Multiple inheritance and covariant return types.

[Regan Heath]

BCS wrote:
> Reply to Frank,
> 
>> Hi,
>>
>> I have another question on multiple inheritance. The following code
>> does not work (tested with DMD 1.020 and 2.003)
>>
>> ---
>> interface A {}
>> interface B {}
>>
>> interface AB: A, B {}
>>
>> abstract class X {
>> abstract A func();
>> }
>> interface Y {
>> B func();
>> }
>> class XY: X, Y {
>> AB func() { return null; }
>> }
>> ---
>> The compiler complains about the line 'AB func() { return null; }':
>> t.d(38): function t.XY.func incompatible covariant types A() and B()
>>
>> but I think it should work, because AB is both, an A and a B, so the
>> return type is covariant with both overridden methods. In fact, the
>> example works fine, if X is an interface instead of an abstract class.
>>
>> Is this a bug or is it my mistake?
>>
>> Thanks,
>> Frank
> 
> The issues is that casting from a class that implements A, B and AB to 
> each of these results in a slightly different result.
> 
> IIRC when a class is cast to an interface, what you get is a pointer to 
> a v-table inside of the class (not the normal one). However because AB 
> inherits from A and B, you end up getting two different v-tables. So 
> returning an AB interface instance would give you something that is an 
> AB and might be an A or B but not both. (This is all at the binary level 
> of course)

So, AB isn't "A or B" but "A and B" and the OP really wants some way to 
say "A or B".

Specifically 'func' needs to return either A or B based on usage, eg.

A a = func();  //returns A
B b = func();  //returns B

Unfortunately you can't overload based on return type or this might have 
worked:

interface A {}
interface B {}

interface AB: A, B {}

abstract class X {
	abstract A func();
}

interface Y {
	B func();
}

template TFunc(T) { T func() { return null; } }

class XY: X, Y {
	mixin TFunc!(A);
	mixin TFunc!(B);
}

void main()
{
	XY xy = new XY;
	A a = xy.func();
	B b = xy.func();
}

Regan