What pattern to use for the following scenario?

I have the following class:

class Base {
public:
    Base(string name) {
        agg = new Aggregate(name);
    }
private:
    Aggregate* agg;
};

Now I need to extend this class:

class Derived : Base {
public:
    Derived(string name) : Base(name) {
        agg2 = new Aggregate2(name);
    }
private:
    Aggregate2* agg2;
};

What I want is when I create a Base object, Aggregate needs to be created and when I create a Derived object only Aggregate2 should be created.

Now this is not happening because Aggregate its created inside the constructor which is called when I create a Derived object and like this Aggregate and Aggregate2 would be created.

I could move the creation to a different method and call that after creating the object.

Is there any other elegant way to do what I want ?