LLVM has a relatively large number of proprietary replacements for standard C++ functions and classes. One of these is dyn_cast to replace dynamic_cast. The two calls appear to be semantically equivalent; the only difference that I can see is that dyn_cast reportedly works on classes that have no v-table [1]. Could someone please explain why I should use dyn_cast instead of dynamic_cast? (I thought all classes have v-tables...) Thanks,
Trevor
Trevor Harmon <Trevor.W.Harmon@nasa.gov> writes:
[snip]
Could someone
please explain why I should use dyn_cast instead of dynamic_cast? (I
thought all classes have v-tables...) Thanks,
For reducing executable size, LLVM builds with RTTI disabled where
possible.
dyn_cast is also much much faster than dynamic_cast, which is really important because llvm uses dyn_cast all over the place.
-Chris
Right, but how does that relate to dyn_cast? I thought v-tables were present even when RTTI is not (because otherwise polymorphism would be impossible).
Trevor
dynamic_cast uses rtti, not vtables. dyn_cast doesn't use either, unless you implement the classof method to use it (which is not typical).
-Chris
Trevor Harmon <trevor.w.harmon@nasa.gov> writes:
Could someone
please explain why I should use dyn_cast instead of dynamic_cast? (I
thought all classes have v-tables...) Thanks,For reducing executable size, LLVM builds with RTTI disabled where
possible.Right, but how does that relate to dyn_cast? I thought v-tables were
present even when RTTI is not (because otherwise polymorphism would be
impossible).
v-tables are used for dispatching method calls. If you want to know if
something points to certain class (or to a derived class of certain
class) you need RTTI.
This is off-topic here. Googling for RTTI will resolve any remaining
doubts.