How to define Iterators for an abstract class in C++

I'm working on a C++ project where I have an abstract class called Aggregate, which represents a container for another abstract class called Primitive.

I want to be able to iterate through an Aggregate, without worrying with the details of how the Primitive objects are actually stored.

Since I'm not really proficient with C++, I have two questions:

First, is it even possible to do something like this?
Second, what exactly should my Aggregate class and its derived classes do in order for this to work?

Any explanation/references are very much appreciated.

Solution

First, is it even possible to do something like this?

Second, what exactly should my Aggregate class and its derived classes do in order for this to work?

Yes, you "simply" add to Aggregate two virtual functions returning a begin() and an end() iterator.

struct Aggregate
{
    struct iterator { /* ... */ };
    virtual ~Aggregate() {}
    virtual iterator begin() { return {}; }
    virtual iterator end() { return begin(); }
};

You can then use range for loops and algorithms from the Standard Library for "free":

for (auto& p : aggregate) {
    p.value = 0;
}

std::copy(aggregate.begin(), aggregate.end(), aggregate_copy.begin());

You'd need some boilerplate code to implement a working iterator though, but you'll manage with a good Google search.

Working demo