Re: Multi-method

Jean-Marc Bourguet wrote:


> Is there such a technique allowing to dispatch on the type of several
> parameters
> - in time proportionnal to the number of dispatching parameter
> - in such a way that independant compilation is possible.


The first move would be getting rid of vtable. The dispatch table
should be associated rather with methods than with objects. For a
method with n dispatching parameters the dispatch table could be a
n-dimensional array of thunks. The type tag could be a pointer to the
type vector that for each method gives an index in the dispatch
table. Thus a dispatching call would require n x indexing the types'
vectors + indexing the method's dispath table.


The major problems are:


1. Maintaining the data structures supporting dispatch at run time.
Provided that overriding may occur in a nested block, consider:


function Funny (...) is
      type AA is new A ... -- A new type derived from A
      function Foo (...) -- Override some method Foo of A
      Object : AA;
begin
      Foo (Object, ...); -- Dispatch to the local version of Foo
end Funny; -- Here AA is finalized and all effects should disappear


2. Freezing rules. Methods cannot be associated with just one type. Thus
they cannot be declared like in C++ within type declarations. The freezing
point in C++ is the closing } of the type declaration. One must relax this
rule, but not very loosely. Because arbitrary overrdinings are near to
impossible to maintain.


--
Regards,
Dmitry Kazakov
www.dmitry-kazakov.de

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