How to implement Cloneable class?

class Cloneable
{
 public:
   virtual Cloneable* clone() const = 0; //return copy of itself
   virtual ~Cloneable() {}
};

class A
{
 public:
   A(const A& a);    
   ~A();  

 private:
   Cloneable* b;
   Cloneable* c;
   std::string* s;     
};

A::~A()  
{
 delete b;
 delete c;
 delete s;
}

Please, help me to understand cloneable pattern. I don't understand how to implement constructor, copy constructor of class A and how can I use it?