Abstract inheritence

I understand that a class containing abstract methods (virtual <type> <method name>(<params>) = 0; ) can only be used to derive from and that the deriving class must implement the abstracted methods, but I'm wondering if it's possible for abstract classes to inherit from other abstract classes. See, in other languages (ones that utilize interfaces rather than multiple inheritence) the interface can inherit another interface(s). I'm wondering if the same can be said for abstract classes in C++.

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;
};

---

Quote:

---

multiple inheritance can cause confusion

---

only if you're confused about how multiple inheritance works.

@ OP:

sure there's no reason you can't do that. you just have to implement both classes' abstract methods before you can instantiate something deriving from them.

Quote:

---

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

i think that's the problem

---

That's not a problem. In fact, if you override a virtual function in a base class, it is automatically virtual whether you specify it or not.