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Inheritance enables you to adapt programming code to your particular needs by defining a new class (a derived class) from an existing class (the base class). Public inheritance models an is-a relationship, meaning that a derived-class object should also be a kind of base-class object. As part of the is-a model, a derived class inherits the data members and most methods of the base class. However, a derived class doesn’t inherit the base-class constructors, destructors, and assignment operators. A derived class can access the public and protected members of the base class directly and the private base-class members via the public and protected base-class methods. You can then add new data members and methods to the class, and you can use the derived class as a base class for further development. Each derived class requires its own constructors. When a program creates a derived-class object, it first calls a base-class constructor and then the derived-class constructor. When a program deletes an object, it first calls the derived-class destructor and then the base-class destructor.

If a class is meant to be a base class, you may choose to use protected members instead of private members so that derived classes can access those members directly. However, using private members, in general, reduces the scope for programming bugs. If you intend that a derived class can redefine a base-class method, you should make it a virtual function by declaring it with the keyword virtual. This enables objects accessed by pointers or references to be handled on the basis of the object type rather than on the basis of the reference type or pointer type. In particular, the destructor for a base class should normally be virtual.

You might want to define an ABC that defines an interface without getting into implementation matters. For example, you could define an abstract Shape class from which particular shape classes, such as Circle and Square, will be derived. An ABC must include at least one pure virtual method. You declare a pure virtual method by placing = 0 before the closing semicolon of the declaration:

virtual double area() const = 0;

You don’t have to define pure virtual methods, and you can’t create an object of a class that contains pure virtual members. Instead, pure virtual methods serve to define a common interface to be used by derived classes.

Chapter Review

1. What does a derived class inherit from a base class?

2. What doesn’t a derived class inherit from a base class?

3. Suppose the return type for the baseDMA::operator=() function were defined as void instead of baseDMA &. What effect, if any, would that have? What if the return type were baseDMA instead of baseDMA &?

4. In what order are class constructors and class destructors called when a derived-class object is created and deleted?

5. If a derived class doesn’t add any data members to the base class, does the derived class require constructors?

6. Suppose a base class and a derived class both define a method with the same name and a derived-class object invokes the method. What method is called?

7. When should a derived class define an assignment operator?

8. Can you assign the address of an object of a derived class to a pointer to the base class? Can you assign the address of an object of a base class to a pointer to the derived class?

9. Can you assign an object of a derived class to an object of the base class? Can you assign an object of a base class to an object of the derived class?

10. Suppose you define a function that takes a reference to a base-class object as an argument. Why can this function also use a derived-class object as an argument?

11. Suppose you define a function that takes a base-class object as an argument (that is, the function passes a base-class object by value). Why can this function also use a derived-class object as an argument?

12. Why is it usually better to pass objects by reference than by value?

13. Suppose Corporation is a base class and PublicCorporation is a derived class. Also suppose that each class defines a head() member function, that ph is a pointer to the Corporation type, and that ph is assigned the address of a PublicCorporation object. How is ph->head() interpreted if the base class defines head() as a

a. Regular nonvirtual method

b. Virtual method

14. What’s wrong, if anything, with the following code?

class Kitchen

{

private:

    double kit_sq_ft;

public:

    Kitchen() {kit_sq_ft = 0.0; }

    virtual double area() const { return kit_sq_ft * kit_sq_ft; }

};

class House : public Kitchen

{

private:

    double all_sq_ft;

public:

    House() {all_sq_ft += kit_sq_ft;}

    double area(const char *s) const { cout << s; return all_sq_ft; }

};

Programming Exercises

1. Start with the following class declaration:

// base class

class Cd {  // represents a CD disk

private:

    char performers[50];

    char label[20];

    int selections;   // number of selections

    double playtime;  // playing time in minutes

public: