Re: [boost] [outcome] To variant, or not to variant?

Subject: Re: [boost] [outcome] To variant, or not to variant?
From: Peter Dimov (lists_at_[hidden])
Date: 2017-06-02 00:43:57

Gavin Lambert wrote:
> On 2/06/2017 00:15, Peter Dimov wrote:
> > That's what I was thinking - that you want to place an assert there - I 
> > just wanted to confirm that this is the only objection. Do we agree that 
> > returning nullptr from operator-> is good for informing the static 
> > analyzer and the optimizer that a value is present?
>
> I assume you meant "not present" there.

No, I do mean "present". If you do `r->x` in your code, the compiler from 
this point on will optimize based on the assumption that a value was 
present - otherwise what you did would have been undefined.

Actually, it will even propagate the "has_value()" assumption backwards from 
the `r->x` point, not just forwards.

Returning a reinterpret_cast'ed pointer, on the other hand, will not do so, 
although maybe an `assert( has_value() )` will, depending. (Last time I 
recall Microsoft tried feeding asserts to the optimizer, they reverted it, 
as it broke too much code, but things may have changed in the meantime.)