Thread: Abstract inheritence

why would you do that ??
abstract class is already an abstract class, one I think should be enough because after all it's generic and should contain all functions

multiple inheritance can cause confusion

Sure it's possible. You might want an abstract subclass to provide a partial default implementation. Even if you don't provide a default impl, you may want to extend the interface.

Code:

#include <iostream>

class A {
public:
    virtual void func1() = 0;
    virtual void func2() = 0;
};

class B : public A {
public:
    virtual void func2() 
    {
	std::cout << "B::func2()" << std::endl;
    }
};

class C : public B {
public:
    virtual void func1() 
    {
	std::cout << "C::func1()" << std::endl;
    }
};

int main()
{
    A * c = new C;
    // Can't do this, because B is abstract
    // A * b = new B;  

    c->func1();
    c->func2();
}

$ g++ abstract.cpp
$ a.out
C::func1()
B::func2()
$

because in A there is func2() which is pure
and you define it again in B not as pure but just virtual

you are trying to make a function virtual while it already exists as virtual

i think that's the problem

It's not a problem if that's what you want. You may want a sub-interface that also cannot be instantiated.

Code:

class Bird {
public:
    virtual ~Bird() {}
    virtual void sing() = 0;
    virtual void eat() = 0;
    virtual void walk() = 0;
};

class FlyingBird : public Bird {
public:
    virtual void fly() = 0;
};