[llvm-dev] RTTI with smart pointers

[Zachary Turner via llvm-dev]

If you don't expect the cast to fail because you have special knowledge
about its derived type, then you should just use std::static_pointer_cast.
If you do think it might fail at runtime and the only way to know is to try
the dynamic_cast, then you have to use the llvm casting infrastructure
because it is basically LLVM's replacement for dynamic_cast and RTTI.  So
it's up to you to add support for classof() etc to the base and derived.
Then you can write:

if (llvm::isa<Derived>(x.get())) {
  auto p = std::static_pointer_cast<Derived>(x);
}

I don't think we have an llvm::shared_dyn_cast<>, but it might be
reasonable to add one that encapsulated the above if-statement so that you
could write:

if (auto p = llvm::shared_dyn_cast<Derived>(x)) {
  ...
}

On Mon, Jul 31, 2017 at 1:55 PM Vedant Kumar <vsk at apple.com> wrote:

> On Jul 31, 2017, at 5:43 AM, Victor Campos via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
> Hi,
>
> I would like to use std::shared_ptr in my pass. However I'm facing a
> problem wrt RTTI. If I have a code like:
>
> std::shared_ptr<BaseClass> x(new DerivedClass());
> ...
> std::shared_ptr<DerivedClass> p =
> std::dynamic_pointer_cast<DerivedClass>(x);
>
> It does not compile since the default RTTI infrastructure is not used by
> LLVM. Also, it's not clear to me if the 'classof' approach works in this
> case (I did try it with no success).
>
> Is it possible to have a dynamic_cast using std smart pointers in LLVM?
>
>
> I don't know of an off-the-shelf way to do this but you could look
> to r300098 for inspiration (it introduced unique_dyn_cast).
>
> vedant
>
>
> Thanks.
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>
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