Clone a class without copy constructor

[jmrag]

Is there any way to create a class through a function that its scope is in the outside??
To be clear i have this

Class A{
public:
      int id;
      //function clone();
};
main(){
A b;
b.clone();// i want this function to create a new object A with the same
          // data member's values of b 

[Gaminic]

Why?

I guess you could use the empty constructor and then manually fill in the data. Assuming a already exists:

A b; 
b.id = a.id;

Or, if it's allowed, use the assignment operator:
A b = a;
If you want to do it inside a function, be wary of scope problems. You'll have to create the "clone" outside the function and pass it by reference, else it's just a temporary and it disappears. Also, if class A has any private members, the function will need to be a friend of the class.

Most importantly, though: Why?

[jmrag]

Because my university's teachers are idiots!

Thanks for answer

[Gaminic]

To be honest, that seems more like a poor excuse than a reason. There is no good reason not to define a copy constructor and call it. You can still wrap it in a clone() function if you want to.

[bradw]

C++ will do a shallow copy by default.

class A {
public:
    int id;
};

int main() {
    A b;
    A c;
    b.id = 15;
    c = b;
    // c.id is 15
}

If you really want to have a clone function, it would have to use the new operator, and delete it afterwards. Another option would be to pass by reference, but that would add a parameter to the clone function (for the clone destination), and I think it is less intuitive.

class A {
public:
    int id;
    A *clone();
};

A *A::clone(){
    A *cloned = new A();
    *cloned = *this;
    cout << "created a clone of " << this << " at " << cloned << endl;
    return cloned;
}

int main()
{
    A b,c,*d;
    b.id = 15;
    c = b;
    d = c.clone();
    
    // show that values are the same
    cout << b.id << endl;
    cout << c.id << endl;
    cout << d->id << endl;

    // show that addresses are different
    cout << &b << endl;
    cout << &c << endl;
    cout << &d << endl;
    cout << d << endl;

    // must delete the cloned copy because we made it with new
    delete d;

    return 0;
}

[Wazzak]

Want a friend?

class A
{
    public:
        friend A Clone();
        void Print()
        {
            std::cout << "this->Member" << " = " << this->Member << std::endl;
        }

    private:
        int Member;
};

A Clone()
{
    A _X;
    _X.Member = 10;
    return(_X);
}

int main()
{
    Clone().Print();
}

Additional information:

A friend function doesn't belong to a class, but is allowed to access the private members. Think of it this way: There's a tray of cakes on the counter. Only the person who baked the cakes (the class), and the baker's friends can eat the cakes. If you're not the baker, or a friend of the baker, on your bike.

_Wazzak