Release Notes

On newer compilers optional is now trivially-copyable for scalar Ts. This uses a different storage (just T rather than aligned_storage). We require the compiler to support defaulted functions.
Changed the implementation of operator== to get rid of the -Wmaybe-uninitialized false-positive warning from GCC.

Added two new in-place constructors. They work similarly to emplace() functions: they initialize the contained value by perfect-forwarding the obtained arguments. One constructor always initializes the contained value, the other based on a boolean condition.
Syntax o = {} now correctly un-initializes optional, just like in std::optional.
Fixed Trac #12203.
Fixed Trac #12563.

Now boost::optional is specialized for reference parameters. This addresses a couple of issues:
- the sizeof of optional reference is that of a pointer,
- some bugs connected to copying optional references are gone,
- all run-time bugs caused by incorrect reference binding on some compilers are now turned into compile-time errors,
- you can swap optional references: it is like swapping pointers: shalow, underlying objects are not affected,
- optional references to abstract types work.
Documented nested typedefs (Trac #5193).
Made the perfect-forwarding constructor SFINAE-friendly, which fixes Trac #12002. However, this only works in the newer platforms that correctly implement C++11 <type_traits>.
Fixed Trac #10445.

Changed the implementation of boost::none again. Now it is a const object with internal linkage (as any other tag). This fixes Trac #11203.

For C++03 compilers, added 0-argument overload for member function emplace(), and therewith removed the dependency on <boost/utility/in_place_factory.hpp>.
Fixed Trac #11241.

boost::none_t is no longer convertible from literal 0. This avoids a bug where optional<rational<int>> oi = 0; would initialize an optional object with no contained value.
Improved the trick that prevents streaming out optional without header optional_io.hpp by using safe-bool idiom. This addresses Trac #10825.
IOStream operators are now mentioned in documentation.
Added a way to manually disable move semantics: just define macro BOOST_OPTIONAL_CONFIG_NO_RVALUE_REFERENCES. This can be used to work around Trac #10399.
It is no longer possible to assign optional<U> to optional<T> when U is not assignable or convertible to T (Trac #11087).
Value accessors now work correctly on rvalues of optional<T&> (Trac #10839).

Git pull #9: "Supply <string> to fix C++03 compile error on logic_error("...")".

Added support for rvalue references. Now optional<T> works with moveable but non-copyable T's,
Improved swap (now uses move operations),
Added function emplace(). This is the last of the requests from Trac #1841,
optional is moveable, including conditional noexcept specifications, which make it move_if_noexcept-friendly,
Using explicit operator bool() on platforms that support it (Trac #4227) (breaking change),
Forward declaration of operator<<(ostream&, optional const&) to prevent inadvertent incorrect serialization of optional objects,
Removed deprecated function reset() from examples (Trac #9005),
Equality comparison with boost::none does not require that T be EqualityComparable,
Optional rvalue references are explicitly disallowed,
Binding temporaries to optional references is explicitly disallowed (breaking change),
More ways to access the contained value, functions value(), value_or(), value_or_eval(),
Updated and reorganized documentation, added tutorial and quick guide sections.