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compatibility.htm
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<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html>
<head>
<title>Smart Pointer Changes</title>
<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1">
</head>
<body text="#000000" bgcolor="#ffffff" link="#0000ff" vlink="#0000ff">
<h1><img height="86" alt="boost.png (6897 bytes)" src="../../boost.png"
width="277" align="middle" border="0">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>
The <b>weak_ptr</b>
template was added.
<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>
A custom deallocator can be passed in when creating a <b>shared_ptr</b> or <b>shared_array</b>.
<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>
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>
The <b>BOOST_SMART_PTR_CONVERSION</b>
feature has been removed.
<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>
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>
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>
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>
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>$Date$</p>
<p><small>Copyright 2002 Darin Adler. Distributed under the Boost Software License, Version
1.0. See accompanying file <A href="../../LICENSE_1_0.txt">LICENSE_1_0.txt</A> or
copy at <A href="http://www.boost.org/LICENSE_1_0.txt">http://www.boost.org/LICENSE_1_0.txt</A>.</small></p>
</body>
</html>