下面的代码有问题，如何改正？且如何将fly，swim动作以接口的形式剥离出来呢？

Question

class Bird
{
virtual void fly();  //birds can fly
void travel() { fly(); } //birds travel by flying
}
class Eagle: public Bird
{
virtual void fly() {/* implement the flying code here */}
}
class Penguin: public Bird
{
virtual void swim(); //penguins can swim
}
Eagle e();
e.travel();  //OK, eagle can fly
Penguin p();
p.travel();  //Error, penguin can't fly

Answer 1

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

class Eagle: public Bird
{
virtual void travel() { fly(); }
void fly() {/* implement the flying code here */}
}

class Penguin: public Bird
{
virtual void travel() { swim(); }
void swim() {}
}
Eagle e();
e.travel();  //OK, eagle can fly
Penguin p();
p.travel();  //Error, penguin can't fly