llvm dev - Dec 2008 - [LLVMdev] dyn_cast really doesn't like multiple inheritance

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

On Dec 9, 2008, at 8:46 PM, 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:
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.  :)  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

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