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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/move constructor and T's
destructor are explicitly used to initialize, copy, move 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.