Question

I have a data structure that accepts an abstract class in the c’tor. I’d like to copy the class (via clone method) to one of the data structure’s private fields so I don’t have to keep storing it outside the structure.
I’ve found this explanation on cplusplus.com by jsmith:

// Depending upon your needs, you might not require a base class  
// clonable concept.  It would only be needed if you need to store  
// clonable objects polymorphically.  
struct clonable {  
    virtual ~clonable() {}  
    virtual clonable* clone() const = 0;  
};  

class Base : public clonable {  
public:  
    virtual Base* clone() const  
        { return new Base( *this ); }  
};  

class Derived : public Base {  
public:  
     virtual Derived* clone() const  
         { return new Derived( *this ); }  
};

Here’s what I have so far:

template <class T>
class AbstractBase {
public:
    virtual AbstractBase<T>* clone() const = 0;
    virtual int operator()(const T& lhs, const T& rhs) const = 0;
};

template <class T>
class Derived : public AbstractBase<T> {
public:
    Derived* clone() { return new Derived(*this); } /* ERROR: can't instantiate abstract class */
    int operator()(const T& lhs, const T& rhs) const { return something; }
};

template <class T>
class DataStructure {
public:
    DataStructure(AbstractBase<T>* base) : base(base->clone()) {}
    //...
private:
    AbstractBase<T>* base;
}

I realise it’s calling the c’tor of AbstractBase. Is there an elegant and/or simple way to fix this while keeping AbstractBase abstract?

Answer 1

You need to override

Derived* clone() const;

Note the const.