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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 V1.75.2" /><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" title="shared_ptr"><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>
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The shared_ptr class template stores a pointer, usually obtained via new,
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and implements shared ownership semantics.
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</p><div class="sect2" title="Requirements"><div class="titlepage"><div><div><h3 class="title"><a id="shared_ptr.req"></a>Requirements</h3></div></div></div><p>
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</p><p>
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The standard deliberately doesn't require a reference-counted
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implementation, allowing other techniques such as a
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circular-linked-list.
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</p><p>
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At the time of writing the C++0x working paper doesn't mention how
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threads affect shared_ptr, but it is likely to follow the existing
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practice set by <code class="classname">boost::shared_ptr</code>. The
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shared_ptr in libstdc++ is derived from Boost's, so the same rules
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apply.
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</p><p>
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</p></div><div class="sect2" title="Design Issues"><div class="titlepage"><div><div><h3 class="title"><a id="shared_ptr.design_issues"></a>Design Issues</h3></div></div></div><p>
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The <code class="classname">shared_ptr</code> code is kindly donated to GCC by the Boost
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project and the original authors of the code. The basic design and
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algorithms are from Boost, the notes below describe details specific to
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the GCC implementation. Names have been uglified in this implementation,
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but the design should be recognisable to anyone familiar with the Boost
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1.32 shared_ptr.
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</p><p>
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The basic design is an abstract base class, <code class="code">_Sp_counted_base</code> that
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does the reference-counting and calls virtual functions when the count
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drops to zero.
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Derived classes override those functions to destroy resources in a context
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where the correct dynamic type is known. This is an application of the
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technique known as type erasure.
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</p></div><div class="sect2" title="Implementation"><div class="titlepage"><div><div><h3 class="title"><a id="shared_ptr.impl"></a>Implementation</h3></div></div></div><div class="sect3" title="Class Hierarchy"><div class="titlepage"><div><div><h4 class="title"><a id="id628890"></a>Class Hierarchy</h4></div></div></div><p>
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A <code class="classname">shared_ptr<T></code> contains a pointer of
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type <span class="type">T*</span> and an object of type
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<code class="classname">__shared_count</code>. The shared_count contains a
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pointer of type <span class="type">_Sp_counted_base*</span> which points to the
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object that maintains the reference-counts and destroys the managed
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resource.
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</p><div class="variablelist"><dl><dt><span class="term"><code class="classname">_Sp_counted_base<Lp></code></span></dt><dd><p>
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The base of the hierarchy is parameterized on the lock policy alone.
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_Sp_counted_base doesn't depend on the type of pointer being managed,
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it only maintains the reference counts and calls virtual functions when
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the counts drop to zero. The managed object is destroyed when the last
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strong reference is dropped, but the _Sp_counted_base itself must exist
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until the last weak reference is dropped.
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</p></dd><dt><span class="term"><code class="classname">_Sp_counted_base_impl<Ptr, Deleter, Lp></code></span></dt><dd><p>
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Inherits from _Sp_counted_base and stores a pointer of type <span class="type">Ptr</span>
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and a deleter of type <code class="code">Deleter</code>. <code class="code">_Sp_deleter</code> is
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used when the user doesn't supply a custom deleter. Unlike Boost's, this
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default deleter is not "checked" because GCC already issues a warning if
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<code class="function">delete</code> is used with an incomplete type.
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This is the only derived type used by <code class="classname">shared_ptr<Ptr></code>
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and it is never used by <code class="classname">shared_ptr</code>, which uses one of
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the following types, depending on how the shared_ptr is constructed.
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</p></dd><dt><span class="term"><code class="classname">_Sp_counted_ptr<Ptr, Lp></code></span></dt><dd><p>
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Inherits from _Sp_counted_base and stores a pointer of type <span class="type">Ptr</span>,
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which is passed to <code class="function">delete</code> when the last reference is dropped.
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This is the simplest form and is used when there is no custom deleter or
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allocator.
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</p></dd><dt><span class="term"><code class="classname">_Sp_counted_deleter<Ptr, Deleter, Alloc></code></span></dt><dd><p>
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Inherits from _Sp_counted_ptr and adds support for custom deleter and
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allocator. Empty Base Optimization is used for the allocator. This class
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is used even when the user only provides a custom deleter, in which case
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<code class="classname">allocator</code> is used as the allocator.
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</p></dd><dt><span class="term"><code class="classname">_Sp_counted_ptr_inplace<Tp, Alloc, Lp></code></span></dt><dd><p>
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Used by <code class="code">allocate_shared</code> and <code class="code">make_shared</code>.
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Contains aligned storage to hold an object of type <span class="type">Tp</span>,
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which is constructed in-place with placement <code class="function">new</code>.
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Has a variadic template constructor allowing any number of arguments to
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be forwarded to <span class="type">Tp</span>'s constructor.
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Unlike the other <code class="classname">_Sp_counted_*</code> classes, this one is parameterized on the
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type of object, not the type of pointer; this is purely a convenience
|
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that simplifies the implementation slightly.
|
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</p></dd></dl></div></div><div class="sect3" title="Thread Safety"><div class="titlepage"><div><div><h4 class="title"><a id="id612958"></a>Thread Safety</h4></div></div></div><p>
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The interface of <code class="classname">tr1::shared_ptr</code> was extended for C++0x
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with support for rvalue-references and the other features from
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N2351. As with other libstdc++ headers shared by TR1 and C++0x,
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boost_shared_ptr.h uses conditional compilation, based on the macros
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<code class="constant">_GLIBCXX_INCLUDE_AS_CXX0X</code> and
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<code class="constant">_GLIBCXX_INCLUDE_AS_TR1</code>, to enable and disable
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features.
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</p><p>
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C++0x-only features are: rvalue-ref/move support, allocator support,
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aliasing constructor, make_shared & allocate_shared. Additionally,
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the constructors taking <code class="classname">auto_ptr</code> parameters are
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deprecated in C++0x mode.
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</p><p>
|
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The
|
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<a class="ulink" href="http://boost.org/libs/smart_ptr/shared_ptr.htm#ThreadSafety" target="_top">Thread
|
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Safety</a> section of the Boost shared_ptr documentation says "shared_ptr
|
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objects offer the same level of thread safety as built-in types."
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The implementation must ensure that concurrent updates to separate shared_ptr
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instances are correct even when those instances share a reference count e.g.
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</p><pre class="programlisting">
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shared_ptr<A> a(new A);
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shared_ptr<A> b(a);
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// Thread 1 // Thread 2
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a.reset(); b.reset();
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</pre><p>
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The dynamically-allocated object must be destroyed by exactly one of the
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threads. Weak references make things even more interesting.
|
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The shared state used to implement shared_ptr must be transparent to the
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user and invariants must be preserved at all times.
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The key pieces of shared state are the strong and weak reference counts.
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Updates to these need to be atomic and visible to all threads to ensure
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correct cleanup of the managed resource (which is, after all, shared_ptr's
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job!)
|
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On multi-processor systems memory synchronisation may be needed so that
|
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reference-count updates and the destruction of the managed resource are
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race-free.
|
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</p><p>
|
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The function <code class="function">_Sp_counted_base::_M_add_ref_lock()</code>, called when
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obtaining a shared_ptr from a weak_ptr, has to test if the managed
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resource still exists and either increment the reference count or throw
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<code class="classname">bad_weak_ptr</code>.
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In a multi-threaded program there is a potential race condition if the last
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reference is dropped (and the managed resource destroyed) between testing
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the reference count and incrementing it, which could result in a shared_ptr
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pointing to invalid memory.
|
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</p><p>
|
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The Boost shared_ptr (as used in GCC) features a clever lock-free
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algorithm to avoid the race condition, but this relies on the
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processor supporting an atomic <span class="emphasis"><em>Compare-And-Swap</em></span>
|
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instruction. For other platforms there are fall-backs using mutex
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locks. Boost (as of version 1.35) includes several different
|
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implementations and the preprocessor selects one based on the
|
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compiler, standard library, platform etc. For the version of
|
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shared_ptr in libstdc++ the compiler and library are fixed, which
|
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makes things much simpler: we have an atomic CAS or we don't, see Lock
|
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Policy below for details.
|
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</p></div><div class="sect3" title="Selecting Lock Policy"><div class="titlepage"><div><div><h4 class="title"><a id="id637767"></a>Selecting Lock Policy</h4></div></div></div><p>
|
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</p><p>
|
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There is a single <code class="classname">_Sp_counted_base</code> class,
|
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which is a template parameterized on the enum
|
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<span class="type">__gnu_cxx::_Lock_policy</span>. The entire family of classes is
|
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parameterized on the lock policy, right up to
|
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<code class="classname">__shared_ptr</code>, <code class="classname">__weak_ptr</code> and
|
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|
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<code class="classname">__enable_shared_from_this</code>. The actual
|
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|
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<code class="classname">std::shared_ptr</code> class inherits from
|
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|
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<code class="classname">__shared_ptr</code> with the lock policy parameter
|
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|
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selected automatically based on the thread model and platform that
|
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|
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libstdc++ is configured for, so that the best available template
|
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specialization will be used. This design is necessary because it would
|
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not be conforming for <code class="classname">shared_ptr</code> to have an
|
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extra template parameter, even if it had a default value. The
|
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available policies are:
|
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|
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</p><div class="orderedlist"><ol class="orderedlist" type="1"><li class="listitem"><p>
|
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<span class="type">_S_Atomic</span>
|
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|
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</p><p>
|
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Selected when GCC supports a builtin atomic compare-and-swap operation
|
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on the target processor (see <a class="ulink" href="http://gcc.gnu.org/onlinedocs/gcc/Atomic-Builtins.html" target="_top">Atomic
|
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|
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Builtins</a>.) The reference counts are maintained using a lock-free
|
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algorithm and GCC's atomic builtins, which provide the required memory
|
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synchronisation.
|
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</p></li><li class="listitem"><p>
|
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|
|
<span class="type">_S_Mutex</span>
|
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|
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</p><p>
|
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The _Sp_counted_base specialization for this policy contains a mutex,
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which is locked in add_ref_lock(). This policy is used when GCC's atomic
|
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|
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builtins aren't available so explicit memory barriers are needed in places.
|
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|
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</p></li><li class="listitem"><p>
|
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|
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<span class="type">_S_Single</span>
|
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|
|
</p><p>
|
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This policy uses a non-reentrant add_ref_lock() with no locking. It is
|
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|
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used when libstdc++ is built without <code class="literal">--enable-threads</code>.
|
167 |
|
|
</p></li></ol></div><p>
|
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|
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For all three policies, reference count increments and
|
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decrements are done via the functions in
|
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<code class="filename">ext/atomicity.h</code>, which detect if the program
|
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is multi-threaded. If only one thread of execution exists in
|
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the program then less expensive non-atomic operations are used.
|
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</p></div><div class="sect3" title="Dual C++0x and TR1 Implementation"><div class="titlepage"><div><div><h4 class="title"><a id="id618991"></a>Dual C++0x and TR1 Implementation</h4></div></div></div><p>
|
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The classes derived from <code class="classname">_Sp_counted_base</code> (see Class Hierarchy
|
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|
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below) and <code class="classname">__shared_count</code> are implemented separately for C++0x
|
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|
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and TR1, in <code class="filename">bits/boost_sp_shared_count.h</code> and
|
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<code class="filename">tr1/boost_sp_shared_count.h</code> respectively. All other classes
|
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including <code class="classname">_Sp_counted_base</code> are shared by both implementations.
|
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</p><p>
|
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The TR1 implementation is considered relatively stable, so is unlikely to
|
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change unless bug fixes require it. If the code that is common to both
|
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C++0x and TR1 modes needs to diverge further then it might be necessary to
|
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duplicate additional classes and only make changes to the C++0x versions.
|
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|
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</p></div><div class="sect3" title="Related functions and classes"><div class="titlepage"><div><div><h4 class="title"><a id="id589355"></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>,
|
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|
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<code class="code">const_pointer_cast</code></span></dt><dd><p>
|
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|
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As noted in N2351, these functions can be implemented non-intrusively using
|
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the alias constructor. However the aliasing constructor is only available
|
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|
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in C++0x mode, so in TR1 mode these casts rely on three non-standard
|
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constructors in shared_ptr and __shared_ptr.
|
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|
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In C++0x mode these constructors and the related tag types are not needed.
|
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|
|
</p></dd><dt><span class="term"><code class="code">enable_shared_from_this</code></span></dt><dd><p>
|
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|
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The clever overload to detect a base class of type
|
193 |
|
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<code class="code">enable_shared_from_this</code> comes straight from Boost.
|
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There is an extra overload for <code class="code">__enable_shared_from_this</code> to
|
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|
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work smoothly with <code class="code">__shared_ptr<Tp, Lp></code> using any lock
|
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policy.
|
197 |
|
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</p></dd><dt><span class="term"><code class="code">make_shared</code>, <code class="code">allocate_shared</code></span></dt><dd><p>
|
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|
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<code class="code">make_shared</code> simply forwards to <code class="code">allocate_shared</code>
|
199 |
|
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with <code class="code">std::allocator</code> as the allocator.
|
200 |
|
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Although these functions can be implemented non-intrusively using the
|
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alias constructor, if they have access to the implementation then it is
|
202 |
|
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possible to save storage and reduce the number of heap allocations. The
|
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newly constructed object and the _Sp_counted_* can be allocated in a single
|
204 |
|
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block and the standard says implementations are "encouraged, but not required,"
|
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to do so. This implementation provides additional non-standard constructors
|
206 |
|
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(selected with the type <code class="code">_Sp_make_shared_tag</code>) which create an
|
207 |
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object of type <code class="code">_Sp_counted_ptr_inplace</code> to hold the new object.
|
208 |
|
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The returned <code class="code">shared_ptr<A></code> needs to know the address of the
|
209 |
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new <code class="code">A</code> object embedded in the <code class="code">_Sp_counted_ptr_inplace</code>,
|
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but it has no way to access it.
|
211 |
|
|
This implementation uses a "covert channel" to return the address of the
|
212 |
|
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embedded object when <code class="code">get_deleter<_Sp_make_shared_tag>()</code>
|
213 |
|
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is called. Users should not try to use this.
|
214 |
|
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As well as the extra constructors, this implementation also needs some
|
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members of _Sp_counted_deleter to be protected where they could otherwise
|
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be private.
|
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|
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</p></dd></dl></div></div></div><div class="sect2" title="Use"><div class="titlepage"><div><div><h3 class="title"><a id="shared_ptr.using"></a>Use</h3></div></div></div><div class="sect3" title="Examples"><div class="titlepage"><div><div><h4 class="title"><a id="id619880"></a>Examples</h4></div></div></div><p>
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Examples of use can be found in the testsuite, under
|
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|
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<code class="filename">testsuite/tr1/2_general_utilities/shared_ptr</code>.
|
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|
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</p></div><div class="sect3" title="Unresolved Issues"><div class="titlepage"><div><div><h4 class="title"><a id="id653780"></a>Unresolved Issues</h4></div></div></div><p>
|
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|
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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>,
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"shared_ptr interface changes for consistency with N1856" will
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need to be implemented after it is accepted into the working
|
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paper. Issue <a class="ulink" href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-active.html#743" target="_top">743</a>
|
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might also require changes.
|
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|
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</p><p>
|
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The <span class="type">_S_single</span> policy uses atomics when used in MT
|
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code, because it uses the same dispatcher functions that check
|
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|
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<code class="function">__gthread_active_p()</code>. This could be
|
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addressed by providing template specialisations for some members
|
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|
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of <code class="classname">_Sp_counted_base<_S_single></code>.
|
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</p><p>
|
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Unlike Boost, this implementation does not use separate classes
|
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|
for the pointer+deleter and pointer+deleter+allocator cases in
|
235 |
|
|
C++0x mode, combining both into _Sp_counted_deleter and using
|
236 |
|
|
<code class="classname">allocator</code> when the user doesn't specify
|
237 |
|
|
an allocator. If it was found to be beneficial an additional
|
238 |
|
|
class could easily be added. With the current implementation,
|
239 |
|
|
the _Sp_counted_deleter and __shared_count constructors taking a
|
240 |
|
|
custom deleter but no allocator are technically redundant and
|
241 |
|
|
could be removed, changing callers to always specify an
|
242 |
|
|
allocator. If a separate pointer+deleter class was added the
|
243 |
|
|
__shared_count constructor would be needed, so it has been kept
|
244 |
|
|
for now.
|
245 |
|
|
</p><p>
|
246 |
|
|
The hack used to get the address of the managed object from
|
247 |
|
|
<code class="function">_Sp_counted_ptr_inplace::_M_get_deleter()</code>
|
248 |
|
|
is accessible to users. This could be prevented if
|
249 |
|
|
<code class="function">get_deleter<_Sp_make_shared_tag>()</code>
|
250 |
|
|
always returned NULL, since the hack only needs to work at a
|
251 |
|
|
lower level, not in the public API. This wouldn't be difficult,
|
252 |
|
|
but hasn't been done since there is no danger of accidental
|
253 |
|
|
misuse: users already know they are relying on unsupported
|
254 |
|
|
features if they refer to implementation details such as
|
255 |
|
|
_Sp_make_shared_tag.
|
256 |
|
|
</p><p>
|
257 |
|
|
tr1::_Sp_deleter could be a private member of tr1::__shared_count but it
|
258 |
|
|
would alter the ABI.
|
259 |
|
|
</p><p>
|
260 |
|
|
Exposing the alias constructor in TR1 mode could simplify the
|
261 |
|
|
*_pointer_cast functions. Constructor could be private in TR1
|
262 |
|
|
mode, with the cast functions as friends.
|
263 |
|
|
</p></div></div><div class="sect2" title="Acknowledgments"><div class="titlepage"><div><div><h3 class="title"><a id="shared_ptr.ack"></a>Acknowledgments</h3></div></div></div><p>
|
264 |
|
|
The original authors of the Boost shared_ptr, which is really nice
|
265 |
|
|
code to work with, Peter Dimov in particular for his help and
|
266 |
|
|
invaluable advice on thread safety. Phillip Jordan and Paolo
|
267 |
|
|
Carlini for the lock policy implementation.
|
268 |
|
|
</p></div><div class="bibliography" title="Bibliography"><div class="titlepage"><div><div><h3 class="title"><a id="shared_ptr.biblio"></a>Bibliography</h3></div></div></div><div class="biblioentry" title="Improving shared_ptr for C++0x, Revision 2"><a id="id678655"></a><p>[<abbr class="abbrev">
|
269 |
|
|
n2351
|
270 |
|
|
</abbr>] <span class="title"><i>
|
271 |
|
|
Improving shared_ptr for C++0x, Revision 2
|
272 |
|
|
</i>. </span><span class="subtitle">
|
273 |
|
|
N2351
|
274 |
|
|
. </span><span class="biblioid">
|
275 |
|
|
<a class="ulink" href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2007/n2351.htm" target="_top">
|
276 |
|
|
</a>
|
277 |
|
|
. </span></p></div><div class="biblioentry" title="C++ Standard Library Active Issues List (Revision R52)"><a id="id678679"></a><p>[<abbr class="abbrev">
|
278 |
|
|
n2456
|
279 |
|
|
</abbr>] <span class="title"><i>
|
280 |
|
|
C++ Standard Library Active Issues List (Revision R52)
|
281 |
|
|
</i>. </span><span class="subtitle">
|
282 |
|
|
N2456
|
283 |
|
|
. </span><span class="biblioid">
|
284 |
|
|
<a class="ulink" href="http://open-std.org/jtc1/sc22/wg21/docs/papers/2007/n2456.html" target="_top">
|
285 |
|
|
</a>
|
286 |
|
|
. </span></p></div><div class="biblioentry" title="Working Draft, Standard for Programming Language C++"><a id="id661793"></a><p>[<abbr class="abbrev">
|
287 |
|
|
n2461
|
288 |
|
|
</abbr>] <span class="title"><i>
|
289 |
|
|
Working Draft, Standard for Programming Language C++
|
290 |
|
|
</i>. </span><span class="subtitle">
|
291 |
|
|
N2461
|
292 |
|
|
. </span><span class="biblioid">
|
293 |
|
|
<a class="ulink" href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2007/n2461.pdf" target="_top">
|
294 |
|
|
</a>
|
295 |
|
|
. </span></p></div><div class="biblioentry" title="Boost C++ Libraries documentation - shared_ptr class template"><a id="id612415"></a><p>[<abbr class="abbrev">
|
296 |
|
|
boostshared_ptr
|
297 |
|
|
</abbr>] <span class="title"><i>
|
298 |
|
|
Boost C++ Libraries documentation - shared_ptr class template
|
299 |
|
|
</i>. </span><span class="subtitle">
|
300 |
|
|
N2461
|
301 |
|
|
. </span><span class="biblioid">
|
302 |
|
|
<a class="ulink" href="http://boost.org/libs/smart_ptr/shared_ptr.htm" target="_top">shared_ptr
|
303 |
|
|
</a>
|
304 |
|
|
. </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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