Re: [boost] [variant2] Formal review

Subject: Re: [boost] [variant2] Formal review
From: Rainer Deyke (rainerd_at_[hidden])
Date: 2019-04-15 06:57:20

On 14.04.19 22:58, Emil Dotchevski via Boost wrote:
> On Sun, Apr 14, 2019 at 12:53 PM Rainer Deyke via Boost <
> boost_at_[hidden]> wrote:
>> Yes, but the question was about the benefits of the never-empty
>> guarantee.  If the never-empty guarantee doesn't help with maintaining
>> higher level invariants, then what benefit does it bring?
> 
> If the design allows for one more state, then that state must be handled:
> various functions in the program must check "is the object empty" and
> define behavior for that case. The benefit of the never-empty guarantee is
> that no checks are needed because the object may not be empty.

No.  A function is not required to check its invariants and 
preconditions.  If a function is defined as taking a non-empty variant, 
then it is up to the caller to make sure the variant is not empty before 
passing it to the function.  If the function is a member of the same 
object as the variant and the object requires that the variant is 
non-empty as an invariant, then it is up to the other member functions 
of the object to maintain that invariant.  In both cases the function 
can just assume that the variant is non-empty.

-- 
Rainer Deyke (rainerd_at_[hidden])