Boost logo

Boost :

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
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
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.

Boost list run by bdawes at, gregod at, cpdaniel at, john at