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<title>Smart Pointers</title>
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2002-02-02 18:36:12 +00:00
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<h1><img src="../../c++boost.gif" alt="c++boost.gif (8819 bytes)" align="middle" width="277" height="86">Smart
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Pointers</h1>
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<p>Smart pointers are objects which store pointers to dynamically allocated
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(heap) objects. They behave much like built-in C++ pointers except that
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they automatically delete the object pointed to at the appropriate
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time. Smart pointers are particularly useful in the face of exceptions as
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they ensure proper destruction of dynamically allocated objects. They can also
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be used to keep track of dynamically allocated objects shared by multiple
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owners.</p>
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<p>Conceptually, smart pointers are seen as owning the object pointed to, and
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thus responsible for deletion of the object when it is no longer needed.</p>
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<p>The smart pointer library provides five smart pointer class templates:</p>
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<div align="left">
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<table border="1" cellpadding="4" cellspacing="4">
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<tr>
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<td><a href="scoped_ptr.htm"><b>scoped_ptr</b></a></td>
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<td><a href="../../boost/scoped_ptr.hpp"><boost/scoped_ptr.hpp></a></td>
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<td>Simple sole ownership of single objects. Noncopyable.</td>
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</tr>
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<tr>
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<td><a href="scoped_array.htm"><b>scoped_array</b></a></td>
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<td><a href="../../boost/scoped_array.hpp"><boost/scoped_array.hpp></a></td>
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<td>Simple sole ownership of arrays. Noncopyable.</td>
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</tr>
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<tr>
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<td><a href="shared_ptr.htm"><b>shared_ptr</b></a></td>
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<td><a href="../../boost/shared_ptr.hpp"><boost/shared_ptr.hpp></a></td>
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<td>Object ownership shared among multiple pointers</td>
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</tr>
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<tr>
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<td><a href="shared_array.htm"><b>shared_array</b></a></td>
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<td><a href="../../boost/shared_array.hpp"><boost/shared_array.hpp></a></td>
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<td>Array ownership shared among multiple pointers.</td>
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</tr>
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<tr>
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<td><a href="weak_ptr.htm"><b>weak_ptr</b></a></td>
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<td><a href="../../boost/weak_ptr.hpp"><boost/weak_ptr.hpp></a></td>
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<td>Non-owning observers of an object owned by <b>shared_ptr</b>.</td>
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</tr>
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</table>
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</div>
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<p>These templates are designed to complement the <b>std::auto_ptr</b> template.</p>
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<p>They are examples of the "resource acquisition is initialization"
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idiom described in Bjarne Stroustrup's "The C++ Programming Language",
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3rd edition, Section 14.4, Resource Management.</p>
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<p>A test program, <a href="smart_ptr_test.cpp">smart_ptr_test.cpp</a>, is
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provided to verify correct operation.</p>
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<p>A page on <a href="compatibility.htm">compatibility</a> with older versions of
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the Boost smart pointer library describes some of the changes since earlier versions
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of the smart pointer implementation.</p>
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<p>A page on <a href="smarttests.htm">smart pointer timings</a> will be of
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interest to those curious about performance issues.</p>
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<h2><a name="Common requirements">Common Requirements</a></h2>
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<p>These smart pointer class templates have a template parameter, <b>T</b>, which
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specifies the type of the object pointed to by the smart pointer. The
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behavior of the smart pointer templates is undefined if the destructor or <b>operator delete</b>
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for objects of type <b>T</b> throw exceptions.</p>
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<p><b>T</b> may be an incomplete type at the point of smart pointer
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declaration. Unless otherwise specified, it is required that <b>T</b>
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be a complete type at points of smart pointer instantiation. Implementations are
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required to diagnose (treat as an error) all violations of this requirement,
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including deletion of an incomplete type.
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See the description of the <a href="../utility/utility.htm#checked_delete"><b>checked_delete</b></a>
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function template.</p>
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<h3>Rationale</h3>
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<p>The requirements on <b>T</b> are carefully crafted to maximize safety
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yet allow handle-body (also called pimpl) and similar idioms. In these idioms a
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smart pointer may appear in translation units where <b>T</b> is an
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incomplete type. This separates interface from implementation and hides
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implementation from translation units which merely use the interface.
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Examples described in the documentation for specific smart pointers illustrate
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use of smart pointers in these idioms.</p>
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<p>Note that <b>scoped_ptr</b> requires that <b>T</b> be a complete type
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at destruction time, but <b>shared_ptr</b> does not.</p>
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<h2>Exception Safety</h2>
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<p>Several functions in these smart pointer classes are specified as having
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"no effect" or "no effect except such-and-such" if an
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exception is thrown. This means that when an exception is thrown by
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an object of one of these classes, the entire program state remains the same as
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it was prior to the function call which resulted in the exception being
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thrown. This amounts to a guarantee that there are no detectable side
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effects. Other functions never throw exceptions. The only exception
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ever thrown by functions which do throw (assuming <b>T</b> meets the
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<a href="#Common requirements">common requirements</a>) is <b>std::bad_alloc</b>,
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and that is thrown only by functions which are explicitly documented as possibly
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throwing <b>std::bad_alloc</b>.</p>
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<h2>Exception-specifications</h2>
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<p>Exception-specifications are not used; see
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<a href="../../more/lib_guide.htm#Exception-specification">exception-specification
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rationale</a>.</p>
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<p>All the smart pointer templates contain member functions which can never throw exceptions,
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because they neither throw exceptions themselves nor call other functions which
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may throw exceptions. These members are indicated by a comment:
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<code>// never throws</code>. </p>
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<p>Functions which destroy objects of the pointed to type are prohibited from
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throwing exceptions by the <a href="#Common requirements">common requirements</a>.</p>
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<h2>History and Acknowledgements</h2>
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<p>January 2002. Peter Dimov reworked all four classes, adding features, fixing bugs,
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and splitting them into four separate headers, and added <b>weak_ptr</b>. See the
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<a href="compatibility.htm">compatibility</a> page for a summary of the changes.</p>
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<p>May 2001. Vladimir Prus suggested requiring a complete type on
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destruction. Refinement evolved in discussions including Dave Abrahams,
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Greg Colvin, Beman Dawes, Rainer Deyke, Peter Dimov, John Maddock, Vladimir Prus,
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Shankar Sai, and others.</p>
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2002-02-27 17:03:30 +00:00
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<p>November 1999. Darin Adler provided <b>operator ==</b>, <b>operator !=</b>, and <b>std::swap</b>
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and <b>std::less</b> specializations for shared types.</p>
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2002-02-27 17:03:30 +00:00
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<p>September 1999. Luis Coelho provided <b>shared_ptr::swap</b> and <b>shared_array::swap</b></p>
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<p>May 1999. In April and May, 1999, Valentin Bonnard and David Abrahams
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made a number of suggestions resulting in numerous improvements.</p>
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<p>October 1998. In 1994 Greg Colvin proposed to the C++ Standards Committee
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classes named <b>auto_ptr</b> and <b>counted_ptr</b> which
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were very similar to what we now call <b>scoped_ptr</b> and <b>shared_ptr</b>.
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The committee document was 94-168/N0555, Exception Safe Smart Pointers. In
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one of the very few cases where the Library Working Group's recommendations were
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not followed by the full committee, <b>counted_ptr</b> was rejected
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and surprising transfer-of-ownership semantics were added to <b>auto_ptr</b>.</p>
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<p>Beman Dawes proposed reviving the original semantics under the names <b>safe_ptr</b>
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and <b>counted_ptr</b> at an October, 1998, meeting of Per Andersson,
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Matt Austern, Greg Colvin, Sean Corfield, Pete Becker, Nico Josuttis, Dietmar
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K<EFBFBD>hl, Nathan Myers, Chichiang Wan and Judy Ward. During the discussion,
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the four class names were finalized, it was decided that there was no need to
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exactly follow the <b>std::auto_ptr</b> interface, and various
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function signatures and semantics were finalized.</p>
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<p>Over the next three months, several implementations were considered for <b>shared_ptr</b>,
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and discussed on the <a href="http://www.boost.org">boost.org</a> mailing
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list. The implementation questions revolved around the reference count
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which must be kept, either attached to the pointed to object, or detached
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elsewhere. Each of those variants have themselves two major variants:
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<ul>
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<li>Direct detached: the shared_ptr contains a pointer to the object, and a
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pointer to the count.</li>
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<li>Indirect detached: the shared_ptr contains a pointer to a helper object,
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which in turn contains a pointer to the object and the count.</li>
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<li>Embedded attached: the count is a member of the object pointed to.</li>
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<li>Placement attached: the count is attached via operator new manipulations.</li>
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</ul>
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<p>Each implementation technique has advantages and disadvantages. We went
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so far as to run various timings of the direct and indirect approaches, and
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found that at least on Intel Pentium chips there was very little measurable
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difference. Kevlin Henney provided a paper he wrote on "Counted Body
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Techniques." Dietmar K<>hl suggested an elegant partial template
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specialization technique to allow users to choose which implementation they
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preferred, and that was also experimented with.</p>
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<p>But Greg Colvin and Jerry Schwarz argued that "parameterization will
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discourage users", and in the end we choose to supply only the direct
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implementation.</p>
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<hr>
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<p>Revised <!--webbot bot="Timestamp" s-type="EDITED" s-format="%d %B %Y" startspan
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-->4 February 2002<!--webbot bot="Timestamp" endspan i-checksum="40737"
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--></p>
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<p>Copyright 1999 Greg Colvin and Beman Dawes. Copyright 2002 Darin Adler.
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Permission to copy, use,
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modify, sell and distribute this document is granted provided this copyright
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notice appears in all copies. This document is provided "as is"
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without express or implied warranty, and with no claim as to its suitability for
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any purpose.</p>
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</body>
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</html>
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