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diff --git a/libstdc++-v3/doc/html/manual/shared_ptr.html b/libstdc++-v3/doc/html/manual/shared_ptr.html new file mode 100644 index 00000000000..21d38d3e3d6 --- /dev/null +++ b/libstdc++-v3/doc/html/manual/shared_ptr.html @@ -0,0 +1,304 @@ +<?xml version="1.0" encoding="UTF-8" standalone="no"?> +<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"> +<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 V1.73.2" /><meta name="keywords" content=" ISO C++ , shared_ptr " /><meta name="keywords" content=" ISO C++ , library " /><link rel="start" href="../spine.html" title="The GNU C++ Library Documentation" /><link rel="up" href="bk01pt04ch11.html" title="Chapter 11. Memory" /><link rel="prev" href="auto_ptr.html" title="auto_ptr" /><link rel="next" href="bk01pt04ch12.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="bk01pt04ch12.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="id461761"></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="id360970"></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="id396141"></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="id420953"></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="id454146"></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="id404984"></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="id398405"></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="id485662"></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="id485686"></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="id403091"></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="id514547"></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="bk01pt04ch11.html">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="bk01pt04ch12.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. 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