boost_1_45_0/libs/optional/doc/implementation_notes.qbk

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    Boost.Optional

    Copyright (c) 2003-2007 Fernando Luis Cacciola Carballal

    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 Implementation Notes]

`optional<T>` is currently implemented using a custom aligned storage facility
built from `alignment_of` and `type_with_alignment` (both from Type Traits). It
uses a separate boolean flag to indicate the initialization state.
Placement new with `T`'s copy constructor and `T`'s destructor are explicitly used
to initialize,copy and destroy optional values.
As a result, `T`'s default constructor is effectively by-passed, but the exception
guarantees are basic.
It is planned to replace the current implementation with another with stronger
exception safety, such as a future `boost::variant`.

[endsect]