C++ Primer, Fifth Edition

15.9.3. The Derived Classes

The most interesting part of the classes derived from Query_base is how they are represented. The WordQuery class is most straightforward. Its job is to hold the search word.

The other classes operate on one or two operands. A NotQuery has a single operand, and AndQuery and OrQuery have two operands. In each of these classes, the operand(s) can be an object of any of the concrete classes derived from Query_base: A NotQuery can be applied to a WordQuery, an AndQuery, an OrQuery, or another NotQuery. To allow this flexibility, the operands must be stored as pointers to Query_base. That way we can bind the pointer to whichever concrete class we need.

However, rather than storing a Query_base pointer, our classes will ...