Re: [boost] Outcome/expected/etc/etc/etc

Subject: Re: [boost] Outcome/expected/etc/etc/etc
From: Peter Dimov (lists_at_[hidden])
Date: 2017-06-05 16:08:13

Gottlob Frege wrote:

> It just means I need to double buffer higher up.  I really want my drawing 
> program to just fail (rollback) the transaction if it can't complete - 
> that is what users really want.

That's true, I understand that, and I tried to communicate the need for 
variant to support the strong guarantee on assignment for this reason.

Normally, if your type was, say, vector<>, you don't need all vectors to be 
double-buffered. You do

    vector<> tmp( v );
    process( tmp );
    v.swap( tmp ); // commit transaction

at the places where you want the strong guarantee.

The problem with variant<> is that the above doesn't quite produce the 
strong guarantee when the types aren't nothrow move constructible. I also 
tried to provide a solution to this problem in the form of pilfering, 
because types can typically be pilfered even when they can't be 
nothrow-moved-from, but this didn't go anywhere.