Re: use of vtbl

"Lieven Marchand" <mal@wyrd.be> writes:


> In some implementations this is done by keeping a table of offsets
> in the vtbl, in others it's done by letting the function in the
> table be a trampoline that adjusts the this pointer before jumping
> to the real function.


There are many more issues to consider:


- how do you represent RTTI, in particular:
    - at what slot do you put RTTI (typically -1 or 0)?
    - does that slot indicate a function or a typeinfo pointer?


- how do you represent covariant return types?


- how do you represent function pointers? The typical solution is to
    take the address of the function. This is a problem for
    position-independent code, though, as that address may be known only
    at run-time, so the dynamic loader would have to fix all vtables.


- where do you put the implicit virtual destructor?


- is there any virtual deallocation function (not mandated by
    standard, but MSVC adds a slot for that, anyway)


- how do you represent virtual bases?


There are probably more issues.


Regards,
Martin

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