">__shared_count</code> are implemented separately for C++0x
+and TR1, in <code class="filename">bits/boost_sp_shared_count.h</code> and
+<code class="filename">tr1/boost_sp_shared_count.h</code> respectively. All other classes
+including <code class="classname">_Sp_counted_base</code> are shared by both implementations.
+</p><p>
+The TR1 implementation is considered relatively stable, so is unlikely to
+change unless bug fixes require it. If the code that is common to both
+C++0x and TR1 modes needs to diverge further then it might be necessary to
+duplicate additional classes and only make changes to the C++0x versions.
+</p></div><div class="sect3" lang="en" xml:lang="en"><div class="titlepage"><div><div><h4 class="title"><a id="id396482"></a>Related functions and classes</h4></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>,
+<code class="code">const_pointer_cast</code></span></dt><dd><p>
+As noted in N2351, these functions can be implemented non-intrusively using
+the alias constructor. However the aliasing constructor is only available
+in C++0x mode, so in TR1 mode these casts rely on three non-standard
+constructors in shared_ptr and __shared_ptr.
+In C++0x mode these constructors and the related tag types are not needed.
+ </p></dd><dt><span class="term"><code class="code">enable_shared_from_this</code></span></dt><dd><p>
+The clever overload to detect a base class of type
+<code class="code">enable_shared_from_this</code> comes straight from Boost.
+There is an extra overload for <code class="code">__enable_shared_from_this</code> to
+work smoothly with <code class="code">__shared_ptr<Tp, Lp></code> using any lock
+policy.
+ </p></dd><dt><span class="term"><code class="code">make_shared</code>, <code class="code">allocate_shared</code></span></dt><dd><p>
+<code class="code">make_shared</code> simply forwards to <code class="code">allocate_shared</code>
+with <code class="code">std::allocator</code> as the allocator.
+Although these functions can be implemented non-intrusively using the
+alias constructor, if they have access to the implementation then it is
+possible to save storage and reduce the number of heap allocations. The
+newly constructed object and the _Sp_counted_* can be allocated in a single
+block and the standard says implementations are "encouraged, but not required,"
+to do so. This implementation provides additional non-standard constructors
+(selected with the type <code class="code">_Sp_make_shared_tag</code>) which create an
+object of type <code class="code">_Sp_counted_ptr_inplace</code> to hold the new object.
+The returned <code class="code">shared_ptr<A></code> needs to know the address of the
+new <code class="code">A</code> object embedded in the <code class="code">_Sp_counted_ptr_inplace</code>,
+but it has no way to access it.
+This implementation uses a "covert channel" to return the address of the
+embedded object when <code class="code">get_deleter<_Sp_make_shared_tag>()</code>
+is called. Users should not try to use this.
+As well as the extra constructors, this implementation also needs some
+members of _Sp_counted_deleter to be protected where they could otherwise
+be private.
+ </p></dd></dl></div></div></div><div class="sect2" lang="en" xml:lang="en"><div class="titlepage"><div><div><h3 class="title"><a id="shared_ptr.using"></a>Use</h3></div></div></div><div class="sect3" lang="en" xml:lang="en"><div class="titlepage"><div><div><h4 class="title"><a id="id397112"></a>Examples</h4></div></div></div><p>
+ Examples of use can be found in the testsuite, under
+ <code class="filename">testsuite/tr1/2_general_utilities/shared_ptr</code>.
+ </p></div><div class="sect3" lang="en" xml:lang="en"><div class="titlepage"><div><div><h4 class="title"><a id="id403999"></a>Unresolved Issues</h4></div></div></div><p>
+ The resolution to C++ Standard Library issue <a class="ulink" href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-active.html#674" target="_top">674</a>,
+ "shared_ptr interface changes for consistency with N1856" will
+ need to be implemented after it is accepted into the working
+ paper. Issue <a class="ulink" href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-active.html#743" target="_top">743</a>
+ might also require changes.
+ </p><p>
+ The <span class="type">_S_single</span> policy uses atomics when used in MT
+ code, because it uses the same dispatcher functions that check
+ <code class="function">__gthread_active_p()</code>. This could be
+ addressed by providing template specialisations for some members
+ of <code class="classname">_Sp_counted_base<_S_single></code>.
+ </p><p>
+ Unlike Boost, this implementation does not use separate classes
+ for the pointer+deleter and pointer+deleter+allocator cases in
+ C++0x mode, combining both into _Sp_counted_deleter and using
+ <code class="classname">allocator</code> when the user doesn't specify
+ an allocator. If it was found to be beneficial an additional
+ class could easily be added. With the current implementation,
+ the _Sp_counted_deleter and __shared_count constructors taking a
+ custom deleter but no allocator are technically redundant and
+ could be removed, changing callers to always specify an
+ allocator. If a separate pointer+deleter class was added the
+ __shared_count constructor would be needed, so it has been kept
+ for now.
+ </p><p>
+ The hack used to get the address of the managed object from
+ <code class="function">_Sp_counted_ptr_inplace::_M_get_deleter()</code>
+ is accessible to users. This could be prevented if
+ <code class="function">get_deleter<_Sp_make_shared_tag>()</code>
+ always returned NULL, since the hack only needs to work at a
+ lower level, not in the public API. This wouldn't be difficult,
+ but hasn't been done since there is no danger of accidental
+ misuse: users already know they are relying on unsupported
+ features if they refer to implementation details such as
+ _Sp_make_shared_tag.
+ </p><p>
+ tr1::_Sp_deleter could be a private member of tr1::__shared_count but it
+ would alter the ABI.
+ </p><p>
+ Exposing the alias constructor in TR1 mode could simplify the
+ *_pointer_cast functions. Constructor could be private in TR1
+ mode, with the cast functions as friends.
+ </p></div></div><div class="sect2" lang="en" xml:lang="en"><div class="titlepage"><div><div><h3 class="title"><a id="shared_ptr.ack"></a>Acknowledgments</h3></div></div></div><p>
+ The original authors of the Boost shared_ptr, which is really nice
+ code to work with, Peter Dimov in particular for his help and
+ invaluable advice on thread safety. Phillip Jordan and Paolo
+ Carlini for the lock policy implementation.
+ </p></div><div class="bibliography"><div class="titlepage"><div><div><h3 class="title"><a id="shared_ptr.biblio"></a>Bibliography</h3></div></div></div><div class="biblioentry"><a id="id399126"></a><p>[<abbr class="abbrev">
+ n2351
+ </abbr>] <span class="title"><i>
+ Improving shared_ptr for C++0x, Revision 2
+ </i>. </span><span class="subtitle">
+ N2351
+ . </span><span class="biblioid">
+ <a class="ulink" href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2007/n2351.htm" target="_top">
+ </a>
+ . </span></p></div><div class="biblioentry"><a id="id406438"></a><p>[<abbr class="abbrev">
+ n2456
+ </abbr>] <span class="title"><i>
+ C++ Standard Library Active Issues List (Revision R52)
+ </i>. </span><span class="subtitle">
+ N2456
+ . </span><span class="biblioid">
+ <a class="ulink" href="http://open-std.org/jtc1/sc22/wg21/docs/papers/2007/n2456.html" target="_top">
+ </a>
+ . </span></p></div><div class="biblioentry"><a id="id406462"></a><p>[<abbr class="abbrev">
+ n2461
+ </abbr>] <span class="title"><i>
+ Working Draft, Standard for Programming Language C++
+ </i>. </span><span class="subtitle">
+ N2461
+ . </span><span class="biblioid">
+ <a class="ulink" href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2007/n2461.pdf" target="_top">
+ </a>
+ . </span></p></div><div class="biblioentry"><a id="id413273"></a><p>[<abbr class="abbrev">
+ boostshared_ptr
+ </abbr>] <span class="title"><i>
+ Boost C++ Libraries documentation - shared_ptr class template
+ </i>. </span><span class="subtitle">
+ N2461
+ . </span><span class="biblioid">
+ <a class="ulink" href="http://boost.org/libs/smart_ptr/shared_ptr.htm" target="_top">shared_ptr
+ </a>
+ . </span></p></div></div></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="auto_ptr.html">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="memory.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">auto_ptr </td><td width="20%" align="center"><a accesskey="h" href="../spine.html">Home</a></td><td width="40%" align="right" valign="top"> Chapter 12. Traits</td></tr></table></div></body></html>
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