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Re: [std-proposals] std::any::base

From: Mihail Mihaylov <mihail.mihailov_at_[hidden]>
Date: Tue, 16 Apr 2024 14:58:36 +0300
On Tue, Apr 16, 2024 at 2:03 PM Sebastian Wittmeier via Std-Proposals <
std-proposals_at_[hidden]> wrote:

> That ambigious base classes can be only detected at runtime, also hints
> that casting any to a base class is probably quite an expensive operation
> (compared to dynamic_cast).
> Mihail: "Then why would it be good practice, if we transform these
> functions to take in an any* or any&, to force them to perform a widening
> any_cast to polymorphic_value<A> followed by a dynamic_cast<B&> or
> dynamic_cast<C&> respectively?"
> Normally the complicated double casts (any_cast + dynamic_cast) are not
> needed: In many situations you would just transfer a polymorphic_value<A>
> without std::any.

We are not discussing cases where there is a common base, only the case of
disjoint types when at least one of them can have descendants.

> But just in case, in those situations (where also totally different types
> are transferred), it would be best practice to chain both casts exactly for
> the following reasons:
> - it is more performant (no scan through inheritance hierarchies for each
> cast needed)

As far as I know, under many circumstances, *dynamic_cast* can be expensive
too. And yes, it's a runtime cast, which is expected to be more costly, we
can assume that the user has weighed the performance cost.

> - ambiguous base classes A (for B and/or C) would be detected at
> compile-time

I don't think that *polymorphic_value<T>* would prevent you from storing in
it a class that has an ambiguous base other than *T*. Besides, "detected at
compile-time" means "forbidden". It should be OK to store in an *any* a
class that uses non-virtual multiple inheritance, as long as no one tries
to cast it to the ambiguous base.

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Received on 2024-04-16 11:58:50