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0 pointsleetcrew5y ago0 comments

> Likewise, I believe I have heard of small string optimisation being impossible with std::string for similar reasons.

not impossible, SSO is implemented to some extent in most mainstream c++ compilers.

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I thought that was impossible because it breaks references when moving strings.

I don't see why this would happen. what problems are you envisioning? moves should leave the original object in a valid but unspecified state. that is, a move should not break a reference to the original object itself, but there are no guarantees as to what data it contains afterwards. in the case of a dynamically allocated string, the moved-from object would probably be empty. with SSO, a move is necessarily a copy, so the moved-from string could either be empty or just contain stale data.

are you maybe thinking of iterators? you have to assume iterators are invalid after a move.

jstimpfle5y ago

No, I think the requirement is that a reference to the actual string buffer must be kept valid when the string is moved. And that property breaks with SSO data, which is not placed in the dynamically allocated buffer - it is placed directly in the object struct (it's an optimization after all), which can't be moved.

EDIT: seems I was wrong, and SSO is allowed for std::string. A similar optimization is illegal for std::vector, though, for the reasons I gave above.

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jstimpfle5y ago

I thought that was impossible because it breaks references when moving strings.

leetcrewOP5y ago

are you maybe thinking of iterators? you have to assume iterators are invalid after a move.

jstimpfle5y ago

EDIT: seems I was wrong, and SSO is allowed for std::string. A similar optimization is illegal for std::vector, though, for the reasons I gave above.

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