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From: "Matthew Heaney" <mheaney@on2.com>
Newsgroups: comp.lang.ada
Subject: Re: Abstract methods in ADA95
Date: Thu, 17 Oct 2002 12:18:35 -0400
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References: <20021017-143635-828420@foorum.com>
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Date: 2002-10-17T12:18:35-04:00
List-Id: <comp.lang.ada>


"Hector Hugo" <hmijares@indra.es> wrote in message
news:20021017-143635-828420@foorum.com...
>
> class Abstract_Class
> {
> public:
>    virtual void Abstract_Method () = 0;  // C++ abstract method.
>
>    ...
> };

This class smells funny, because you didn't make the dtor virtual:

class AC
{
public:
   virtual ~AC();
   virtual void am() = 0;
};

If you try to delete an object of your original type, your program is
undefined; at a minimum you'll have a memory leak.

Note the the abstract root type must *always* define the dtor --not just
declare it-- even if the dtor is marked as pure virtual.

Also, you should *always* define the dtor as a non-inline function, so that
the vtables have a home.

Ahhh, isn't C++ fun...


> class Son_Class: public Abstract_Class   // C++ tagged type extension.
> {
>  public:
>    void Abstract_Method ();
>
>    ...
> };
>
> void main ()
> {
>    Abstract_Class* p_abstract = new Son_Class;
>
>    p_abstract->Abstract_Method ();
> }


package P is
   type T is abstract tagged null record;
   procedure Op (O : in out T) is abstract;
end P;

package P.C is
   type NT is new T with null record;
   procedure Op (O : in out NT);
end P.C;

declare
   O : T'Class := NT'(T with null record);
begin
   Op (O);
end;

Alternatively, you can allocate off the heap as you did in your C++ example:

declare
   type T_Class_Access is access all T'Class;
   O : T_CLass_Access := new NT;
begin
   Op (O.all);
end;

Or you could rewrite your C++ example to eliminate the heap:

{
   Son_Class x;
   Abstract_Class* const p = &x;
   p->Abstract_Method();  //dispatches
}

Or you could rewrite the C++ example to eliminate the pointer:

{
   Son_Class x;
   Abstract_Class& y = x;
   y.Abstract_Method();  //dispatches
}

Or you could write the example so that the call is statically bound:

{
   Son_Class x;
   x.Abstract_Method();  //static binding
}