Sams Teach Yourself C++ in 21 Days

Implementing Inheritance 397

12

access it in the same way you have been using the base class to access the virtual meth-

ods? There shouldn’t be a name conflict because only the derived class has the method.

If the Dogobject had a method,WagTail(), which is not in Mammal, you could not use the

pointer to Mammalto access that method. Because WagTail()is not a virtual function,

and because it is not in a Mammalobject, you can’t get there without either a Dogobject or

a Dogpointer.

You could cast the Mammalto act as a Dog; however, this is not safe if the Mammalis not a

Dog. Although this would transform the Mammalpointer into a Dogpointer, a much better

and safer way exists to call the WagTail()method. Besides, C++ frowns on explicit casts

because they are error-prone. This subject is addressed in depth when multiple inheri-

tance is covered on Day 15, and again when templates are covered on Day 20, “Handling

Errors and Exceptions.”

Slicing ............................................................................................................

Note that the virtual function magic operates only on pointers and references. Passing an

object by value does not enable the virtual functions to be invoked. Listing 12.10 illus-

trates this problem.

LISTING12.10 Data Slicing When Passing by Value

1: //Listing 12.10 Data slicing with passing by value

2: #include <iostream>

3: using namespace std;

4:

5: class Mammal

6: {

7: public:

8: Mammal():itsAge(1) { }

9: virtual ~Mammal() { }

10: virtual void Speak() const { cout << “Mammal speak!\n”; }

11:

12: protected:

13: int itsAge;

14: };

15:

16: class Dog : public Mammal

17: {

18: public:

19: void Speak()const { cout << “Woof!\n”; }

20: };

21:

22: class Cat : public Mammal

23: {

24: public: