Copy object - keep polymorphism

Question

The following code tries to copy an object and keep the original type. Unfortunately it does not work (every copied object will become a Super instead of being

Answer 1

What you want is generally implemented using an abstract clone method in the base class. Special will typically implement this method by returning new Special(*this).

Also note that it is considered a best practice to make base classes uncopyable.

Answer 2

Just for the record, this is in the C++ FAQ:

http://www.dietmar-kuehl.de/mirror/c++-faq/abcs.html#faq-22.5

Answer 3

This is what you need :

class Super
{
    public:
        Super()
        {
        }

        virtual Super* clone() const
        {
            return( new Super(*this) );
        };
};


class Special : public Super
{
    public:
        Special() : Super()
        {
        };
        Special(const Special& _rhs) : Super(_rhs)
        {
        };
        virtual Special* clone() const
        {
            return( new Special( *this ) );
        };
};

int main()
{
    Special a;
    Super &c( a );
    Super *b1 = c.clone();
    Special *b2 = a.clone();
    Super *b3 = a.clone();
}

One of previous examples has the clone for derived class wrong. The above is correct way of implementing the clone method.

Answer 4

Try this:

class Super
{
public:
    Super();// regular ctor
    Super(const Super& _rhs); // copy constructor
    virtual Super* clone() const {return(new Super(*this));};
}; // eo class Super


class Special : public Super
{
public:
    Special() : Super() {};
    Special(const Special& _rhs) : Super(_rhs){};
    virtual Special* clone() const {return(new Special(*this));};
}; // eo class Special

Note that we have implemented a clone() function that Special (and any other derivative of Super) overrides to create the correct copy.

e.g:

Super* s = new Super();
Super* s2 = s->clone(); // copy of s
Special* a = new Special();
Special* b = a->clone(); // copy of a

EDIT: As other commentator pointed out, *this, not this. That'll teach me to type quickly.

EDIT2: Another correction.

EDIT3: I really should not post so quickly when in the middle of work. Modified return-type of Special::clone() for covariant return-types.