15.8. Containers and Inheritance

When we use a container to store objects from an inheritance hierarchy, we generally must store those objects indirectly. We cannot put objects of types related by inheritance directly into a container, because there is no way to define a container that holds elements of differing types.

As an example, assume we want to define a vector to hold several books that a customer wants to buy. It should be easy to see that we can’t use a vector that holds Bulk_quote objects. We can’t convert Quote objects to Bulk_quote (§15.2.3, p. 602), so we wouldn’t be able to put Quote objects into that vector.

It may be somewhat less obvious that we also can’t use a vector that holds objects of type Quote. In this case, we can put Bulk_quote objects into the container. However, those objects would no longer be Bulk_quote objects:

vector<Quote> basket;
basket.push_back(Quote("0-201-82470-1", 50));
// ok, but copies only the Quote part of the object into basket
basket.push_back(Bulk_quote("0-201-54848-8", 50, 10, .25));
// calls version defined by Quote, prints 750, i.e., 15 * $50
cout << basket.back().net_price(15) << endl;

The elements in basket are Quote objects. When we add a Bulk_quote object to the vector its derived part is ignored (§15.2.3, p. 603).