Re: Question about OO programming in Ada

[Hyman Rosen <hyrosen@mail.com>]

Dmitry A. Kazakov wrote:
> Dispatch table is a property of a dispatching subroutine.

No it's not, at least not in the nearly universal C++ implementation.
In that approach, there exists one dispatch table per tagged class
with slots for each virtual function of that class. Then every object
of that class contains a pointer to this single dispatch table. To
make a dispatching call, the compiler accesses the dispatch table
using the index corresponding to the function being called, and calls
the method found there. (The model is more complicated due to virtual
and multiple inheritance, so that an object may contain pointers to
more than one dispatch table, and the addresses are sometimes to fixup
thunks, but that doesn't matter here.)

I'm pretty sure that at least GNAT implements Ada dispatching the
same way.

> It is a complementary view. Either we say that the type of an object
> is mutating or that the type of a formal parameter does (= changing
> the dispatch table = changing the subroutine parameter profile). The
> effect is same. It is no more strongly typed.

In C++, the type of 'this' in a method is always 'pointer-to-class-type-
of-this-method', or a const variant of that. The (dynamic) type of the
object pointed to by 'this' can change as control passes through *tors.
Note that for typical C++ implementations we are not just "saying" that
the type of the object changes; the generated code actually modifies the
dispatch table pointer stored within the object as it progresses through
its construction and destruction. (In fact, I believe Microsoft's C++
compiler offers an option to disable this for efficiency when you can
assure it that it won't matter.)