design problem with inheritance

I am reading a design pattern book.

the example is a class duck that a method fly

class duck
{
    virtual void fly(){......}
    void display();

}

class FakeDuck: public duck()
{
    void fly(){ override to nothing...}

}

if I have a FakeDuck class it's need to override the fly method to
nothing because a FakeDuck can't fly.
And so on for every subclass of duck that have no fly feature.

book propose the pattern strategy and to extern the fly behavior as a
external class

class FlyBehavior
{
     virtual void fly()=0;
}

class NoFly: public FlyBehavior
{
    virtual void fly(){ printf("can't fly")};

and the class duck became

class duck
{
    FlyBehavior * bhv;
    void performFly(){ bhv->fly(); }
}

I have doubts about the design proposed because book says
encapsulating the algorithms in a class hierarchy and make them
interchangeable.
But ok if I need the private data about duck class to implement the
fly method of NoFly class how can do it??
What's the best solutions?

Thanks in advance.

Ale.