[Lldb-commits] [PATCH] D62934: [LanguageRuntime] Introdce LLVM-style casts

Pavel Labath via Phabricator via lldb-commits lldb-commits at lists.llvm.org
Thu Jun 6 00:53:15 PDT 2019

labath added a comment.

Implementing `classof` via an enum is the simplest and most common solution, but it is not the only thing possible. The central enum thingy is fine when all the subclasses are also defined centrally, but for a more distributed scenario like this one, it begins to smell, as now the `LanguageRuntime` class suddenly needs to be aware of all of its subclasses.

A way to implement that without the all-encompassing enum would be via something like:

  struct Runtime: {
  virtual bool isA(const void *ClassID) const { return ClassID == &ID; }
  static char ID;
  struct DerivedRuntime: ParentRuntime {
  bool isA(const void *ClassID) const override { return ClassID == &ID || ParentRuntime::isA(ClassID); }
  static bool classof(const Runtime *R) { return R->isA(&ID); }
  static char ID;

It takes a while to wrap your head around this, but what it basically does is use pointer equality instead of enum comparisons. Then, if you have a multi level hierarchy (like we have here), it uses the virtual `isA` function to check for subclasses instead of doing a range comparison: The desired target class type is captured in the `classof` call. Then the `|| Parent::isA` chain builds up a list of actual classes that this object is an instance of. If the captured type is found in this list, then the cast can proceed. You can see this in action in `llvm/Support/Error.h` (though the ErrorInfo class does not implement the `classof` method, because it wants to do casting differently.)

This is a bit more complex set up, but I think the overall result is worth it.



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