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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
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<title>Smart Pointer Changes</title>
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<h1><img src="../../boost.png" alt="boost.png (6897 bytes)" align="middle" width="277" height="86">Smart
Pointer Changes</h1>
<p>The February 2002 change to the Boost smart pointers introduced a number
of changes. Since the previous version of the smart pointers was in use for
a long time, it's useful to have a detailed list of what changed from a library
user's point of view.</p>
<p>Note that for compilers that don't support member templates well enough,
a separate implementation is used that lacks many of the new features and is
more like the old version.</p>
<h2>Features Requiring Code Changes to Take Advantage</h2>
<ul>
<li>The smart pointer class templates now each have their own header file.
For compatibility, the
<a href="../../boost/smart_ptr.hpp"><boost/smart_ptr.hpp></a>
header now includes the headers for the four classic smart pointer class templates.</li>
<li>The <b>weak_ptr</b> template was added.</li>
<li>The new <b>shared_ptr</b> and <b>shared_array</b> relax the requirement that the pointed-to object's
destructor must be visible when instantiating the <b>shared_ptr</b> destructor.
This makes it easier to have shared_ptr members in classes without explicit destructors.</li>
<li>A custom deallocator can be passed in when creating a <b>shared_ptr</b> or <b>shared_array</b>.</li>
<li><b>shared_static_cast</b> and <b>shared_dynamic_cast</b> function templates are
provided which work for <b>shared_ptr</b> and <b>weak_ptr</b> as <b>static_cast</b> and
<b>dynamic_cast</b> do for pointers.</li>
<li>The self-assignment misfeature has been removed from <b>shared_ptr::reset</b>,
although it is still present in <b>scoped_ptr</b>, and in <b>std::auto_ptr</b>.
Calling <b>reset</b> with a pointer to the object that's already owned by the
<b>shared_ptr</b> results in undefined behavior
(an assertion, or eventually a double-delete if assertions are off).</li>
<li>The <b>BOOST_SMART_PTR_CONVERSION</b> feature has been removed.</li>
<li><b>shared_ptr<void></b> is now allowed.</li>
</ul>
<h2>Features That Improve Robustness</h2>
<ul>
<li>The manipulation of use counts is now <a name="threadsafe">thread safe</a> on Windows, Linux, and platforms
that support pthreads. See the
<a href="../../boost/detail/atomic_count.hpp"><boost/detail/atomic_count.hpp></a>
file for details</li>
<li>The new shared_ptr will always delete the object using the pointer it was originally constructed with.
This prevents subtle problems that could happen if the last <b>shared_ptr</b> was a pointer to a sub-object
of a class that did not have a virtual destructor.</li>
</ul>
<h2>Implementation Details</h2>
<ul>
<li>Some bugs in the assignment operator implementations and in <b>reset</b>
have been fixed by using the "copy and swap" idiom.</li>
<li>Assertions have been added to check preconditions of various functions;
however, since these use the new
<a href="../../boost/assert.hpp"><boost/assert.hpp></a>
header, the assertions are disabled by default.</li>
<li>The partial specialization of <b>std::less</b> has been replaced by <b>operator<</b>
overloads which accomplish the same thing without relying on undefined behavior.</li>
<li>The incorrect overload of <b>std::swap</b> has been replaced by <b>boost::swap</b>, which
has many of the same advantages for generic programming but does not violate the C++ standard.</li>
</ul>
<hr>
<p>Revised 1 February 2002</p>
<p>Copyright 2002 Darin Adler.
Permission to copy, use,
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>
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