Implementing abstract class members in a parent class

Question

Is it possible to implement an abstract base class with members inherited from another parent class in C++?

It works in C#, so I tried doing it in C++:

// Virtual destructors omitted for brevity

class ITalk
{
public:
    virtual void SayHi() = 0;
};

class Parent
{
public:
    void SayHi();
};

class Child : public Parent, public ITalk
{
};

void Parent::SayHi()
{
    std::printf("Hi\n");
}

My compiler didn't really like it though:

ITalk* lChild = new Child(); // You idiot, Child is an abstract class!
lChild->SayHi();

I can't add public ITalk to the Parent class because "base-class 'ITalk' is already a base-class of 'Parent'." I could move public ITalk up to the Parent class, but in my particular scenario that complicates a lot of things.

Answer 1

No because what you really have is two base classes without any knowledge of each other.

Italk    Parent
 / \       / \
  |         |
  +---------+
       |
     Child

If Parent and Italk had two variables named i, there'd be two instances of "i", ITalk::i and Parent::i. To access them you'd have to fully qualify which one you wanted.

The same is true of methods, lChild has two methods called SayHi and you need to clarify which one you mean when calling SayHi because the multiple inheritance has made it ambiguous.

You have Parent's SayHi

lChild->Parent::SayHi();

and Italk's SayHi:

lChild->ITalk::SayHi(); 

The latter is pure virtual and because its abstract needs to be overridden locally in Child. To satisfy this you'll need to define

Child::SayHi();

Which would now hide Parent::SayHi() when invoking SayHi without scoping it to the class:

lChild->SayHi() //parent's now hidden, invoke child's

Of course Child::SayHi() could call Parent::SayHi():

void Child::SayHi()
{
     Parent::SayHi();
}

which would solve your problem.