The Slicing Problem

1. The Slicing Problem CS240 Computer Science II

2. Some relationship between base and derived classes: data members • Derived class inherits data members of its base class (or classes in the case of multiple inheritance). Therefore, a derived class object contains data members of it base class. • The reverse is not true, i.e., the base class object does not contain additional data members defined in the derived class.

3. Example • Based class point contains int data members x and y, its derived class circle defined an additional int data member radius. • Then point p; // object p contains x and y circle c; // object c contains x, y, and radius • Obviously, c is a “larger” object for it contains an additional data member.

4. The “Slicing” problem • In the context of inheritance: when a larger circle object c is copied to a smaller point object p by value in a function call, the extra data member radius is discarded or “sliced off”: a situation known as the slicing problem. • With passing by reference or through a pointer, the slicing problem does not occur because the address of the larger object instead of the larger object itself is passed to the function.

5. Static binding: some observations • Assuming that there is a print( ) function in both the point and circle classes. point p, *pPtr = &p; circle c, *cPtr = &c; p.print( ); // print function defined in point class c.print( ); // print function defined in circle class pPtr->print( ); // print function defined in point class cPtr->print( ); // print function defined in circle class pPtr = &c; // now pPtr points to a circle obj! pPtr->print( ); // still print function defined in clircle // class! Why??? Because of static // binding! At compile time, the // compiler uses the point type (class) // to generate the code!

6. Dynamic binding through virtual functions • The whole situation is quite different if print function is declared virtual in the class (implying print will be virtual in the derived class as well). The binding of a function with the calling object occurs at run time, and the print function defined in the class (ADT) of the calling object will be activated.