Correct definition of operator unspecified-bool-type.  Make reference for is_initialized point to this operator.  Fixes #6364.

svn path=/trunk/libs/optional/; revision=82911
Replace deprecated function reset in examples.  Fixes #765.

svn path=/trunk/libs/optional/; revision=82912
Work around msvc bug when explicitly destroying a class with a virtual base.  Fixes #5940.

svn path=/trunk/boost/optional/; revision=82915