Re: [boost] [variant2] Formal review

Subject: Re: [boost] [variant2] Formal review
From: Phil Endecott (spam_from_boost_dev_at_[hidden])
Date: 2019-04-15 14:46:37

Emil Dotchevski wrote:
> 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.

I think that's often not really the case; at least, it is not a large 
burden.

It seems to me that in many/most cases, the empty state is essentially 
transient; it can exist between the throw in the assignment and the 
end of the variant's scope:

void f() {
  variant<...> v{42};
  v = something;  // throw in assignment; v is empty
  foo();  // skipped
  blah(); // skipped
  // v is destructed
}

You can only observe the empty state if you have a try/catch inside the 
scope of the variant.  Or possibly something with a dtor that accesses 
the variant.  If you limit yourself to not doing that, then you can 
ignore the possibility of empty in the rest of your logic.

What do others think?  Do you believe that it would be common to 
catch the exception thrown during the variant assignment and not 
"fix up" the variant's value, such that code after the catch could 
see the variant in its empty state?

Regards, Phil.