Smart Pointer Changes

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.

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.

Features Requiring Code Changes to Take Advantage

• The smart pointer class templates now each have their own header file. For compatibility, the <boost/smart_ptr.hpp> header now includes the headers for the four classic smart pointer class templates.
• The weak_ptr template was added.
• The new shared_ptr and shared_array relax the requirement that the pointed-to object's destructor must be visible when instantiating the shared_ptr destructor. This makes it easier to have shared_ptr members in classes without explicit destructors.
• A custom deallocator can be passed in when creating a shared_ptr or shared_array.
• shared_static_cast and shared_dynamic_cast function templates are provided which work for shared_ptr and weak_ptr as static_cast and dynamic_cast do for pointers.
• The self-assignment misfeature has been removed from shared_ptr::reset, although it is still present in scoped_ptr, and in std::auto_ptr. Calling reset with a pointer to the object that's already owned by the shared_ptr results in undefined behavior (an assertion, or eventually a double-delete if assertions are off).
• The BOOST_SMART_PTR_CONVERSION feature has been removed.
• shared_ptr<void> is now allowed.

Features That Improve Robustness

• The manipulation of use counts is now thread safe on Windows, Linux, and platforms that support pthreads. See the <boost/detail/atomic_count.hpp> file for details
• 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 shared_ptr was a pointer to a sub-object of a class that did not have a virtual destructor.

Implementation Details

• Some bugs in the assignment operator implementations and in reset have been fixed by using the "copy and swap" idiom.
• Assertions have been added to check preconditions of various functions; however, since these use the new <boost/assert.hpp> header, the assertions are disabled by default.
• The partial specialization of std::less has been replaced by operator< overloads which accomplish the same thing without relying on undefined behavior.
• The incorrect overload of std::swap has been replaced by boost::swap, which has many of the same advantages for generic programming but does not violate the C++ standard.

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Revised 1 February 2002

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.