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Peter Dimov suggested the name 'optional', and was the first to point out
the need for aligned storage.
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Douglas Gregor developed 'type_with_alignment', and later Eric Friedman coded
'aligned_storage', which are the core of the optional class implementation.
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Andrei Alexandrescu and Brian Parker also worked with aligned storage techniques
and their work influenced the current implementation.
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Gennadiy Rozental made extensive and important comments which shaped the
design.
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Vesa Karvonen and Douglas Gregor made quite useful comparisons between optional,
variant and any; and made other relevant comments.
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Douglas Gregor and Peter Dimov commented on comparisons and evaluation in
boolean contexts.
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Eric Friedman helped understand the issues involved with aligned storage,
move/copy operations and exception safety.
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Many others have participated with useful comments: Aleksey Gurotov, Kevlin
Henney, David Abrahams, and others I can't recall.
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William Kempf carefully considered the originally proposed interface and
suggested the new interface which is currently used. He also started and
fueled the discussion about the analogy optional<>/smart pointer and
about relational operators.
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Peter Dimov, Joel de Guzman, David Abrahams, Tanton Gibbs and Ian Hanson
focused on the relational semantics of optional (originally undefined); concluding
with the fact that the pointer-like interface doesn't make it a pointer so
it shall have deep relational operators.
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Augustus Saunders also explored the different relational semantics between
optional<> and a pointer and developed the OptionalPointee concept
as an aid against potential conflicts on generic code.
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Joel de Guzman noticed that optional<> can be seen as an API on top
of variant<T,nil_t>.
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Dave Gomboc explained the meaning and usage of the Haskell analog to optional<>:
the Maybe type constructor (analogy originally pointed out by David Sankel).
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Other comments were posted by Vincent Finn, Anthony Williams, Ed Brey, Rob
Stewart, and others.
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Joel de Guzman made the case for the support of references and helped with
the proper semantics.
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Mat Marcus shown the virtues of a value-oriented interface, influencing the
current design, and contributed the idea of "none".