Re: [boost] [variant2] documentation request

On Sat, Mar 2, 2019 at 1:20 PM Peter Dimov via Boost wrote:

Niall Douglas wrote:

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It so happens that the strong guarantee is unachievable with variant (without too much double buffering.) You can either have basic, or noexcept.

You surprise me, given this is a design capable of double buffering.

For a variant2, where both A and B have throwing move constructors and assignment, surely if the variant has state A in buffer1, setting it to state B would use buffer2. If B throws during move, we simply don't change the currently selected buffer to buffer2. The variant's A state remains untouched i.e. strong guarantee.

I guess you haven't written out anywhere how and when the double buffering comes into play.

That is actually the one thing I have written, twice, once in the README and one in the Overview section of the documentation. :-)

"To avoid going into a valueless-by-exception state, this implementation falls back to using double storage unless

* one of the alternatives is the type monostate, * one of the alternatives has a nonthrowing default constructor, or * all the contained types are nothrow move constructible."

So, yes, variant will be double-buffered, but variant and variant won't be.

For variant, can I assign a T that has an operator int()? Is assignment templatized? What happens with struct Bad { operator int() { throw false; } }; variant v = (short)10; v = Bad();