Re: [boost] [variant] Why boost::get<Base>(derived) not supported?

Subject: Re: [boost] [variant] Why boost::get<Base>(derived) not supported?
From: TONGARI J (tongari95_at_[hidden])
Date: 2013-12-07 10:46:36

2013/12/7 Antony Polukhin <antoshkka_at_[hidden]>

> 2013/12/7 TONGARI J <tongari95_at_[hidden]>
> <...>
>
> > Maybe we can promote this idea to something more general, like
> > 'variant_cast', not necessary have to be in base-derived relationship but
> > just something convertible.
> >
> > Does that make sense?
> >
>
> I'd prefer not mixing functionalities. We can make polymorphic_get<> work
> only for polymorphic casts (Base and Derived can be checked using type
> trait).
>

I just looked into the code of get, and it also uses visitor, a bit
disappointed.
Since the current boost::get requires exact type match (except const),
there's only one viable way from the types, I thought it might do better
under the hood (i.e. single if-check instead of switch), but it didn't,
it's just a more restrictive visitor than what I shown in my sample code.

And for type-checking, I think the user should use .which(), but I don't
know how many ones already did that with get, so it's a valid concern not
to break others' code...otherwise I'm inclined to make it the default
behavior of boost::get.

I'm OK with polymorphic_get (except the long name, though), since that's my
use-case.
But variant_cast seems more attractive.
You can do variant_cast<Base&>(derived) or variant_cast<Base*>(&derived) in
that case, and variant_cast<Compatible>(v) for other conversion.
If you want to go further, you can ensure that at least one viable
conversion exists at compile time by some metaprogramming tricks.