10leave

11ret

Figure 7.20: Assembly Code for Simple Inheritance

7.2.5Inheritance and Polymorphism

Inheritance allows one class to inherit the data and methods of another.

For example, consider the code in Figure 7.19. It shows two classes, A and

B, where class B inherits from A. The output of the program is:

Size of a: 4 Offset of ad: 0

Size of b: 8 Offset of ad: 0 Offset of bd: 4

A::m()

A::m()

Notice that the ad data members of both classes (B inherits it from A) are at the same o set. This is important since the f function may be passed a pointer to either an A object or any object of a type derived (i.e. inherited from) A. Figure 7.20 shows the (edited) asm code for the function (generated by gcc).

Note that in the output that A’s m method was called for both the a and b objects. From the assembly, one can see that the call to A::m() is hardcoded into the function. For true object-oriented programming, the method called should depend on what type of object is passed to the function. This is known as polymorphism. C++ turns this feature o by default. One uses the virtual keyword to enable it. Figure 7.21 shows how the two classes would be changed. None of the other code needs to be changed. Polymorphism can be implemented many ways. Unfortunately, gcc’s implementation is in transition at the time of this writing and is becoming significantly more complicated than its initial implementation. In the interest of simplifying this discussion, the author will only cover the implementation of polymorphism which the Windows based Microsoft and Borland compilers use. This

4int ad;

5 };

6

7 class B : public A {

8 public:

9virtual void cdecl m() { cout << ”B::m()” << endl; }

10int bd;

11};

Figure 7.21: Polymorphic Inheritance

implementation has not changed in many years and probably will not change in the foreseeable future.

With these changes, the output of the program changes:

Size of a: 8 Offset of ad: 4

Size of b: 12 Offset of ad: 4 Offset of bd: 8

A::m()

B::m()

Now the second call to f calls the B::m() method because it is passed a B object. This is not the only change however. The size of an A is now 8

(and B is 12). Also, the o set of ad is 4, not 0. What is at o set 0? The answer to these questions are related to how polymorphism is implemented.

A C++ class that has any virtual methods is given an extra hidden field that is a pointer to an array of method pointers13. This table is often called the vtable. For the A and B classes this pointer is stored at o set 0. The

Windows compilers always put this pointer at the beginning of the class at the top of the inheritance tree. Looking at the assembly code (Figure 7.22) generated for function f (from Figure 7.19) for the virtual method version of the program, one can see that the call to method m is not to a label. Line 9 finds the address of the vtable from the object. The address of the object is pushed on the stack in line 11. Line 12 calls the virtual method by branching to the first address in the vtable14. This call does not use a label, it branches to the code address pointed to by EDX. This type of call is an

13For classes without virtual methods C++ compilers always make the class compatible with a normal C struct with the same data members.

14Of course, this value is already in the ECX register. It was put there in line 8 and line 10 could be removed and the next line changed to push ECX. The code is not very e cient because it was generated without compiler optimizations turned on.