One essential feature of objects is that they can be classified. Each object belongs to a class. This is a straightforward relationship between an object and class with a simple, single-inheritance design.

With multiple inheritance, the classification problem can become complex. When we look at real-world objects, such as coffee cups, we can classify them as containers without too much difficulty. That is, after all, their primary use case. The problem they solve is that of holding coffee. However, in another context, we may be interested in other use cases. In a decorative collection of ceramic mugs, we might be more interested in size, shape, and glaze than we are in the coffee-carrying aspect of a cup.

Most objects have a straightforward is-a relationship with a class. In our coffee-holding problem domain, the mug sitting on the desk is a coffee cup as well as a container. Objects may also have several acts-as relationships with other classes. A mug acts as a piece of...