Re: [boost] Strong guarantee variant (Was: Outcome/expected/etc/etc/etc)

Andrzej Krzemienski wrote:

2017-06-06 14:20 GMT+02:00 Peter Dimov via Boost :

...

Types don't generally provide the strong guarantee on assignment as it doesn't compose. If type T has strong assign, type U has strong assign, struct { T t; U u; } no longer does. So it's more efficient to just do basic everywhere, which does compose, and then at the specific point you want the strong guarantee, use f.ex. the copy and swap trick to get it.

I think the reasoning behind Niall's position is that a type `X` can have a custom guarantee: upon its assignment it guarantees that after a throw the type is either unchanged or goes to fallback state `0`. This guarantee is stronger than basic, and weaker than strong.

Types _could_ provide such a guarantee, but don't, for the same reason they don't provide strong - as outlined in the quoted paragraph above, if types X and Y provide it, struct { X x; Y y; } no longer does. So it's additional work, and buys nothing much.