Put (Smart) Pointers, Not Objects, in Containers

When we need a container that holds objects related by inheritance, we typically define the container to hold pointers (preferably smart pointers (§12.1, p. 450)) to the base class. As usual, the dynamic type of the object to which those pointers point might be the base-class type or a type derived from that base:

vector<shared_ptr<Quote>> basket;
basket.push_back(make_shared<Quote>("0-201-82470-1", 50));
basket.push_back(
    make_shared<Bulk_quote>("0-201-54848-8", 50, 10, .25));
// calls the version defined by Quote; prints 562.5, i.e., 15 * $50 less the discount
cout << basket.back()->net_price(15) << endl;

Because basket holds shared_ptrs, we must dereference the value returned by basket.back() to get the object on which to run net_price. We do so by using -> in the call to net_price. As usual, the version of net_price that is called depends on the dynamic type of the object to which that pointer points.

It is worth noting that we defined basket as shared_ptr<Quote>, yet in the second push_back we passed a shared_ptr to a Bulk_quote object. Just as we can convert an ordinary pointer to a derived type to a pointer to an base-class type (§15.2.2, p. 597), we can also convert a smart pointer to a derived type to a smart pointer to an base-class type. Thus, make_shared<Bulk_quote> returns a shared_ptr<Bulk_quote> object, which is converted to shared_ptr<Quote> when we call push_back. As a result, despite appearances, all of the elements of basket have the same type.