class B {
public:
      virtual ~B (){}
};

class D : virtual public B {
public:
      void f () {}
};

class Base {
public:
  int i;
};

class A : virtual public Base {
};

class B : virtual public Base {
};

class C : public A, public B {
};

int main(int argc, char* argv[]) {
  C c;    
  c.i = 10;  // без virtual будет ругаться, т.к. у класса С будет два члена i - один от A, другой от B
             
  return 0;
}

class Base {
public:
  int i;
};

class A : virtual public Base {
};
...

struct IPicture {
    //  ...
};

struct IAnimatedPicture : virtual IPicture {
    //  ...
};

class IPictureImpl : public virtual IPicture {
    //  ...
};

class IAnimatedPictureImpl : public [virtual] IAnimatdPicture, IPictureImpl {
    //  ...
};

class A;
class B1: A;
class B2: A;
class C: B1, B2;

#include <iostream>


struct IPicture
{
    virtual void foo() = 0;
};

struct IAnimalPicture : public IPicture
{
    virtual void goo() = 0;
};

struct IAnimatedPicture : public IPicture
{
    virtual void hoo() = 0;
};

class AnimatedAnimalPicture : public IAnimalPicture, public IAnimatedPicture
{
public:
    void foo() { std::cout << "foo" << std::endl; }
    void goo() { std::cout << "goo" << std::endl; }
    void hoo() { std::cout << "hoo" << std::endl; }
};

int main()
{
    AnimatedAnimalPicture pikcha;
    pikcha.foo();
    pikcha.goo();
    pikcha.hoo();
    
    IAnimalPicture * pAnimal = &pikcha;
    pAnimal->foo();
    pAnimal->goo();
    
    IAnimatedPicture * pAnimated = &pikcha;
    pAnimated->foo();
    pAnimated->hoo();
    
    IPicture * pPicture = pAnimated;
    pPicture->foo();
    
    pPicture = pAnimal;
    pPicture->foo();

    return 0;
}