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

From: Fernando Cacciola (fcacciola_at_[hidden])
Date: 2001-08-30 17:38:06

----- Original Message -----
From: Douglas Gregor <gregod_at_[hidden]>
To: <boost_at_[hidden]>
Sent: Thursday, August 30, 2001 4:43 PM
Subject: Re: [boost] Re: optional vs variant vs any

> 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>().
>
Not quite so. AFAIK, any_cast<> and .as<> are rvalues.

There is a problem, anyway, in supporting a non-explicit direct
initialization:

optional<double> opt = foo();
if ( opt == 3.14 )  // This will automatically create a temporary for the
3.14, thus violating the strict pointer semantic I am trying to keep strict.

> >
> >   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.

Perhaps I am biased by the coding style of having only one return statatent
at the end. So direct initialization won't usually be of much help to me in
a return statement.

There is a situation, though, in which direct initialization is
*significantly* useful:

void foo ( optional<point> where ) ;

with direct initialization:   foo ( optional<point>( point(2,3) ) ) ;

currently:  optional<point> p ; *p = point(2,3) ; foo(p);

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

>
> > 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.
>
I'll consider adding direct initialization through explicit ctor.

Fernando Cacciola
Sierra s.r.l.
fcacciola_at_[hidden]
www.gosierra.com