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source: downloads/boost_1_34_1/libs/smart_ptr/smart_ptr.htm @ 33

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1	<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
2	<html>
3	<head>
4	<title>Smart Pointers</title>
5	<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1">
6	</head>
7	<body bgcolor="#ffffff" text="#000000">
8	<h1><A href="../../index.htm"><img src="../../boost.png" alt="boost.png (6897 bytes)" align="middle" width="277" height="86"
9	border="0"></A>Smart Pointers</h1>
10	<p><a href="#Introduction">Introduction</a><br>
11	<a href="#common_requirements">Common Requirements</a><br>
12	<a href="#Exception_Safety">Exception Safety</a><br>
13	<a href="#Exception-specifications">Exception-specifications</a><br>
14	<a href="#History">History and Acknowledgements</a><br>
15	<a href="#References">References</a></p>
16	<h2><a name="Introduction">Introduction</a></h2>
17	<p>Smart pointers are objects which store pointers to dynamically allocated (heap)
18	objects. They behave much like built-in C++ pointers except that they
19	automatically delete the object pointed to at the appropriate time. Smart
20	pointers are particularly useful in the face of exceptions as they ensure
21	proper destruction of dynamically allocated objects. They can also be used to
22	keep track of dynamically allocated objects shared by multiple owners.</p>
23	<p>Conceptually, smart pointers are seen as owning the object pointed to, and thus
24	responsible for deletion of the object when it is no longer needed.</p>
25	<p>The smart pointer library provides five smart pointer class templates:</p>
26	<div align="left">
27	<table border="1" cellpadding="4" cellspacing="0">
28	<tr>
29	<td><a href="scoped_ptr.htm"><b>scoped_ptr</b></a></td>
30	<td><a href="../../boost/scoped_ptr.hpp"><boost/scoped_ptr.hpp></a></td>
31	<td>Simple sole ownership of single objects. Noncopyable.</td>
32	</tr>
33	<tr>
34	<td><a href="scoped_array.htm"><b>scoped_array</b></a></td>
35	<td><a href="../../boost/scoped_array.hpp"><boost/scoped_array.hpp></a></td>
36	<td>Simple sole ownership of arrays. Noncopyable.</td>
37	</tr>
38	<tr>
39	<td><a href="shared_ptr.htm"><b>shared_ptr</b></a></td>
40	<td><a href="../../boost/shared_ptr.hpp"><boost/shared_ptr.hpp></a></td>
41	<td>Object ownership shared among multiple pointers</td>
42	</tr>
43	<tr>
44	<td><a href="shared_array.htm"><b>shared_array</b></a></td>
45	<td><a href="../../boost/shared_array.hpp"><boost/shared_array.hpp></a></td>
46	<td>Array ownership shared among multiple pointers.</td>
47	</tr>
48	<tr>
49	<td><a href="weak_ptr.htm"><b>weak_ptr</b></a></td>
50	<td><a href="../../boost/weak_ptr.hpp"><boost/weak_ptr.hpp></a></td>
51	<td>Non-owning observers of an object owned by <b>shared_ptr</b>.</td>
52	</tr>
53	<tr>
54	<td><a href="intrusive_ptr.html"><b>intrusive_ptr</b></a></td>
55	<td><a href="../../boost/intrusive_ptr.hpp"><boost/intrusive_ptr.hpp></a></td>
56	<td>Shared ownership of objects with an embedded reference count.</td>
57	</tr>
58	</table>
59	</div>
60	<p>These templates are designed to complement the <b>std::auto_ptr</b> template.</p>
61	<p>They are examples of the "resource acquisition is initialization" idiom
62	described in Bjarne Stroustrup's "The C++ Programming Language", 3rd edition,
63	Section 14.4, Resource Management.</p>
64	<p>A test program, <a href="test/smart_ptr_test.cpp">smart_ptr_test.cpp</a>, is
65	provided to verify correct operation.</p>
66	<p>A page on <a href="compatibility.htm">compatibility</a> with older versions of
67	the Boost smart pointer library describes some of the changes since earlier
68	versions of the smart pointer implementation.</p>
69	<p>A page on <a href="smarttests.htm">smart pointer timings</a> will be of interest
70	to those curious about performance issues.</p>
71	<P>A page on <A href="sp_techniques.html">smart pointer programming techniques</A> lists
72	some advanced applications of <code>shared_ptr</code> and <code>weak_ptr</code>.</P>
73	<h2><a name="common_requirements">Common Requirements</a></h2>
74	<p>These smart pointer class templates have a template parameter, <b>T</b>, which
75	specifies the type of the object pointed to by the smart pointer. The behavior
76	of the smart pointer templates is undefined if the destructor or <b>operator delete</b>
77	for objects of type <b>T</b> throw exceptions.</p>
78	<p><b>T</b> may be an incomplete type at the point of smart pointer declaration.
79	Unless otherwise specified, it is required that <b>T</b> be a complete type at
80	points of smart pointer instantiation. Implementations are required to diagnose
81	(treat as an error) all violations of this requirement, including deletion of
82	an incomplete type. See the description of the <a href="../utility/utility.htm#checked_delete">
83	<b>checked_delete</b></a> function template.</p>
84	<P>Note that <STRONG>shared_ptr</STRONG> does not have this restriction, as most of
85	its member functions do not require <STRONG>T</STRONG> to be a complete type.</P>
86	<h3>Rationale</h3>
87	<p>The requirements on <b>T</b> are carefully crafted to maximize safety yet allow
88	handle-body (also called pimpl) and similar idioms. In these idioms a smart
89	pointer may appear in translation units where <b>T</b> is an incomplete type.
90	This separates interface from implementation and hides implementation from
91	translation units which merely use the interface. Examples described in the
92	documentation for specific smart pointers illustrate use of smart pointers in
93	these idioms.</p>
94	<p>Note that <b>scoped_ptr</b> requires that <b>T</b> be a complete type at
95	destruction time, but <b>shared_ptr</b> does not.</p>
96	<h2><a name="Exception_Safety">Exception Safety</a></h2>
97	<p>Several functions in these smart pointer classes are specified as having "no
98	effect" or "no effect except such-and-such" if an exception is thrown. This
99	means that when an exception is thrown by an object of one of these classes,
100	the entire program state remains the same as it was prior to the function call
101	which resulted in the exception being thrown. This amounts to a guarantee that
102	there are no detectable side effects. Other functions never throw exceptions.
103	The only exception ever thrown by functions which do throw (assuming <b>T</b> meets
104	the <a href="#common_requirements">common requirements</a>) is <b>std::bad_alloc</b>,
105	and that is thrown only by functions which are explicitly documented as
106	possibly throwing <b>std::bad_alloc</b>.</p>
107	<h2><a name="Exception-specifications">Exception-specifications</a></h2>
108	<p>Exception-specifications are not used; see <a href="../../more/lib_guide.htm#Exception-specification">
109	exception-specification rationale</a>.</p>
110	<p>All the smart pointer templates contain member functions which can never throw
111	exceptions, because they neither throw exceptions themselves nor call other
112	functions which may throw exceptions. These members are indicated by a comment: <code>
113	// never throws</code>.
114	</p>
115	<p>Functions which destroy objects of the pointed to type are prohibited from
116	throwing exceptions by the <a href="#common_requirements">common requirements</a>.</p>
117	<h2><a name="History">History</a> and Acknowledgements</h2>
118	<p>January 2002. Peter Dimov reworked all four classes, adding features, fixing
119	bugs, and splitting them into four separate headers, and added <b>weak_ptr</b>.
120	See the <a href="compatibility.htm">compatibility</a> page for a summary of the
121	changes.</p>
122	<p>May 2001. Vladimir Prus suggested requiring a complete type on destruction.
123	Refinement evolved in discussions including Dave Abrahams, Greg Colvin, Beman
124	Dawes, Rainer Deyke, Peter Dimov, John Maddock, Vladimir Prus, Shankar Sai, and
125	others.</p>
126	<p>November 1999. Darin Adler provided <b>operator ==</b>, <b>operator !=</b>, and <b>std::swap</b>
127	and <b>std::less</b> specializations for shared types.</p>
128	<p>September 1999. Luis Coelho provided <b>shared_ptr::swap</b> and <b>shared_array::swap</b></p>
129	<p>May 1999. In April and May, 1999, Valentin Bonnard and David Abrahams made a
130	number of suggestions resulting in numerous improvements.</p>
131	<p>October 1998. Beman Dawes proposed reviving the original semantics under the
132	names <b>safe_ptr</b> and <b>counted_ptr</b>, meeting of Per Andersson, Matt
133	Austern, Greg Colvin, Sean Corfield, Pete Becker, Nico Josuttis, Dietmar Kühl,
134	Nathan Myers, Chichiang Wan and Judy Ward. During the discussion, the four new
135	class names were finalized, it was decided that there was no need to exactly
136	follow the <b>std::auto_ptr</b> interface, and various function signatures and
137	semantics were finalized.</p>
138	<p>Over the next three months, several implementations were considered for <b>shared_ptr</b>,
139	and discussed on the <a href="http://www.boost.org">boost.org</a> mailing list.
140	The implementation questions revolved around the reference count which must be
141	kept, either attached to the pointed to object, or detached elsewhere. Each of
142	those variants have themselves two major variants:
143	<ul>
144	<li>
145	Direct detached: the shared_ptr contains a pointer to the object, and a pointer
146	to the count.
147	<li>
148	Indirect detached: the shared_ptr contains a pointer to a helper object, which
149	in turn contains a pointer to the object and the count.
150	<li>
151	Embedded attached: the count is a member of the object pointed to.
152	<li>
153	Placement attached: the count is attached via operator new manipulations.</li>
154	</ul>
155	<p>Each implementation technique has advantages and disadvantages. We went so far
156	as to run various timings of the direct and indirect approaches, and found that
157	at least on Intel Pentium chips there was very little measurable difference.
158	Kevlin Henney provided a paper he wrote on "Counted Body Techniques." Dietmar
159	Kühl suggested an elegant partial template specialization technique to allow
160	users to choose which implementation they preferred, and that was also
161	experimented with.</p>
162	<p>But Greg Colvin and Jerry Schwarz argued that "parameterization will discourage
163	users", and in the end we choose to supply only the direct implementation.</p>
164	<p>Summer, 1994. Greg Colvin proposed to the C++ Standards Committee classes named <b>auto_ptr</b>
165	and <b>counted_ptr</b> which were very similar to what we now call <b>scoped_ptr</b>
166	and <b>shared_ptr</b>. <a href="#Col-94">[Col-94]</a> In one of the very few
167	cases where the Library Working Group's recommendations were not followed by
168	the full committee, <b>counted_ptr</b> was rejected and surprising
169	transfer-of-ownership semantics were added to <b>auto_ptr</b>.</p>
170	<h2><a name="References">References</a></h2>
171	<p>[<a name="Col-94">Col-94</a>] Gregory Colvin, <a href="http://std.dkuug.dk/jtc1/sc22/wg21/docs/papers/1994/N0555.pdf">
172	Exception Safe Smart Pointers</a>, C++ committee document 94-168/N0555,
173	July, 1994.</p>
174	<p>[<a name="E&D-94">E&D-94</a>] John R. Ellis & David L. Detlefs, <a href="http://www.usenix.org/publications/library/proceedings/c++94/full_papers/ellis.a">
175	Safe, Efficient Garbage Collection for C++</a>, Usenix Proceedings,
176	February, 1994. This paper includes an extensive discussion of weak pointers
177	and an extensive bibliography.</p>
178	<hr>
179	<p>$Date: 2005/09/25 21:54:19 $</p>
180	<p>Copyright 1999 Greg Colvin and Beman Dawes. Copyright 2002 Darin Adler.
181	Permission to copy, use, modify, sell and distribute this document is granted
182	provided this copyright notice appears in all copies. This document is provided
183	"as is" without express or implied warranty, and with no claim as to its
184	suitability for any purpose.</p>
185	</body>
186	</html>

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