The only problem is to ensure the entire team agrees to only use it like an interface and nothing else. But I guess that’s the only proper way to do it in C++, for now.

Yet I still had an urge to explain an obvious thing. Because it's C++, so everyhing goes. There are even tools to auto-generate C++ interfaces, because of course someone decided that C++ is inadequate and must be improved using some kind of poorly-documented ad-hoc extension language on top of C++.

In your example, the declaration of ArrayList look like:

public class ArrayList extends AbstractList implements List {
}

The dependence on AbstractList is public. Any public method in AbstractList is also accessible from the outside. It opens up for tricky dependencies that can be difficult to unravel.

Compare it with my solution:

public class ArrayList implements List {
    private AbstractList = new AbstractList();
}

Nothing about the internals of ArrayList is exposed. You’re free to change the internals however you want. There’s no chance any outside code will depend on this implementation detail.