Question about multin inheritance in C ++?

Question

I have the following code:

#include "stdafx.h" #include <iostream> #include <conio.h> using namespace std; #define MNAME 30 class Person { public: char name[MNAME + 1]; }; class Student : public Person { }; class Staff : public Person { }; class Faculty : public Student, public Staff { }; int _tmain(int argc, _TCHAR* argv[]) { Faculty faculty; cout << "Address of faculty.Person::name: " << &faculty.Person::name << endl; cout << "Address of faculty.Student::name: " << &faculty.Student::name << endl; cout << "Address of faculty.Staff::name: " << &faculty.Staff::name << endl; getch(); return 0; } 

When executed, the program gives the results:

 Address of faculty.Person::name: 0012FF20 // **Line 1** Address of faculty.Student::name: 0012FF20 // **Line 2** Address of faculty.Staff::name: 0012FF3F // **Line 3** 

I do not understand. Why is the address in Line 1 and Line 2 different from Line 3 , and both Student and Staff inherit the name from Person?

Answer 1

With regular multiple inheritance, you get multiple copies of the common base classes. If you need one copy, use virtual inheritance.

Well explained on Wikipedia

 class Student : public virtual Person { }; class Staff : public virtual Person { }; 

You will get what you expected

Answer 2

When you do multiple inheritance, you get two copies of the grandparent class. This is a classic dreaded diamond problem where you are trying to do this:

         Person
        / \
      Student staff
        \ /
         Faculty

but through regular inheritance, you really get this:

     Person person
       |  |
      Student staff
        \ /
         Faculty

So, there really are 2 people in the copy of the faculty, that is, you will get 2 names.

To get the diamond in the first diagram above, you want to use virtual inheritance .

 class Staff : public virtual Person { }; class Student : public virtual Person { }; 

Answer 3

you inherit from two different classes separately.

you must use virtual inheritance

Answer 4

You are faced with the problem of classic diamond inheritance. Due to how many inheritances work in C ++, there are actually two different copies of name in Faculty . You can usually solve this using virtual inheritance like this, so you only have one instance of Person and its members:

 class Student : public virtual Person { }; class Staff : public virtual Person { }; 

I am sure of this, but you do not want to do this. It seems reasonable to assume that each Faculty also a member of Student AND a Staff , so you should not represent it that way. It seems plausible that Faculty will always be Staff , so you can use single inheritance to model this relationship. Then, if necessary, discard (in free functions or a separate class) the general code from the student, which is also necessary at the Faculty .

Answer 5

class Faculty inherits two subclasses of class Person , one through class Student and the other through class Staff .

&faculty.Staff::name returns the address of the helper class Person object obtained through class Staff .

&faculty.Student::name returns the address of the class Person sub-object obtained through class Student .

Both are different subordinate objects and therefore different addresses.

Answer 6

For multiple inheritance, the faculty derived class has 2 copies of Person . 1 to Student and 2 to Staff .

When you refer to faculty.Person::name , it means either through Student or through Staff . This situation is ambiguous and will not even compile with g ++.

In MSVC, it seems that since faculty first inherits Student and then Staff , it refers to faculty.Person::name as facutlty ==> Student ==> Person ==> name . Therefore, the output of the first two lines is the same, and the third line is different.

Answer 7

A little off topic, but .... Most experienced developers avoid multiple inheritance. Difficult to maintain and fraught with danger.