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<body bgcolor="#ffffff" text="#000000">
<h1><img src="../../c++boost.gif" alt="c++boost.gif (8819 bytes)" align="middle" width="277" height="86">Smart
Pointers</h1>
<p><a href="#Introduction">Introduction</a><br>
<a href="#common_requirements">Common Requirements</a><br>
<a href="#Exception_Safety">Exception Safety</a><br>
<a href="#Exception-specifications">Exception-specifications</a><br>
<a href="#History">History and Acknowledgements</a><br>
<a href="#References">References</a></p>
<h2><a name="Introduction">Introduction</a></h2>
<p>Smart pointers are objects which store pointers to dynamically allocated (heap)
objects. They behave much like built-in C++ pointers except that they
automatically delete the object pointed to at the appropriate time. Smart
@@ -79,7 +86,7 @@
these idioms.</p>
<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>
<h2><a name="Exception_Safety">Exception Safety</a></h2>
<p>Several functions in these smart pointer classes are specified as having "no
effect" or "no effect except such-and-such" if an exception is thrown. This
means that when an exception is thrown by an object of one of these classes,
@@ -90,7 +97,7 @@
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>
<h2>Exception-specifications</h2>
<h2><a name="Exception-specifications">Exception-specifications</a></h2>
<p>Exception-specifications are not used; see <a href="../../more/lib_guide.htm#Exception-specification">
exception-specification rationale</a>.</p>
<p>All the smart pointer templates contain member functions which can never throw
@@ -100,7 +107,7 @@
</p>
<p>Functions which destroy objects of the pointed to type are prohibited from
throwing exceptions by the <a href="#common_requirements">common requirements</a>.</p>
<h2>History and Acknowledgements</h2>
<h2><a name="History">History</a> 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
@@ -114,18 +121,11 @@
<p>September 1999. Luis Coelho provided <b>shared_ptr::swap</b> and <b>shared_array::swap</b></p>
<p>May 1999. In April and May, 1999, Valentin Bonnard and David Abrahams made a
number of suggestions resulting in numerous improvements.</p>
<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 one of the very
few cases where the Library Working Group's recommendations were 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, Matt
<p>October 1998. Beman Dawes proposed reviving the original semantics under the names <b>safe_ptr</b>
and <b>counted_ptr</b>, meeting of Per Andersson, Matt
Austern, Greg Colvin, Sean Corfield, Pete Becker, Nico Josuttis, Dietmar K<>hl,
Nathan Myers, Chichiang Wan and Judy Ward. During the discussion, the four
class names were finalized, it was decided that there was no need to exactly
new class names were finalized, it was decided that there was no need to exactly
follow the <b>std::auto_ptr</b> interface, and various function signatures and
semantics were finalized.</p>
<p>Over the next three months, several implementations were considered for <b>shared_ptr</b>,
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experimented with.</p>
<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>
<p>Summer, 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>.
<a href="#Col-94">[Col-94]</a> In one of the very
few cases where the Library Working Group's recommendations were 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>
<h2><a name="References">References</a></h2>
<p>[<a name="Col-94">Col-94</a>] Gregory Colvin,
<a href="http://std.dkuug.dk/jtc1/sc22/wg21/docs/papers/1994/N0555.pdf">
Exception Safe Smart Pointers</a>, C++ committee document 94-168/N0555,
July, 1994.</p>
<p>[<a name="E&amp;D-94">E&amp;D-94</a>] John R. Ellis &amp; David L. Detlefs,
<a href="http://www.usenix.org/publications/library/proceedings/c++94/full_papers/ellis.a">
Safe, Efficient Garbage Collection for C++</a>, Usenix Proceedings,
February, 1994. This paper includes an extensive discussion of weak
pointers and an extensive bibliography.</p>
<hr>
<p>Revised <!--webbot bot="Timestamp" s-type="EDITED" s-format="%d %B %Y" startspan
-->
4 February 2002<!--webbot bot="Timestamp" endspan i-checksum="40737"
-->15 January 2003<!--webbot bot="Timestamp" endspan i-checksum="32303"
--></p>
<p>Copyright 1999 Greg Colvin and Beman Dawes. Copyright 2002 Darin Adler.
Permission to copy, use, modify, sell and distribute this document is granted
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"as is" without express or implied warranty, and with no claim as to its
suitability for any purpose.</p>
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