Re: [boost] variant2 never empty guarantees (was: Re: Outcome/expected/etc/etc/etc)

Gottlob Frege wrote:

On Wed, Jun 7, 2017 at 10:28 AM, Peter Dimov via Boost wrote:

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Gottlob Frege wrote:

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Agreed. But I don't see much value in the never-empty guarantee if it doesn't give you the strong guarantee.

I'm not sure I understand this fully; could you please explain from what expressions, and under what conditions, you expect the strong guarantee?

variant v1, v2; X x;

v1= v2; // do you expect strong guarantee here? v1 = std::move(v2); // here? v1 = x; // here? v1 = std::move(x); // here? v1.emplace<X>(); // here?

How about "all of the above"? At least when X and Y each offer the strong guarantee?

I'm interested in a practical answer, not a theoretically sound one which is of no use. Suppose that X is something that occurs in practice, such as std::vector, not some hypothetical X with a strong assignment, which doesn't. Unless of course you only put types with strong assignment operators into your variants, which in practice confines you to built-ins, in which case all of the above will indeed be not just strong, but nonthrowing as well.