Multiple Inheritance Mark Hennessy Dept. Computer Science NUI Maynooth C++ Workshop 18 th – 22 nd September 2006

class Student { private: std::string id; public: std::string get_id(); }; class Employee { private: std::string id; public: std::string get_id(); };

Example:Syntax class TeachingAssistant:public Student, public Employee { … } Teaching Assistant maintains the attributes and the methods associated with both base classes Student, and Employee

Polymorphic Assignment TeachingAssistant * Mark = new TeachingAssistant; Employee * E = Mark; //Legal as a Teaching Assistant is-an Employee Student * S = Mark; //Legal as a Teaching Assistant is-a Student

Common Misuse of Multiple Inheritance A common error is to use Multiple Inheritance as composition rather than specialisation (is-a): E.G. The following is incorrect: class car: public Engine, public Transmission, public Wheels { … }

Problems with Multiple Inheritance:  Name Ambiguity: –Similar names can be used for different operations in the base classes e.g. an employee might have an id_number and a student might also have an id_number field. Both base classes might have a similar get_id() method. The C++ compiler cannot determine which version to use in the TeachingAssistant class: the get_id() in the Employee class or the get_id () in the Student class.

class TeachingAssistant { }; TeachingAssistant * Mark; Mark->get_id(); /* Which class? */

Problems with Multiple Inheritance: Name Ambiguity Solution 1: Use a fully qualified function name: TeachingAssistant * Mark = new TeachingAssistant; cout << “ The TeachingAssistant is” << Mark -> Employee::get_id() << “\n”;

Problems with Multiple Inheritance: Name Ambiguity Solution 2: Redefine the ambiguous function in the new class and hide the qualified name in a method body: class TeachingAssistant:public Student, public Employee { public: string get_id(); public: string student_id(); }

string TeachingAssistant::get_id() { return Employee::get_id(); } string TeachingAssistant::student_id() { return Student::get_id(); }

Problems with Multiple Inheritance:  Replicated Base Classes: Base classes might be inherited more than once (indirectly) due to a class inheritance hierarchy causing duplicated attributes.  In our example suppose the following holds: class Employee: public Person {..} class Student: public Person {..} class TeachingAssistant:public Student, public Employee {..}

Problems with Multiple Inheritance: Replicated Base Classes: Attributes in the Person class get inherited twice! Eg. A teaching Assistant will have two names. To merge any common base classes into one single copy the inheritance should be written as virtual: class Employee: virtual public Person {..} class Student: virtual public Person {..} class TeachingAssistant:public Student, public Employee {..}

TeachingAssistant * Mark; = new TeachingAssistant; Student * s = Mark; // Legal due to is-a relationship Person * p = s; // Legal due to is-a relationship Employee * e = Mark; // Legal due to is-a relationship Person * p1 = e; // Legal due to is-a relationship Person * p2 = Mark; // Legal only if Virtual Inheritance is used so that the compiler knows which version of person to use, error otherwise