dyn_cast really doesn't like multiple inheritance

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Been having a bit of a problem with dyn_cast: Suppose I have a class A that inherits from two base classes, both of which support dyn_cast. In order to use dyn_cast on A, I need to do a bunch of extra work:

1) Since dyn_cast uses reinterpret_cast rather than static_cast, the pointer value won't get adjusted by the cast operation, making the pointer invalid. I end up having to redefine "cast_convert_val" and other parts of the casting machinery for my type, so that it uses static_cast.

2) In every class B which derives from A, it seems like I have to have 4 overloads of 'classof': One for B, one for A, and one for each of A's top-most ancestors. Otherwise I get ambiguity errors.

What I am wondering is, is this use case supported? And could it be made easier?

-- Talin

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Been having a bit of a problem with dyn_cast: Suppose I have a class A
that inherits from two base classes, both of which support dyn_cast. In
order to use dyn_cast on A, I need to do a bunch of extra work:

Hi Talin,

I don't really have any experience with using dyn_cast in this sort of scenario.

1) Since dyn_cast uses reinterpret_cast rather than static_cast, the
pointer value won't get adjusted by the cast operation, making the
pointer invalid. I end up having to redefine "cast_convert_val" and
other parts of the casting machinery for my type, so that it uses
static_cast.

Is it a problem to just use static_cast everywhere?

2) In every class B which derives from A, it seems like I have to have 4
overloads of 'classof': One for B, one for A, and one for each of A's
top-most ancestors. Otherwise I get ambiguity errors.

What I am wondering is, is this use case supported? And could it be made
easier?

I don't think anyone has ever tried it before. :slight_smile: It seems like it should work, if you have changes to the machinery that would make it more general it would be great to improve Casting.h.

-Chris

3

Talin wrote:

Been having a bit of a problem with dyn_cast: Suppose I have a class A that inherits from two base classes, both of which support dyn_cast. In order to use dyn_cast on A, I need to do a bunch of extra work:

1) Since dyn_cast uses reinterpret_cast rather than static_cast, the pointer value won't get adjusted by the cast operation, making the pointer invalid. I end up having to redefine "cast_convert_val" and other parts of the casting machinery for my type, so that it uses static_cast.

2) In every class B which derives from A, it seems like I have to have 4 overloads of 'classof': One for B, one for A, and one for each of A's top-most ancestors. Otherwise I get ambiguity errors.

What I am wondering is, is this use case supported? And could it be made easier?
  
There is a use case in clang, take a look at this source file: http://llvm.org/svn/llvm-project/cfe/trunk/include/clang/AST/DeclBase.h
Contains the Decl class and DeclContext class and has "isa_impl_wrap" and "cast_convert_val" specializations.
Classes can inherit from both Decl and DeclContext, Decl is the "main" base class.
Subclasses that also inherit from DeclContext need to have two more methods: castToDeclContext/castFromDeclContext, like this:

  static DeclContext *castToDeclContext(const TranslationUnitDecl *D) {
    return static_cast<DeclContext *>(const_cast<TranslationUnitDecl*>(D));
  }
  static TranslationUnitDecl *castFromDeclContext(const DeclContext *DC) {
    return static_cast<TranslationUnitDecl *>(const_cast<DeclContext*>(DC));
  }

It's a fuss to setup and use multiple inheritance with dyn_cast, if you can simplify things let us know.

-Argiris