Re: Merits of re-dispatching [LONG]

[brangdon@cix.co.uk (Dave Harris)]

dmitry@elros.cbb-automation.de (Dmitry A. Kazakov) wrote (abridged):
> That is one of several possible interpretation of what happens in C++
> using Ada terms. However, I prefer mine, because it is consistent with
> the fact that the type tag [= vtab] is constant, thus the actual
> specific type is also constant.
> [...]
> When you pass it to some other routine an implicit specific->class
> wide conversion happens, so the routine can dispatch again.

This doesn't work. If you look at the 3 calls to call_foo() in Hyman 
Rosen's example, you'll see it dispatches to 3 different places. Therefore 
we cannot view the object has having a constant type with 2 kinds of 
dispatching. A specific->class-wide conversion would have it printing 
C::foo 2 or 3 times. Your view doesn't match the visible behaviour.

If you think in terms of a conversion, the different calls to call_foo() 
must convert to different types. It's not enough to switch the vtable on 
and off, you need different conceptual vtables.

Given this, its simpler to think of the conversion as happening as soon as 
the constructor is entered, rather than when the constructor calls out.


> An Ada 95 equivalent would be:

Your example only shows 2 types, so doesn't capture the richness of 
behaviour of Hyman's example which has 3. I suspect you have misunderstood 
it.

  Dave Harris, Nottingham, UK | "Weave a circle round him thrice,
      brangdon@cix.co.uk      |   And close your eyes with holy dread,
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 http://www.bhresearch.co.uk/ |   And drunk the milk of Paradise."