Re: Announcing Xrtti - Extended Runtime Type Information for C++

Mon, 7 May 2007 23:38:05 CST
On May 8, 1:04 pm, Old Wolf <> wrote:

Then I found out that if I did this cheesy hack:

class Rtti
     virtual ~Rtti() { }

void SomeMethod(void *pObject)
     const std::type_info &info = typeid(*((Rtti *) pObject);

Then this works for getting the real type_info for whatever object
happens to be pointed to by the pObject pointer.

Obviously this code has undefined behaviour and could well die
in a screaming heap if you change compiler switches, or use
a different compiler, or whatever.

What I'm wondering though, is in what sane implementation of a
compiler would the above *not* work?

I imagine that for the above not to work, the compiler would have to,
instead of using a simple principle like putting the RTTI type
information in the same place in every vtable for every class, put it
in different places for different classes depending upon some unknown
criteria. Does anybody know of a case where a compiler actually does

Also - is there any binary compatibility between different compilers
with respect to this? If code generated by one compiler creates an
object (and sets up its vtable and rtti structures), and this object
is passed to code produced by another compiler, does the second piece
of code have any hope whatsoever of being able to extract rtti
information from it?

I have this feeling, although I will probably be proven wrong by
somebody who knows the C++ standard much better than I do, that some
of the "disallowed" actions in C++ are disallowed only in theory only;
only in that the C++ standard says "you can't do this". But that in
reality, there is no compiler that would ever make the thing not work
as expected, because it would be almost nonsensical to do so.

The RTTI hack I mentioned is one such example. Here is another:

offsetof for non-POD types. Why won't C++ support this? Surely the
*compiler* must know how to find a class member given a pointer to a
class, why can't it tell *me* what offset it used to do that? Why
does the C++ standard forbid this? Is it trying to leave the door
open for compiler implementations that somehow figure out how to
locate a member within a class instance using runtime computations
instead of compile-time computations? If so, what's the point? Would
any compiler ever have a reason to do this? And is leaving that door
open so that some theoretical compiler can do something really
bizarre, worth eliminating some convenient functionality (i.e.

Another example I encountered: you are not supposed to #define private
public and #define protected public. Doing so lets you compile code
that has access to data members of classes that it wouldn't otherwise
have access to (my Xrtti system depends on this). So I have read that
the C++ standard says "you are not allowed to do this." But why? I
realize that it would only be useful in a very small number of cases,
but why make me have to fight the language to do something like this?
Access mechanisms for class members are just conveniences to allow the
compiler to help enforce encapsulation. But they are a tool. And I
don't see why they ought to be circumventable in those rare cases that
a programmer needs to circumvent them.

I have my flame-retardant suit on and am ready to be toasted by the C+
+ gurus out there who know all of the reasons that my simplistic
reasoning falls down, and that the hubris I demonstrate in questioning
the C++ standard deserves to be smacked down hard.


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