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From: Ole-Hjalmar Kristensen <ole-hjalmar.kristensen@sun.com>
Newsgroups: comp.lang.ada
Subject: Re: Is the Writing on the Wall for Ada?
Date: 18 Sep 2003 10:28:14 +0200
Organization: Sun Microsystems
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>>>>> "JSA" == Jacob Sparre Andersen <sparre@crs4.it> writes:

    JSA> Olehjalmar Kristensen wrote:
    >>>>>>> "RIE" == Robert I Eachus <rieachus@attbi.com> writes:

    RIE> interfaces  and mix-ins.  But  type inheritance  cannot  be from  two
    RIE> concrete parents,  no matter what  the language--one parent has  to be
    RIE> abstract. So  anyone who condemns Ada  for not adding  what cannot be
    RIE> done needs to get a life.
    >> What do you mean by "concrete parents"?


    JSA> Although  I agree  that  "non-abstract parents"  is  the most  obvious
    JSA> interpretation   of   "concrete   parents",   I   understood   it   as
    JSA> "data-containing non-abstract parents".

I understand now, by see below.

    >> class a {
    >> public:
    >> void foo();
    >> };
    >> class b {

    >> public:
    >> void bar();
    >> };
    >> class c: public a, public b{

    >> };
    >> Works just fine,


    JSA> Looks that  way to me  too. But  if a and  b had contained  data, for
    JSA> example  position  data  in  respectively  spherical  and  rectangular
    JSA> coordinates (to reuse an earlier  example), then you would most likely
    JSA> end up in trouble.


    JSA> Jacob
    JSA> -- 
    JSA> Rent-a-Minion Inc. Because good help is so hard to find.

Depends on what you expect. As you can see, there is no conflict as
such. On the other hand, a and b can not access each others variables
either.
In the general case I agree, there may be problems, but I think that
the compiler rejecting cases it cannot resolve properly is quite acceptable.

file multi.cc:

#include <stdio.h>

class a
{
public:
  int x;
  a() : x(13) {}
  void foo(){printf("%p %d\n",&x,x);};
};

class b
{
public:
  int x;
  b() : x(666) {}
  void bar(){printf("%p %d\n",&x,x);};
};

class c : public a, public b
{
};

main()
{
  c cc;
  cc.foo();
  cc.bar();
}

ok145024@khepri06:~/div> ./multi
8046abc 13
8046ac0 666

To let the two classes use each others functions and data you can do
the following:

file multi2.cc:

#include <stdio.h>

class base
{
public:
  virtual int x1() = 0;
  virtual int x2() = 0;
};

class a : public virtual base
{
public:
  int x;
  a() : x(13) {}
  int x1() {return x;}
  void foo(){printf("%p %d %d\n",&x,x,x2());};
};

class b : public virtual base
{
public:
  int x;
  b() : x(666) {}
  int x2() {return x;}
  void bar(){printf("%p %d %d\n",&x,x,x1());};
};

class c : public a, public b
{
};

main()
{
  c cc;
  cc.foo();
  cc.bar();
}

ok145024@khepri06:~/div> ./multi2
8046ab4 13 666
8046abc 666 13

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