result covariant

[Tony Johansson]

Hello!!

When you have result covariance is it then necessary that you have
inheritance.

I mean can you have result covariance without having inheritance.

//Tony

 

[mlimber]

Hello!!

When you have result covariance is it then necessary that you have
inheritance.

I mean can you have result covariance without having inheritance.

//Tony

Inheritance is necessary. Even void* won't work. Try this:

struct A
{
virtual A* Create() const { return new A; }
virtual int* GetVal() const { return new int(); }
};

struct B : public A
{
virtual B* Create() const { return new B; } // Ok
virtual char* GetVal() const { return new char(); } //Error!
};

Cheers! --M

 

[Victor Bazarov]

When you have result covariance is it then necessary that you have
inheritance.

I mean can you have result covariance without having inheritance.

"Covariance" by definition requires inheritance. Is it necessary for
a bicycle to have wheels?

 

[mlimber]

"Covariance" by definition requires inheritance. Is it necessary for
a bicycle to have wheels?

But, Victor, that's "has-a" inheritance! Surely, you meant something
like "Is it necessary for a bicycle to be a mountain bike?" ;-)

Cheers! --M

 

[Victor Bazarov]

But, Victor, that's "has-a" inheritance! Surely, you meant something
like "Is it necessary for a bicycle to be a mountain bike?" ;-)

I think you're confusing what my analogy is for. It is necessary by
definition for a bicycle to have wheels to be a bicycle. It is necessary
by definition for covariance to involve inheritance. I wasn't implying
that a bicycle was covariant with wheels. Yes, a mountain bike would
probably be covariant with a bike if it inherited from it publicly.

V