C++ Primer, Fifth Edition

15.9.2. The Query_base and Query Classes

We’ll start our implementation by defining the Query_base class:

// abstract class acts as a base class for concrete query types; all members are privateclass Query_base {    friend class Query;protected:    using line_no = TextQuery::line_no; // used in the eval functions    virtual ~Query_base() = default;private:    // eval returns the QueryResult that matches this Query    virtual QueryResult eval(const TextQuery&) const = 0;    // rep is a string representation of the query    virtual std::string rep() const = 0;};

Both eval and rep are pure virtual functions, which makes Query_base an abstract base class (§15.4, p. 610). Because we don’t intend users, or the derived classes, to use Query_base directly, ...