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The partial specialization of std::less has been replaced by operator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.
$Date$
Copyright 2002 Darin Adler. Distributed under the Boost Software License, Version
1.0. See accompanying file LICENSE_1_0.txt or
copy at http://www.boost.org/LICENSE_1_0.txt.