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90 lines
4.2 KiB
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<title>Smart Pointer Changes</title>
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<h1><A href="../../index.htm"><img src="../../boost.png" alt="boost.png (6897 bytes)" align="middle" width="277" height="86"
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border="0"></A>Smart Pointer Changes</h1>
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<p>The February 2002 change to the Boost smart pointers introduced a number of
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changes. Since the previous version of the smart pointers was in use for a long
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time, it's useful to have a detailed list of what changed from a library user's
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point of view.</p>
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<p>Note that for compilers that don't support member templates well enough, a
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separate implementation is used that lacks many of the new features and is more
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like the old version.</p>
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<h2>Features Requiring Code Changes to Take Advantage</h2>
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<ul>
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<li>
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The smart pointer class templates now each have their own header file. For
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compatibility, the <a href="../../boost/smart_ptr.hpp"><boost/smart_ptr.hpp></a>
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header now includes the headers for the four classic smart pointer class
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templates.
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<li>
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The <b>weak_ptr</b>
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template was added.
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<li>
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The new <b>shared_ptr</b> and <b>shared_array</b> relax the requirement that
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the pointed-to object's destructor must be visible when instantiating the <b>shared_ptr</b>
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destructor. This makes it easier to have shared_ptr members in classes without
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explicit destructors.
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<li>
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A custom deallocator can be passed in when creating a <b>shared_ptr</b> or <b>shared_array</b>.
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<li>
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<b>shared_static_cast</b> and <b>shared_dynamic_cast</b> function templates are
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provided which work for <b>shared_ptr</b> and <b>weak_ptr</b> as <b>static_cast</b>
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and <b>dynamic_cast</b>
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do for pointers.
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<li>
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The self-assignment misfeature has been removed from <b>shared_ptr::reset</b>,
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although it is still present in <b>scoped_ptr</b>, and in <b>std::auto_ptr</b>.
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Calling <b>reset</b> with a pointer to the object that's already owned by the <b>shared_ptr</b>
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results in undefined behavior (an assertion, or eventually a double-delete if
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assertions are off).
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<li>
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The <b>BOOST_SMART_PTR_CONVERSION</b>
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feature has been removed.
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<li>
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<b>shared_ptr<void></b> is now allowed.</li>
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</ul>
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<h2>Features That Improve Robustness</h2>
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<ul>
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<li>
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The manipulation of use counts is now <a name="threadsafe">thread safe</a> on
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Windows, Linux, and platforms that support pthreads. See the <a href="../../boost/detail/atomic_count.hpp">
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<boost/detail/atomic_count.hpp></a>
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file for details
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<li>
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The new shared_ptr will always delete the object using the pointer it was
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originally constructed with. This prevents subtle problems that could happen if
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the last <b>shared_ptr</b> was a pointer to a sub-object of a class that did
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not have a virtual destructor.</li>
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</ul>
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<h2>Implementation Details</h2>
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<ul>
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<li>
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Some bugs in the assignment operator implementations and in <b>reset</b>
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have been fixed by using the "copy and swap" idiom.
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<li>
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Assertions have been added to check preconditions of various functions;
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however, since these use the new <a href="../../boost/assert.hpp"><boost/assert.hpp></a>
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header, the assertions are disabled by default.
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<li>
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The partial specialization of <b>std::less</b> has been replaced by <b>operator<</b>
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overloads which accomplish the same thing without relying on undefined
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behavior.
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<li>
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The incorrect overload of <b>std::swap</b> has been replaced by <b>boost::swap</b>,
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which has many of the same advantages for generic programming but does not
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violate the C++ standard.</li>
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</ul>
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<hr>
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<p>Revised 1 February 2002</p>
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<p>Copyright 2002 Darin Adler. Permission to copy, use, modify, sell and distribute
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this document is granted provided this copyright notice appears in all copies.
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This document is provided "as is" without express or implied warranty, and with
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no claim as to its suitability for any purpose.</p>
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