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docs: optional<T> == nont_t requirements

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Andrzej Krzemienski
2014-06-08 20:51:55 +02:00
parent befd3970d7
commit 07bdbc3743
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doc/13_relational_operators.qbk
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@ -29,6 +29,8 @@ This mixed comparison has a practical interpretation, which is occasionally usef
In the above example, the meaning of the comparison is 'user chose number 2'. If user chose nothing, he didn't choose number 2.
In case where `optional<T>` is compared to `none`, it is not required that `T` be __SGI_EQUALITY_COMPARABLE__.
In a similar manner, type `optional<T>` is __SGI_LESS_THAN_COMPARABLE__ whenever `T` is __SGI_LESS_THAN_COMPARABLE__. The optional object containing no value is compared less than any value of `T`. To illustrate this, if the default ordering of `size_t` is {`0`, `1`, `2`, ...}, the default ordering of `optional<size_t>` is {`boost::none`, `0`, `1`, `2`, ...}. This order does not have a practical interpretation. The goal is to have any semantically correct default ordering in order for `optional<T>` to be usable in ordered associative containers (wherever `T` is usable).
Mixed relational operators are the only case where the contained value of an optional object can be inspected without the usage of value accessing function (`operator*`, `value`, `value_or`).