In article <1994Jul27.103403.5291@leeds.ac.uk> garyt@resumix.portal.com writes:
>I have an abstract base class, an ABC, with abstract functions. what I want
>to know is why can't abstract functions return abstract classes of
>the same type as the abstract class or one of it's base classes from
>a virtual function.
>
>otherwise how do you use an abc as pure interface class when some of you
>normal ops such as operator + are illegal

 You cant. You couldnt implement + anyhow.
It would be possible to implement a unary operator polymorphically,
the problem is storage management. If C++ had garbage collection,

 class X {
  X& operator!()const=0; // return new object
 };

would make sense in conjunction with covariant returns. C++ doesnt
have garbage collection. Binary operators cant be polymorphic.
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I have an abstract base class, an ABC, with abstract functions. what I want
to know is why can't abstract functions return abstract classes of the same type as the abstract class or one of it's base classes from a virtual function. ie has this been made legal

class A
{
 virtual A func() = 0;  /// declare interface
};

so that in a superclass you can write

class B
{
 B func(); //redeclare interface
};

B B::func() // make it real
{}


otherwise how do you use an abc as pure interface class when some of you normal ops such as operator + are illegal


gary

In article <1994Jul27.103403.5291@leeds.ac.uk>, garyt@resumix.portal.com (Gary Thompson) writes:

> I have an abstract base class, an ABC, with abstract functions. what
> I want to know is why can't abstract functions return abstract
> classes of the same type as the abstract class or one of it's base
> classes from a virtual function.

The following has been made legal:

struct A {
 virtual A *func() = 0;
};

struct B: public A {
 B *func();
};

And it is also legal for references.  This can only be done for the
return type though.  (ARM, chapter 19, section 10.2)

-- Olaf Weber

garyt@resumix.portal.com (Gary Thompson) writes:

>I have an abstract base class, an ABC, with abstract functions. what I want
>to know is why can't abstract functions return abstract classes of the same type as the abstract class or one of it's base classes from a virtual function. ie has this been made legal

>class A
>{
> virtual A func() = 0;  /// declare interface
>};

>so that in a superclass you can write

>class B
>{
> B func(); //redeclare interface
>};

>B B::func() // make it real
>{}


>otherwise how do you use an abc as pure interface class when some of you normal ops such as operator + are illegal


You must change your virtual function to return a A* (pointer to A)
or a A& (reference to A) if you wish to have a child class change
the return type.