1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
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 V1.74.3" /><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="id737043"></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="id656080"></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="id618309"></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="id679959"></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="id680859"></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="id715170"></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="id686787"></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="id622100"></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="id622124"></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="id635143"></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="id685426"></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>
|
