C++ Primer, Fifth Edition

Classes with Pure Virtuals Are Abstract Base Classes

A class containing (or inheriting without overridding) a pure virtual function is an abstract base class. An abstract base class defines an interface for subsequent classes to override. We cannot (directly) create objects of a type that is an abstract base class. Because Disc_quote defines net_price as a pure virtual, we cannot define objects of type Disc_quote. We can define objects of classes that inherit from Disc_quote, so long as those classes override net_price:

// Disc_quote declares pure virtual functions, which Bulk_quote will overrideDisc_quote discounted; // error: can't define a Disc_quote objectBulk_quote bulk;       // ok: Bulk_quote has no pure virtual functions

Classes that ...