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3 <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>
4 The shared_ptr class template stores a pointer, usually obtained via new,
5 and implements shared ownership semantics.
6 </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>
8 The standard deliberately doesn't require a reference-counted
9 implementation, allowing other techniques such as a
12 At the time of writing the C++0x working paper doesn't mention how
13 threads affect shared_ptr, but it is likely to follow the existing
14 practice set by <code class="classname">boost::shared_ptr</code>. The
15 shared_ptr in libstdc++ is derived from Boost's, so the same rules
18 </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>
19 The <code class="classname">shared_ptr</code> code is kindly donated to GCC by the Boost
20 project and the original authors of the code. The basic design and
21 algorithms are from Boost, the notes below describe details specific to
22 the GCC implementation. Names have been uglified in this implementation,
23 but the design should be recognisable to anyone familiar with the Boost
26 The basic design is an abstract base class, <code class="code">_Sp_counted_base</code> that
27 does the reference-counting and calls virtual functions when the count
29 Derived classes override those functions to destroy resources in a context
30 where the correct dynamic type is known. This is an application of the
31 technique known as type erasure.
32 </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="id407519"></a>Class Hierarchy</h4></div></div></div><p>
33 A <code class="classname">shared_ptr<T></code> contains a pointer of
34 type <span class="type">T*</span> and an object of type
35 <code class="classname">__shared_count</code>. The shared_count contains a
36 pointer of type <span class="type">_Sp_counted_base*</span> which points to the
37 object that maintains the reference-counts and destroys the managed
39 </p><div class="variablelist"><dl><dt><span class="term"><code class="classname">_Sp_counted_base<Lp></code></span></dt><dd><p>
40 The base of the hierarchy is parameterized on the lock policy alone.
41 _Sp_counted_base doesn't depend on the type of pointer being managed,
42 it only maintains the reference counts and calls virtual functions when
43 the counts drop to zero. The managed object is destroyed when the last
44 strong reference is dropped, but the _Sp_counted_base itself must exist
45 until the last weak reference is dropped.
46 </p></dd><dt><span class="term"><code class="classname">_Sp_counted_base_impl<Ptr, Deleter, Lp></code></span></dt><dd><p>
47 Inherits from _Sp_counted_base and stores a pointer of type <span class="type">Ptr</span>
48 and a deleter of type <code class="code">Deleter</code>. <code class="code">_Sp_deleter</code> is
49 used when the user doesn't supply a custom deleter. Unlike Boost's, this
50 default deleter is not "checked" because GCC already issues a warning if
51 <code class="function">delete</code> is used with an incomplete type.
52 This is the only derived type used by <code class="classname">shared_ptr<Ptr></code>
53 and it is never used by <code class="classname">shared_ptr</code>, which uses one of
54 the following types, depending on how the shared_ptr is constructed.
55 </p></dd><dt><span class="term"><code class="classname">_Sp_counted_ptr<Ptr, Lp></code></span></dt><dd><p>
56 Inherits from _Sp_counted_base and stores a pointer of type <span class="type">Ptr</span>,
57 which is passed to <code class="function">delete</code> when the last reference is dropped.
58 This is the simplest form and is used when there is no custom deleter or
60 </p></dd><dt><span class="term"><code class="classname">_Sp_counted_deleter<Ptr, Deleter, Alloc></code></span></dt><dd><p>
61 Inherits from _Sp_counted_ptr and adds support for custom deleter and
62 allocator. Empty Base Optimization is used for the allocator. This class
63 is used even when the user only provides a custom deleter, in which case
64 <code class="classname">allocator</code> is used as the allocator.
65 </p></dd><dt><span class="term"><code class="classname">_Sp_counted_ptr_inplace<Tp, Alloc, Lp></code></span></dt><dd><p>
66 Used by <code class="code">allocate_shared</code> and <code class="code">make_shared</code>.
67 Contains aligned storage to hold an object of type <span class="type">Tp</span>,
68 which is constructed in-place with placement <code class="function">new</code>.
69 Has a variadic template constructor allowing any number of arguments to
70 be forwarded to <span class="type">Tp</span>'s constructor.
71 Unlike the other <code class="classname">_Sp_counted_*</code> classes, this one is parameterized on the
72 type of object, not the type of pointer; this is purely a convenience
73 that simplifies the implementation slightly.
74 </p></dd></dl></div></div><div class="sect3" lang="en" xml:lang="en"><div class="titlepage"><div><div><h4 class="title"><a id="id419862"></a>Thread Safety</h4></div></div></div><p>
75 The interface of <code class="classname">tr1::shared_ptr</code> was extended for C++0x
76 with support for rvalue-references and the other features from
77 N2351. As with other libstdc++ headers shared by TR1 and C++0x,
78 boost_shared_ptr.h uses conditional compilation, based on the macros
79 <code class="constant">_GLIBCXX_INCLUDE_AS_CXX0X</code> and
80 <code class="constant">_GLIBCXX_INCLUDE_AS_TR1</code>, to enable and disable
83 C++0x-only features are: rvalue-ref/move support, allocator support,
84 aliasing constructor, make_shared & allocate_shared. Additionally,
85 the constructors taking <code class="classname">auto_ptr</code> parameters are
86 deprecated in C++0x mode.
89 <a class="ulink" href="http://boost.org/libs/smart_ptr/shared_ptr.htm#ThreadSafety" target="_top">Thread
90 Safety</a> section of the Boost shared_ptr documentation says "shared_ptr
91 objects offer the same level of thread safety as built-in types."
92 The implementation must ensure that concurrent updates to separate shared_ptr
93 instances are correct even when those instances share a reference count e.g.
94 </p><pre class="programlisting">
95 shared_ptr<A> a(new A);
96 shared_ptr<A> b(a);
98 // Thread 1 // Thread 2
101 The dynamically-allocated object must be destroyed by exactly one of the
102 threads. Weak references make things even more interesting.
103 The shared state used to implement shared_ptr must be transparent to the
104 user and invariants must be preserved at all times.
105 The key pieces of shared state are the strong and weak reference counts.
106 Updates to these need to be atomic and visible to all threads to ensure
107 correct cleanup of the managed resource (which is, after all, shared_ptr's
109 On multi-processor systems memory synchronisation may be needed so that
110 reference-count updates and the destruction of the managed resource are
113 The function <code class="function">_Sp_counted_base::_M_add_ref_lock()</code>, called when
114 obtaining a shared_ptr from a weak_ptr, has to test if the managed
115 resource still exists and either increment the reference count or throw
116 <code class="classname">bad_weak_ptr</code>.
117 In a multi-threaded program there is a potential race condition if the last
118 reference is dropped (and the managed resource destroyed) between testing
119 the reference count and incrementing it, which could result in a shared_ptr
120 pointing to invalid memory.
122 The Boost shared_ptr (as used in GCC) features a clever lock-free
123 algorithm to avoid the race condition, but this relies on the
124 processor supporting an atomic <span class="emphasis"><em>Compare-And-Swap</em></span>
125 instruction. For other platforms there are fall-backs using mutex
126 locks. Boost (as of version 1.35) includes several different
127 implementations and the preprocessor selects one based on the
128 compiler, standard library, platform etc. For the version of
129 shared_ptr in libstdc++ the compiler and library are fixed, which
130 makes things much simpler: we have an atomic CAS or we don't, see Lock
131 Policy below for details.
132 </p></div><div class="sect3" lang="en" xml:lang="en"><div class="titlepage"><div><div><h4 class="title"><a id="id395234"></a>Selecting Lock Policy</h4></div></div></div><p>
134 There is a single <code class="classname">_Sp_counted_base</code> class,
135 which is a template parameterized on the enum
136 <span class="type">__gnu_cxx::_Lock_policy</span>. The entire family of classes is
137 parameterized on the lock policy, right up to
138 <code class="classname">__shared_ptr</code>, <code class="classname">__weak_ptr</code> and
139 <code class="classname">__enable_shared_from_this</code>. The actual
140 <code class="classname">std::shared_ptr</code> class inherits from
141 <code class="classname">__shared_ptr</code> with the lock policy parameter
142 selected automatically based on the thread model and platform that
143 libstdc++ is configured for, so that the best available template
144 specialization will be used. This design is necessary because it would
145 not be conforming for <code class="classname">shared_ptr</code> to have an
146 extra template parameter, even if it had a default value. The
147 available policies are:
148 </p><div class="orderedlist"><ol type="1"><li><p>
149 <span class="type">_S_Atomic</span>
151 Selected when GCC supports a builtin atomic compare-and-swap operation
152 on the target processor (see <a class="ulink" href="http://gcc.gnu.org/onlinedocs/gcc/Atomic-Builtins.html" target="_top">Atomic
153 Builtins</a>.) The reference counts are maintained using a lock-free
154 algorithm and GCC's atomic builtins, which provide the required memory
157 <span class="type">_S_Mutex</span>
159 The _Sp_counted_base specialization for this policy contains a mutex,
160 which is locked in add_ref_lock(). This policy is used when GCC's atomic
161 builtins aren't available so explicit memory barriers are needed in places.
163 <span class="type">_S_Single</span>
165 This policy uses a non-reentrant add_ref_lock() with no locking. It is
166 used when libstdc++ is built without <code class="literal">--enable-threads</code>.
167 </p></li></ol></div><p>
168 For all three policies, reference count increments and
169 decrements are done via the functions in
170 <code class="filename">ext/atomicity.h</code>, which detect if the program
171 is multi-threaded. If only one thread of execution exists in
172 the program then less expensive non-atomic operations are used.
173 </p></div><div class="sect3" lang="en" xml:lang="en"><div class="titlepage"><div><div><h4 class="title"><a id="id427440"></a>Dual C++0x and TR1 Implementation</h4></div></div></div><p>
174 The classes derived from <code class="classname">_Sp_counted_base</code> (see Class Hierarchy
175 below) and <code class="classname">__shared_count</code> are implemented separately for C++0x
176 and TR1, in <code class="filename">bits/boost_sp_shared_count.h</code> and
177 <code class="filename">tr1/boost_sp_shared_count.h</code> respectively. All other classes
178 including <code class="classname">_Sp_counted_base</code> are shared by both implementations.
180 The TR1 implementation is considered relatively stable, so is unlikely to
181 change unless bug fixes require it. If the code that is common to both
182 C++0x and TR1 modes needs to diverge further then it might be necessary to
183 duplicate additional classes and only make changes to the C++0x versions.
184 </p></div><div class="sect3" lang="en" xml:lang="en"><div class="titlepage"><div><div><h4 class="title"><a id="id420169"></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>,
185 <code class="code">const_pointer_cast</code></span></dt><dd><p>
186 As noted in N2351, these functions can be implemented non-intrusively using
187 the alias constructor. However the aliasing constructor is only available
188 in C++0x mode, so in TR1 mode these casts rely on three non-standard
189 constructors in shared_ptr and __shared_ptr.
190 In C++0x mode these constructors and the related tag types are not needed.
191 </p></dd><dt><span class="term"><code class="code">enable_shared_from_this</code></span></dt><dd><p>
192 The clever overload to detect a base class of type
193 <code class="code">enable_shared_from_this</code> comes straight from Boost.
194 There is an extra overload for <code class="code">__enable_shared_from_this</code> to
195 work smoothly with <code class="code">__shared_ptr<Tp, Lp></code> using any lock
197 </p></dd><dt><span class="term"><code class="code">make_shared</code>, <code class="code">allocate_shared</code></span></dt><dd><p>
198 <code class="code">make_shared</code> simply forwards to <code class="code">allocate_shared</code>
199 with <code class="code">std::allocator</code> as the allocator.
200 Although these functions can be implemented non-intrusively using the
201 alias constructor, if they have access to the implementation then it is
202 possible to save storage and reduce the number of heap allocations. The
203 newly constructed object and the _Sp_counted_* can be allocated in a single
204 block and the standard says implementations are "encouraged, but not required,"
205 to do so. This implementation provides additional non-standard constructors
206 (selected with the type <code class="code">_Sp_make_shared_tag</code>) which create an
207 object of type <code class="code">_Sp_counted_ptr_inplace</code> to hold the new object.
208 The returned <code class="code">shared_ptr<A></code> needs to know the address of the
209 new <code class="code">A</code> object embedded in the <code class="code">_Sp_counted_ptr_inplace</code>,
210 but it has no way to access it.
211 This implementation uses a "covert channel" to return the address of the
212 embedded object when <code class="code">get_deleter<_Sp_make_shared_tag>()</code>
213 is called. Users should not try to use this.
214 As well as the extra constructors, this implementation also needs some
215 members of _Sp_counted_deleter to be protected where they could otherwise
217 </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="id457752"></a>Examples</h4></div></div></div><p>
218 Examples of use can be found in the testsuite, under
219 <code class="filename">testsuite/tr1/2_general_utilities/shared_ptr</code>.
220 </p></div><div class="sect3" lang="en" xml:lang="en"><div class="titlepage"><div><div><h4 class="title"><a id="id457768"></a>Unresolved Issues</h4></div></div></div><p>
221 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>,
222 "shared_ptr interface changes for consistency with N1856" will
223 need to be implemented after it is accepted into the working
224 paper. Issue <a class="ulink" href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-active.html#743" target="_top">743</a>
225 might also require changes.
227 The <span class="type">_S_single</span> policy uses atomics when used in MT
228 code, because it uses the same dispatcher functions that check
229 <code class="function">__gthread_active_p()</code>. This could be
230 addressed by providing template specialisations for some members
231 of <code class="classname">_Sp_counted_base<_S_single></code>.
233 Unlike Boost, this implementation does not use separate classes
234 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
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
257 tr1::_Sp_deleter could be a private member of tr1::__shared_count but it
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" 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>
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"><div class="titlepage"><div><div><h3 class="title"><a id="shared_ptr.biblio"></a>Bibliography</h3></div></div></div><div class="biblioentry"><a id="id419191"></a><p>[<abbr class="abbrev">
270 </abbr>] <span class="title"><i>
271 Improving shared_ptr for C++0x, Revision 2
272 </i>. </span><span class="subtitle">
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">
277 . </span></p></div><div class="biblioentry"><a id="id405886"></a><p>[<abbr class="abbrev">
279 </abbr>] <span class="title"><i>
280 C++ Standard Library Active Issues List (Revision R52)
281 </i>. </span><span class="subtitle">
283 . </span><span class="biblioid">
284 <a class="ulink" href="http://open-std.org/jtc1/sc22/wg21/docs/papers/2007/n2456.html" target="_top">
286 . </span></p></div><div class="biblioentry"><a id="id411352"></a><p>[<abbr class="abbrev">
288 </abbr>] <span class="title"><i>
289 Working Draft, Standard for Programming Language C++
290 </i>. </span><span class="subtitle">
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">
295 . </span></p></div><div class="biblioentry"><a id="id400175"></a><p>[<abbr class="abbrev">
297 </abbr>] <span class="title"><i>
298 Boost C++ Libraries documentation - shared_ptr class template
299 </i>. </span><span class="subtitle">
301 . </span><span class="biblioid">
302 <a class="ulink" href="http://boost.org/libs/smart_ptr/shared_ptr.htm" target="_top">shared_ptr
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="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. Traits</td></tr></table></div></body></html>
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