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Subject: Re: [boost] [optional] Thoughts on disallowing assignment for wrapped references.
From: Stewart, Robert (Robert.Stewart_at_[hidden])
Date: 2011-09-07 07:01:31

Fernando Cacciola wrote:
> >> optional<T>
> >> f()
> >> {
> >> optional<T> result;
> >> // do stuff
> >> return result;
> >> }
> >
> > However, in the latter case, there is no loss of generality
> > since the above would still be a no-no where T a reference
> > type...
> > Or did I miss something in your response?
> You totally missed his point.


> In Robert's example, it is easy to see--or should be easy to
> see--that the fact that the optional<T> is being returned is
> secondary, perhaps even irrelevant, to the point he is making
> His point is about the shortcoming of lacking assignment, not
> about object lifetimes (and BTW, is the same point Nevin and
> I made earlier).

I wouldn't go quite that far...

> Perhaps a less distracting example would be:
> optional<T> o ;
> switch ( cond )
> {
> case a : o = foo1(); break ;
> case b : o = foo2(); break ;
> case c : o = foo3(); break ;
> case e : o = foo4(); break ;
> case e : o = foo5(); break ;
> default : // o remains uninitialized
> }
> Notice that is quite possible to encounter the above example
> in real-life, as it is common that a variable remains
> uninitialized in the default case of a switch (or, for that
> matter, in any branch of a complex conditional).
> In fact, I even wonder if that example couldn't be considered
> as the canonical use case for optional<>'s assignment
> operation.

The one thing you overlook in your example is that there are reasonable alternatives to using optional within a local scope. However, when returning a value from a function, ownership must be transferred to the caller, which reduces the safe options.

Regardless, disallowing assignment for reference types which, in effect, "reseats a reference," is surprising in too many cases. A new, non-assignable optional type might gain traction, but removing assignment from optional<T&> is not acceptable.

Rob Stewart robert.stewart_at_[hidden]
Software Engineer using std::disclaimer;
Dev Tools & Components
Susquehanna International Group, LLP


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