2002-02-02 18:36:12 +00:00
|
|
|
|
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
|
|
|
|
|
|
|
2000-07-27 14:27:00 +00:00
|
|
|
|
<html>
|
|
|
|
|
|
|
|
|
|
|
|
<head>
|
|
|
|
|
|
<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1">
|
2002-02-02 18:36:12 +00:00
|
|
|
|
<title>Smart Pointers</title>
|
2000-07-27 14:27:00 +00:00
|
|
|
|
</head>
|
|
|
|
|
|
|
|
|
|
|
|
<body bgcolor="#FFFFFF" text="#000000">
|
|
|
|
|
|
|
2002-02-02 18:36:12 +00:00
|
|
|
|
<h1><img src="../../c++boost.gif" alt="c++boost.gif (8819 bytes)" align="middle" width="277" height="86">Smart
|
2000-07-27 14:27:00 +00:00
|
|
|
|
Pointers</h1>
|
2002-02-02 18:36:12 +00:00
|
|
|
|
|
|
|
|
|
|
<p>Smart pointers are objects which store pointers to dynamically allocated
|
|
|
|
|
|
(heap) objects. They behave much like built-in C++ pointers except that
|
2000-07-27 14:27:00 +00:00
|
|
|
|
they automatically delete the object pointed to at the appropriate
|
2002-02-02 18:36:12 +00:00
|
|
|
|
time. Smart pointers are particularly useful in the face of exceptions as
|
2000-07-27 14:27:00 +00:00
|
|
|
|
they ensure proper destruction of dynamically allocated objects. They can also
|
|
|
|
|
|
be used to keep track of dynamically allocated objects shared by multiple
|
|
|
|
|
|
owners.</p>
|
2002-02-02 18:36:12 +00:00
|
|
|
|
|
2000-07-27 14:27:00 +00:00
|
|
|
|
<p>Conceptually, smart pointers are seen as owning the object pointed to, and
|
|
|
|
|
|
thus responsible for deletion of the object when it is no longer needed.</p>
|
2002-02-02 18:36:12 +00:00
|
|
|
|
|
|
|
|
|
|
<p>The smart pointer library provides five smart pointer class templates:</p>
|
|
|
|
|
|
|
2000-07-27 14:27:00 +00:00
|
|
|
|
<div align="left">
|
|
|
|
|
|
<table border="1" cellpadding="4" cellspacing="4">
|
|
|
|
|
|
<tr>
|
2002-02-02 18:36:12 +00:00
|
|
|
|
<td><a href="scoped_ptr.htm"><b>scoped_ptr</b></a></td>
|
2002-02-04 18:51:16 +00:00
|
|
|
|
<td><a href="../../boost/scoped_ptr.hpp"><boost/scoped_ptr.hpp></a></td>
|
2001-05-22 18:58:21 +00:00
|
|
|
|
<td>Simple sole ownership of single objects. Noncopyable.</td>
|
2000-07-27 14:27:00 +00:00
|
|
|
|
</tr>
|
|
|
|
|
|
<tr>
|
2002-02-02 18:36:12 +00:00
|
|
|
|
<td><a href="scoped_array.htm"><b>scoped_array</b></a></td>
|
2002-02-04 18:51:16 +00:00
|
|
|
|
<td><a href="../../boost/scoped_array.hpp"><boost/scoped_array.hpp></a></td>
|
2002-02-02 18:36:12 +00:00
|
|
|
|
<td>Simple sole ownership of arrays. Noncopyable.</td>
|
2000-07-27 14:27:00 +00:00
|
|
|
|
</tr>
|
|
|
|
|
|
<tr>
|
2002-02-02 18:36:12 +00:00
|
|
|
|
<td><a href="shared_ptr.htm"><b>shared_ptr</b></a></td>
|
2002-02-04 18:51:16 +00:00
|
|
|
|
<td><a href="../../boost/shared_ptr.hpp"><boost/shared_ptr.hpp></a></td>
|
2000-07-27 14:27:00 +00:00
|
|
|
|
<td>Object ownership shared among multiple pointers</td>
|
|
|
|
|
|
</tr>
|
|
|
|
|
|
<tr>
|
2002-02-02 18:36:12 +00:00
|
|
|
|
<td><a href="shared_array.htm"><b>shared_array</b></a></td>
|
2002-02-04 18:51:16 +00:00
|
|
|
|
<td><a href="../../boost/shared_array.hpp"><boost/shared_array.hpp></a></td>
|
2000-07-27 14:27:00 +00:00
|
|
|
|
<td>Array ownership shared among multiple pointers.</td>
|
|
|
|
|
|
</tr>
|
2002-02-02 18:36:12 +00:00
|
|
|
|
<tr>
|
|
|
|
|
|
<td><a href="weak_ptr.htm"><b>weak_ptr</b></a></td>
|
2002-02-04 18:51:16 +00:00
|
|
|
|
<td><a href="../../boost/weak_ptr.hpp"><boost/weak_ptr.hpp></a></td>
|
2002-02-02 18:36:12 +00:00
|
|
|
|
<td>Non-owning observers of an object owned by <b>shared_ptr</b>.</td>
|
|
|
|
|
|
</tr>
|
2000-07-27 14:27:00 +00:00
|
|
|
|
</table>
|
|
|
|
|
|
</div>
|
2002-02-02 18:36:12 +00:00
|
|
|
|
|
|
|
|
|
|
<p>These templates are designed to complement the <b>std::auto_ptr</b> template.</p>
|
|
|
|
|
|
|
2000-07-27 14:27:00 +00:00
|
|
|
|
<p>They are examples of the "resource acquisition is initialization"
|
|
|
|
|
|
idiom described in Bjarne Stroustrup's "The C++ Programming Language",
|
|
|
|
|
|
3rd edition, Section 14.4, Resource Management.</p>
|
2002-02-02 18:36:12 +00:00
|
|
|
|
|
|
|
|
|
|
<p>A test program, <a href="smart_ptr_test.cpp">smart_ptr_test.cpp</a>, is
|
2000-07-27 14:27:00 +00:00
|
|
|
|
provided to verify correct operation.</p>
|
2002-02-02 18:36:12 +00:00
|
|
|
|
|
|
|
|
|
|
<p>A page on <a href="compatibility.htm">compatibility</a> with older versions of
|
|
|
|
|
|
the Boost smart pointer library describes some of the changes since earlier versions
|
|
|
|
|
|
of the smart pointer implementation.</p>
|
|
|
|
|
|
|
|
|
|
|
|
<p>A page on <a href="smarttests.htm">smart pointer timings</a> will be of
|
2000-07-27 14:27:00 +00:00
|
|
|
|
interest to those curious about performance issues.</p>
|
2002-02-02 18:36:12 +00:00
|
|
|
|
|
|
|
|
|
|
<h2><a name="Common requirements">Common Requirements</a></h2>
|
|
|
|
|
|
|
|
|
|
|
|
<p>These smart pointer class templates have a template parameter, <b>T</b>, which
|
|
|
|
|
|
specifies the type of the object pointed to by the smart pointer. The
|
|
|
|
|
|
behavior of the smart pointer templates is undefined if the destructor or <b>operator delete</b>
|
|
|
|
|
|
for objects of type <b>T</b> throw exceptions.</p>
|
|
|
|
|
|
|
|
|
|
|
|
<p><b>T</b> may be an incomplete type at the point of smart pointer
|
|
|
|
|
|
declaration. Unless otherwise specified, it is required that <b>T</b>
|
2001-05-24 18:42:25 +00:00
|
|
|
|
be a complete type at points of smart pointer instantiation. Implementations are
|
|
|
|
|
|
required to diagnose (treat as an error) all violations of this requirement,
|
2002-02-02 18:36:12 +00:00
|
|
|
|
including deletion of an incomplete type.
|
|
|
|
|
|
See the description of the <a href="../utility/utility.htm#checked_delete"><b>checked_delete</b></a>
|
|
|
|
|
|
function template.</p>
|
|
|
|
|
|
|
2001-05-22 18:58:21 +00:00
|
|
|
|
<h3>Rationale</h3>
|
2002-02-02 18:36:12 +00:00
|
|
|
|
|
|
|
|
|
|
<p>The requirements on <b>T</b> are carefully crafted to maximize safety
|
|
|
|
|
|
yet allow handle-body (also called pimpl) and similar idioms. In these idioms a
|
|
|
|
|
|
smart pointer may appear in translation units where <b>T</b> is an
|
|
|
|
|
|
incomplete type. This separates interface from implementation and hides
|
|
|
|
|
|
implementation from translation units which merely use the interface.
|
2001-05-24 18:42:25 +00:00
|
|
|
|
Examples described in the documentation for specific smart pointers illustrate
|
|
|
|
|
|
use of smart pointers in these idioms.</p>
|
2002-02-02 18:36:12 +00:00
|
|
|
|
|
|
|
|
|
|
<p>Note that <b>scoped_ptr</b> requires that <b>T</b> be a complete type
|
|
|
|
|
|
at destruction time, but <b>shared_ptr</b> does not.</p>
|
|
|
|
|
|
|
|
|
|
|
|
<h2>Exception Safety</h2>
|
|
|
|
|
|
|
2000-07-27 14:27:00 +00:00
|
|
|
|
<p>Several functions in these smart pointer classes are specified as having
|
|
|
|
|
|
"no effect" or "no effect except such-and-such" if an
|
2002-02-02 18:36:12 +00:00
|
|
|
|
exception is thrown. This means that when an exception is thrown by
|
2000-07-27 14:27:00 +00:00
|
|
|
|
an object of one of these classes, the entire program state remains the same as
|
|
|
|
|
|
it was prior to the function call which resulted in the exception being
|
2002-02-02 18:36:12 +00:00
|
|
|
|
thrown. This amounts to a guarantee that there are no detectable side
|
|
|
|
|
|
effects. Other functions never throw exceptions. The only exception
|
|
|
|
|
|
ever thrown by functions which do throw (assuming <b>T</b> meets the
|
|
|
|
|
|
<a href="#Common requirements">common requirements</a>) is <b>std::bad_alloc</b>,
|
|
|
|
|
|
and that is thrown only by functions which are explicitly documented as possibly
|
|
|
|
|
|
throwing <b>std::bad_alloc</b>.</p>
|
|
|
|
|
|
|
2000-07-27 14:27:00 +00:00
|
|
|
|
<h2>Exception-specifications</h2>
|
2002-02-02 18:36:12 +00:00
|
|
|
|
|
|
|
|
|
|
<p>Exception-specifications are not used; see
|
|
|
|
|
|
<a href="../../more/lib_guide.htm#Exception-specification">exception-specification
|
2000-07-27 14:27:00 +00:00
|
|
|
|
rationale</a>.</p>
|
2002-02-02 18:36:12 +00:00
|
|
|
|
|
|
|
|
|
|
<p>All the smart pointer templates contain member functions which can never throw exceptions,
|
2000-07-27 14:27:00 +00:00
|
|
|
|
because they neither throw exceptions themselves nor call other functions which
|
2002-02-02 18:36:12 +00:00
|
|
|
|
may throw exceptions. These members are indicated by a comment:
|
|
|
|
|
|
<code>// never throws</code>. </p>
|
|
|
|
|
|
|
2000-07-27 14:27:00 +00:00
|
|
|
|
<p>Functions which destroy objects of the pointed to type are prohibited from
|
2002-02-02 18:36:12 +00:00
|
|
|
|
throwing exceptions by the <a href="#Common requirements">common requirements</a>.</p>
|
|
|
|
|
|
|
|
|
|
|
|
<h2>History and Acknowledgements</h2>
|
|
|
|
|
|
|
|
|
|
|
|
<p>January 2002. Peter Dimov reworked all four classes, adding features, fixing bugs,
|
|
|
|
|
|
and splitting them into four separate headers, and added <b>weak_ptr</b>. See the
|
|
|
|
|
|
<a href="compatibility.htm">compatibility</a> page for a summary of the changes.</p>
|
|
|
|
|
|
|
|
|
|
|
|
<p>May 2001. Vladimir Prus suggested requiring a complete type on
|
|
|
|
|
|
destruction. Refinement evolved in discussions including Dave Abrahams,
|
2001-05-22 18:58:21 +00:00
|
|
|
|
Greg Colvin, Beman Dawes, Rainer Deyke, Peter Dimov, John Maddock, Vladimir Prus,
|
|
|
|
|
|
Shankar Sai, and others.</p>
|
2002-02-02 18:36:12 +00:00
|
|
|
|
|
|
|
|
|
|
<p>November 1999. Darin Adler provided operator ==, operator !=, and std::swap
|
2000-07-27 14:27:00 +00:00
|
|
|
|
and std::less specializations for shared types.</p>
|
2002-02-02 18:36:12 +00:00
|
|
|
|
|
|
|
|
|
|
<p>September 1999. Luis Coelho provided shared_ptr::swap and shared_array::swap</p>
|
|
|
|
|
|
|
|
|
|
|
|
<p>May 1999. In April and May, 1999, Valentin Bonnard and David Abrahams
|
|
|
|
|
|
made a number of suggestions resulting in numerous improvements. See the
|
|
|
|
|
|
revision history in <a href="../../boost/smart_ptr.hpp"><b>smart_ptr.hpp</b></a>
|
2000-07-27 14:27:00 +00:00
|
|
|
|
for the specific changes made as a result of their constructive criticism.</p>
|
2002-02-02 18:36:12 +00:00
|
|
|
|
|
|
|
|
|
|
<p>October 1998. In 1994 Greg Colvin proposed to the C++ Standards Committee
|
|
|
|
|
|
classes named <b>auto_ptr</b> and <b>counted_ptr</b> which
|
|
|
|
|
|
were very similar to what we now call <b>scoped_ptr</b> and <b>shared_ptr</b>.
|
|
|
|
|
|
The committee document was 94-168/N0555, Exception Safe Smart Pointers. In
|
2000-07-27 14:27:00 +00:00
|
|
|
|
one of the very few cases where the Library Working Group's recommendations were
|
2002-02-02 18:36:12 +00:00
|
|
|
|
not followed by the full committee, <b>counted_ptr</b> was rejected
|
|
|
|
|
|
and surprising transfer-of-ownership semantics were added to <b>auto_ptr</b>.</p>
|
|
|
|
|
|
|
|
|
|
|
|
<p>Beman Dawes proposed reviving the original semantics under the names <b>safe_ptr</b>
|
|
|
|
|
|
and <b>counted_ptr</b> at an October, 1998, meeting of Per Andersson,
|
2000-07-27 14:27:00 +00:00
|
|
|
|
Matt Austern, Greg Colvin, Sean Corfield, Pete Becker, Nico Josuttis, Dietmar
|
2002-02-02 18:36:12 +00:00
|
|
|
|
K<EFBFBD>hl, Nathan Myers, Chichiang Wan and Judy Ward. During the discussion,
|
2000-07-27 14:27:00 +00:00
|
|
|
|
the four class names were finalized, it was decided that there was no need to
|
2002-02-02 18:36:12 +00:00
|
|
|
|
exactly follow the <b>std::auto_ptr</b> interface, and various
|
2000-07-27 14:27:00 +00:00
|
|
|
|
function signatures and semantics were finalized.</p>
|
2002-02-02 18:36:12 +00:00
|
|
|
|
|
|
|
|
|
|
<p>Over the next three months, several implementations were considered for <b>shared_ptr</b>,
|
2000-07-27 14:27:00 +00:00
|
|
|
|
and discussed on the <a href="http://www.boost.org">boost.org</a> mailing
|
2002-02-02 18:36:12 +00:00
|
|
|
|
list. The implementation questions revolved around the reference count
|
2000-07-27 14:27:00 +00:00
|
|
|
|
which must be kept, either attached to the pointed to object, or detached
|
|
|
|
|
|
elsewhere. Each of those variants have themselves two major variants:
|
2002-02-02 18:36:12 +00:00
|
|
|
|
|
2000-07-27 14:27:00 +00:00
|
|
|
|
<ul>
|
|
|
|
|
|
<li>Direct detached: the shared_ptr contains a pointer to the object, and a
|
|
|
|
|
|
pointer to the count.</li>
|
|
|
|
|
|
<li>Indirect detached: the shared_ptr contains a pointer to a helper object,
|
|
|
|
|
|
which in turn contains a pointer to the object and the count.</li>
|
|
|
|
|
|
<li>Embedded attached: the count is a member of the object pointed to.</li>
|
|
|
|
|
|
<li>Placement attached: the count is attached via operator new manipulations.</li>
|
|
|
|
|
|
</ul>
|
2002-02-02 18:36:12 +00:00
|
|
|
|
|
|
|
|
|
|
<p>Each implementation technique has advantages and disadvantages. We went
|
2000-07-27 14:27:00 +00:00
|
|
|
|
so far as to run various timings of the direct and indirect approaches, and
|
|
|
|
|
|
found that at least on Intel Pentium chips there was very little measurable
|
2002-02-02 18:36:12 +00:00
|
|
|
|
difference. Kevlin Henney provided a paper he wrote on "Counted Body
|
|
|
|
|
|
Techniques." Dietmar K<>hl suggested an elegant partial template
|
2000-07-27 14:27:00 +00:00
|
|
|
|
specialization technique to allow users to choose which implementation they
|
|
|
|
|
|
preferred, and that was also experimented with.</p>
|
2002-02-02 18:36:12 +00:00
|
|
|
|
|
2000-07-27 14:27:00 +00:00
|
|
|
|
<p>But Greg Colvin and Jerry Schwarz argued that "parameterization will
|
|
|
|
|
|
discourage users", and in the end we choose to supply only the direct
|
|
|
|
|
|
implementation.</p>
|
2002-02-02 18:36:12 +00:00
|
|
|
|
|
2000-07-27 14:27:00 +00:00
|
|
|
|
<hr>
|
2002-02-02 18:36:12 +00:00
|
|
|
|
|
|
|
|
|
|
<p>Revised <!--webbot bot="Timestamp" s-type="EDITED" s-format="%d %B %Y" startspan
|
2002-02-04 22:52:48 +00:00
|
|
|
|
-->4 February 2002<!--webbot bot="Timestamp" endspan i-checksum="40737"
|
2000-07-27 14:27:00 +00:00
|
|
|
|
--></p>
|
2002-02-02 18:36:12 +00:00
|
|
|
|
|
|
|
|
|
|
<p>Copyright 1999 Greg Colvin and Beman Dawes. Copyright 2002 Darin Adler.
|
|
|
|
|
|
Permission to copy, use,
|
2000-07-27 14:27:00 +00:00
|
|
|
|
modify, sell and distribute this document is granted provided this copyright
|
|
|
|
|
|
notice appears in all copies. This document is provided "as is"
|
|
|
|
|
|
without express or implied warranty, and with no claim as to its suitability for
|
|
|
|
|
|
any purpose.</p>
|
|
|
|
|
|
|
|
|
|
|
|
</body>
|
|
|
|
|
|
|
|
|
|
|
|
</html>
|
