Question

I need to copy-construct an object simultaneously changing it’s type to another class being a member of the same class-hierarchy. I’ve read about polymorphic copy-constructors and (hopefully) understand the idea behind it. Yet, I still don’t know if this pattern applies to my case and, if so, how to implement it. I think it’s best if I show what I need on an example.

There is a Base class and two child classes, Child1 and Child2. I need to create an object of type Child2 basing on Child1, ie. most of all, I need to copy the object p_int is pointing to from Child1 to Child2. I’ve written a simple program to illustrate it:

#include <iostream>
using namespace std;

class Base {
public:
    Base() { p_int = new int; *p_int = 0; }
    ~Base() { delete p_int; }
    virtual Base* clone() const = 0;

    void setpInt(int val) { *p_int = val; }
    void setInt(int val) { a = val; }
    virtual void print() {
        cout << "Base: ";
        cout << (long)p_int << ":" << *p_int << " " << a << endl;
    }
protected:
    int* p_int;
    int a;
};

class Child1 : public Base {
public:
    Child1() {};
    Child1(const Child1& child) {
        p_int = new int (*child.p_int);
        a = child.a + 1;
    }

    Base* clone() const { return new Child1(*this); }

    void print() {
        cout << "Child1: ";
        cout << (long)p_int << ":" << *p_int << " " << a << endl;
    }
};

class Child2 : public Base {
public:
    Child2() {};
    Child2(const Child2& child) {
        p_int = new int (*child.p_int);
        a = child.a + 1;
    }

    Base* clone() const { return new Child2(*this); }

    void print() {
        cout << "Child2: ";
        cout << (long)p_int << ":" << *p_int << " " << a << endl;
    }
};

int main() {
    Child1* c1 = new Child1();
    Child2* c2;

    c1->setpInt(4);
    c1->print();

    c2 = (Child2*)c1->clone();
    c2->print();
}

Unfortunately, the outcome is as below, ie. there is no type conversion:

Child1: 162611224:4 0
Child1: 162611272:4 1

What exactly do I need to implement, to be able to achieve what I need? I’m starting to think there is a type-conversion mechanism I need to implement rather than a polymorphic copy-constructor, but I’m confused already.

EDIT: Asked a follow up here

Answer 1

Simplest solution would probably be to implement a Child2 constructor taking a Child1& as parameter. Then you could simply call:

Child2* c2 = new Child2(*c1);