Re: [boost] Re: optional vs variant vs any

From: Douglas Gregor (gregod_at_[hidden])
Date: 2001-08-30 14:43:25

On Thursday 30 August 2001 02:48, you wrote:
> > optional<T> foo() {
> >   if (everything_is_happy_in_the_world) {
> >     return optional<T>(some_value);
> >   }
> >   else {
> >     return optional<T>();
> >   }
> > }
>
> Hmm. Good point again.
> Still, I think this shouldn't be the proper usage. I would write the above
> like this:
>
> optional<T> foo() {
>
>    optional<T> result ;
>
>    if (everything_is_happy_in_the_world) {
>      *result =some_value;
>    }
>
>    return result ;
> }
>
> Supporting direct initialization has the drawback that both statements
> below become equivalent, which I find confusing:
>
>   optional<int> v ;
>
>   v = 3 ;
>   *v = 4 ;

I personally don't mind this equivalence. It's the same thing one deals with 
when using any or a variant type, except that instead of *v we have something 
like any_cast<T>(v) or v.as<T>().

> unless the constructor is made explicit, but even then I find it confusing:
>
>   optional<int> v ( 3 ) ;
>   *v = 4 ;

This might be a good compromise. It looks strange when you name v (why would 
it be optional if you have a value for it?), but it's not quite so 
strange in a return statement.

> I might be happy with a third alternative, a named constructor:
>
>   optional<int> v = optional<int>::create_initialized(3) ;
>   *v = 4 ;

Yuck :)

> I still find odd to use it like you used it. I prefer no direct
> initialization.
> But if there is general agreement to support direct initialization I can
> add it (but only explicit or with a named constructor)

I don't have any actual experience with the optional type, so I'm just trying 
to imagine how I would use it. Perhaps having direct initialization would be 
confusing - you would know better than I for sure.

        Doug