C++ Primer, Fifth Edition

Pure Virtual Functions

Thinking about the question in this detail reveals that our problem is not just that we don’t know how to define net_price. In practice, we’d like to prevent users from creating Disc_quote objects at all. This class represents the general concept of a discounted book, not a concrete discount strategy.

We can enforce this design intent—and make it clear that there is no meaning for net_price—by defining net_price as a pure virtual function. Unlike ordinary virtuals, a pure virtual function does not have to be defined. We specify that a virtual function is a pure virtual by writing = 0 in place of a function body (i.e., just before the semicolon that ends the declaration). The = 0 may appear only on the declaration of a virtual ...