A Derived Class Constructor Initializes Its Direct Base Class Only

Now we can reimplement Bulk_quote to inherit from Disc_quote rather than inheriting directly from Quote:

// the discount kicks in when a specified number of copies of the same book are sold
// the discount is expressed as a fraction to use to reduce the normal price
class Bulk_quote : public Disc_quote {
public:
    Bulk_quote() = default;
    Bulk_quote(const std::string& book, double price,
              std::size_t qty, double disc):
          Disc_quote(book, price, qty, disc) { }
    // overrides the base version to implement the bulk purchase discount policy
    double net_price(std::size_t) const override;
};

This version of Bulk_quote has a direct base class, Disc_quote, and an indirect base class, Quote. Each Bulk_quote object has three subobjects: an (empty) Bulk_quote part, a Disc_quote subobject, and a Quote subobject.

As we’ve seen, each class controls the initialization of objects of its type. Therefore, even though Bulk_quote has no data members of its own, it provides the same four-argument constructor as in our original class. Our new constructor passes its arguments to the Disc_quote constructor. That constructor in turn runs the Quote constructor. The Quote constructor initializes the bookNo and price members of bulk. When the Quote constructor ends, the Disc_quote constructor runs and initializes the quantity and discount members. At this point, the Bulk_quote constructor resumes. That constructor has no further initializations or any other work to do.