[llvm-dev] RFC: Towards unified semantic for casts

[Dmitriy Borisenkov via llvm-dev]

Hi everyone,

I have an idea that should allow reducing code duplication in Casting.h
while making llvm::isa, llvm::cast, llvm::dyn_cast, etc more generic. Since
we added unique pointers support for these template functions (see
ab480f45cd23c08cb9aa3f427aad072df249135f) I propose to generalize their
semantics to deal with any pointer-like type (i.e. dereferenceable and
implicitly convertible to bool) so that:

   - for each fancy_pointer<T> t: isa<G>(t) returns true iff current
   implementation of isa<G>(nonfancy_t) returns true where
   decltype(nonfancy_t) is T*
   - all old cast operations should return a raw pointer of type typename
   std::pointer_traits<fancy_pointer<T>>::element_type * and do not perform
   ownership transfer.
   - move_[dyn_]cast should do what unique_dyn_cast is currently doing in a
   more generic manner: it moves to an object of type typename
   std::pointer_traits<fancy_pointer<T>>::rebind<G>

N.B. std::pointer_traits is a conception that I use to explain the
behaviour - not necessary the way I plan to implement it.


An example implementation of the proposal for llvm::isa :
https://reviews.llvm.org/D42027


--

Kind regards, Dmitry
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