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

On 12/07/13 10:37, Larry Evans wrote:

On 12/07/13 10:21, TONGARI J wrote:

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2013/12/7 TONGARI J

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2013/12/7 Antony Polukhin

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2013/12/7 TONGARI J <...>

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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(derived) or variant_cast(&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.

Aside: Do we have something like unconditional_get that doesn't perform the runtime check?

Wouldn't reinterpret_cast do?

OOPS. The storage_ member variable is stored after the which_ tag as shown on lines 1237 and 1275 of here: https://github.com/boostorg/variant/blob/master/include/boost/variant/varian... Also, the storage_ member variable maybe does not start with the actual storage. Instead, looking at some of the code shows it's reached by: storage_.address() So, I was wrong about using reinterpret_cast, and I've no idea how to do it without modifying the variant code, maybe by providing acciess to storage_.address(). -regards, Larry