[/
    Boost.Optional

    Copyright (c) 2015, 2016 Andrzej Krzemienski

    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)
]


[section:relnotes Release Notes]

[heading Boost Release 1.67]

* Fixed [@https://github.com/boostorg/optional/issues/46 issue #46].
* Fixed `-Wzero-as-null-pointer-constant` warnings.


[heading Boost Release 1.66]

* On newer compilers `optional` is now trivially-copyable for scalar `T`s. 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. 

[heading Boost Release 1.63]
* 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 [@https://svn.boost.org/trac/boost/ticket/12203 Trac #12203].
* Fixed [@https://svn.boost.org/trac/boost/ticket/12563 Trac #12563].


[heading Boost Release 1.62]

* Fixed [@https://svn.boost.org/trac/boost/ticket/12179 Trac #12179].


[heading Boost Release 1.61]

* 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 ([@https://svn.boost.org/trac/boost/ticket/5193 Trac #5193]).
* Made the perfect-forwarding constructor SFINAE-friendly, which fixes [@https://svn.boost.org/trac/boost/ticket/12002 Trac #12002]. However, this only works in the newer platforms that correctly implement C++11 `<type_traits>`.
* Fixed [@https://svn.boost.org/trac/boost/ticket/10445 Trac #10445].


[heading Boost Release 1.60]

* Changed the implementation of `boost::none` again. Now it is a const object with internal linkage (as any other tag). This fixes [@https://svn.boost.org/trac/boost/ticket/11203 Trac #11203].


[heading Boost Release 1.59]

* 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 [@https://svn.boost.org/trac/boost/ticket/11241 Trac #11241].

[heading Boost Release 1.58]

* `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 [@https://svn.boost.org/trac/boost/ticket/10825 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 [@https://svn.boost.org/trac/boost/ticket/10399 Trac #10399].
* It is no longer possible to assign `optional<U>` to `optional<T>` when `U` is not assignable or convertible to `T` ([@https://svn.boost.org/trac/boost/ticket/11087 Trac #11087]).
* Value accessors now work correctly on rvalues of `optional<T&>` ([@https://svn.boost.org/trac/boost/ticket/10839 Trac #10839]).  


[heading Boost Release 1.57]

* [@https://github.com/boostorg/optional/pull/9 Git pull #9]: ['"Supply `<string>` to fix C++03 compile error on `logic_error("...")`"].


[heading Boost Release 1.56]

* 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 [@https://svn.boost.org/trac/boost/ticket/1841 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 ([@https://svn.boost.org/trac/boost/ticket/4227 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 ([@https://svn.boost.org/trac/boost/ticket/9005 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.



[endsect][/ relnotes]
