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From: swillden@fcom.cc.utah.edu (Shawn Willden)
Newsgroups: 
 comp.lang.ada,comp.lang.c,comp.programming,comp.lang.c++,comp.object
Subject: Re: What is OO (Was Why don't large companies use Ada?)
Followup-To: 
 comp.lang.ada,comp.lang.c,comp.programming,comp.lang.c++,comp.object
Date: 29 Nov 1994 15:48:24 GMT
Organization: University Of Utah Computer Center
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Xref: bga.com comp.lang.ada:8057 comp.lang.c:32381 comp.programming:5461
 comp.lang.c++:38712 comp.object:9089
Date: 1994-11-29T15:48:24+00:00
List-Id: <comp.lang.ada>

Andrew Dunstan (andrew@bugalugs.apana.org.au) wrote:
: In article <5918@gec-mrc.co.uk>, paj@gec-mrc.co.uk (Paul Johnson) writes:

: |> Yes, but you missed out inheritance.  What you have described is the
: |> object *based* techniques.  Object *oriented* techniques allow the
: |> programmer to describe new classes of objects in terms of their
: |> similarity to existing classes.  This is impossible in some
: |> traditional languages (e.g Ada) and very difficult in others (e.g C).
: |> That was the main push behind the development of OO languages.

: Your statement implies that this is more possible in C than in Ada83.
: Would you care to justify that? I've written a lot in both, and AFAIK
: there is nothing at all more you can do in this line in C than in Ada83.
: Rather the reverse.

It's definitely *easier* to do in C, however.  For example:

typedef struct abstractBaseClassStruct
    {
    int (*foo)(struct abstractBaseClassStruct*);
    } AbstraceBaseClass;

typedef struct
    {
    int (*foo)(AbstractBaseClass*);
    int x;
    } DerivedClass;
int derivedFoo(AbstractBaseClass* a) { return ((DerivedClass*)a)->x; }

void initBase(struct AbstractBaseClass* a) { a.foo = 0; }
void initDerived(DerivedClass* a)   { a.foo = derivedFoo; }

int main()
    {
    DerivedClass d;
    AbstractBaseClass* a;

    initDerived(&d);
    a = (AbstractBaseClass*)&d;   /* Okay since structures are same */

    a->foo(a);                    /* calls derivedFoo */
    }

Doing this in C for a non-trivial program is hard.  Doing it in Ada
would be even more difficult, involving some sort of selectors that
would be passed to a global function to emulate function pointers,
and I'm not sure it could even be guaranteed to work (are Ada records
guaranteed to always have the same layout?).

--
Shawn Willden
swillden@icarus.weber.edu