6 Inheritance

The relationship between the proposed classes representing different kinds of vehicles is now further illustrated. The figure shows the object hierarchy in vertical direction: an Auto is a special case of a Land vehicle, which in turn is a special case of a Vehicle.

The class Vehicle is thus the `greatest common denominator' in the classification system. For the sake of the example we implement in this class the functionality to store and retrieve the weight of a vehicle:

    class Vehicle
    {
        public:
            // constructors
            Vehicle ();
            Vehicle (int wt);

            // interface
            int getweight () const;
            void setweight (int wt);
            
        private:
            // data
            int weight;
    }

Using this class, the weight of a vehicle can be defined as soon as the corresponding object is created. At a later stage the weight can be re-defined or retrieved.

To represent vehicles which travel over land, a new class Land can be defined with the functionality of a Vehicle, but in addition its own specific information. For the sake of the example we assume that we are interested in the speed of land vehicles and in their weight. The relationship between Vehicles and Lands could of course be represented with composition, but that would be awkward: composition would suggest that a Land vehicle contains a vehicle, while the relationship should be that the Land vehicle is a special case of a vehicle.

A relationship in terms of composition would also introduce needless code. E.g., consider the following code fragment which shows a class Land using composition (only the setweight() functionality is shown):

    class Land
    {
        public:
            void setweight (int wt);
        private:
            Vehicle v;      // composed Vehicle
    };

    void Land::setweight (int wt)
    {
        v.setweight (wt);
    }

Using composition, the setweight() function of the class Land would only serve to pass its argument to Vehicle::setweight(). Thus, as far as weight handling is concerned, Land::setweight() would introduce no extra functionality, just extra code. Clearly this code duplication is redundant: a Land should be a Vehicle, and not: a Land should contain a Vehicle.

The relationship is better achieved with inheritance: Land is derived from Vehicle, in which Vehicle is the base class of the derivation.

    class Land: public Vehicle
    {
        public:
            // constructors
            Land ();
            Land (int wt, int sp);

            // interface
            void setspeed (int sp);
            int getspeed () const;
            
        private:
            // data
            int speed;
    };

By postfixing the class name Land in its definition by public Vehicle the derivation is defined: the class Land now contains all the functionality of its base class Vehicle plus its own specific information. The extra functionality consists here of a constructor with two arguments and interface functions to access the speed data member. (The derivation in this example mentions the keyword public. C++ also implements private derivation, which is not often used and which we will therefore leave to the reader to uncover.)

To illustrate the usage of the derived class Land consider the following example:

    Land
        veh (1200, 145);

    int  main ()
    {
        printf ("Vehicle weighs %d\n"
                "Speed is %d\n",
            veh.getweight (), veh.getspeed ());
        return (0);
    }

This example shows two features of derivation. First, getweight() is no direct member of a Land; nevertheless it is used in veh.getweight(). This member function is an implicit part of the class, inherited from its `parent' vehicle.

Second, although the derived class Land now contains the functionality of Vehicle, the private fields of Vehicle remain private in the sense that they can only be accessed by member functions of Vehicle itself. This means that the member functions of Land must use the interface functions (getweight(), setweight()) to address the weight field; just as any other code outside the Vehicle class. This restriction is necessary so that the aspect of data hiding thus remains ensured. The class Vehicle could, e.g., be recoded and recompiled, after which the program could be relinked. The class Land itself could remain unchanged.

In this example we assume that the class Auto, which represents automobiles, should be able to represent the weight, speed and name of a car. This class is therefore derived from Land:

    class Auto: public Land
    {
        public:
            // constructors
            Auto ();
            Auto (int wt, int sp, char const *nm);

            // copy constructor
            Auto (Auto const &other);

            // assignment
            Auto const &operator= (Auto const &other);

            // destructor
            ~Auto ();

            // interface
            char const *getname () const;
            void setname (char const *nm);

        private:
            // data
            char const *name;
    };

In the above class definition, Auto is derived from Land, which in its turn is derived from Vehicle. We speak here of nested derivation: Land is Auto's direct base class, while Vehicle is the indirect base class.

Note the presence of a destructor, a copy constructor and overloaded assignment function in the class Auto. Since this class uses a pointer to address allocated memory, these tools are needed.

As mentioned previously, a derived class inherits the functionality of its base class. In this section we shall describe the effects of the inheritance on the constructor of a derived class.

As can be seen from the definition of the class Land, a constructor exists to set both the weight and the speed of an object. The poor-man's implementation of this constructor could be:

    Land::Land (int wt, int sp)
    {
        setweight (wt);
        setspeed (sp);
    }

The actions of all functions which are defined in a base class (and which are therefore also available in derived classes) can be redefined. This feature is illustrated in this section.

Let's assume that the vehicle classification system should be able to represent trucks, which consist of a two parts: the front engine, which pulls a trailer. Both the front part and the trailer have their own weight; but the getweight() function should return the combined weights.

The definition of a Truck therefore starts with the class definition, derived from Auto but expanded to hold one more int field to represent additional weight information. Here we choose to represent the weight of the front part of the truck in the Auto class and to store the weight of the trailer as the additional field:

    class Truck: public Auto
    {
        public:
            // constructors
            Truck ();
            Truck (int engine_wt, int sp, char const *nm,
                   int trailer_wt);

            // interface: to set two weight fields
            void setweight (int engine_wt, int trailer_wt);
            // and to return combined weight
            int getweight () const;

        private:
            // data
            int trailer_weight;
    };

    // example of constructor
    Truck::Truck (int engine_wt, int sp, char const *nm,
                  int trailer_wt)
        : Auto (engine_wt, sp, nm)
    {
        trailer_weight = trailer_wt;
    }

Note that the class Truck now contains two functions which are already present in the base class:

• The function setweight() is already defined in Vehicle. The redefinition in Truck poses no problem: this functionality is simply redefined to perform actions which are specific to a Truck object.

  The definition of a new version of setweight() in the class Truck will hide the version of Vehicle: for a Truck only a setweight() function with two int arguments can be used.

• The function getweight() is also already defined in Vehicle, with the same argument list as in Truck. In this case, the class Truck redefines this member function.

The following code fragment presents the redefined function getweight():

    int Truck::getweight () const
    {
        return
            (                           // sum of:
                Auto::getweight () +    //   engine part plus
                trailer_weight          //   the trailer
            );
    }

Note that in this function the call Auto::getweight() explicitly selects the getweight() function of the class Auto. An implementation like

    return (getweight () + trailer_weight);

would not be correct: this statement would lead to infinite recursion, and hence to an error in the program execution.

In the previously described derivations, a class was always derived from one base class. C++ also implements multiple derivation, in which a class is derived from several base classes and hence inherits the functionality of more than one `parent' at the same time.

For example, let's assume that a class Engine exists with the functionality to store information about an engine: the serial number, the power, the type of fuel, etc..:

    class Engine
    {
        public:
            // constructors and such
            Engine ();
            Engine (char const *serial_nr, int power,
                    char const *fuel_type);

            // tools needed 'cuz we have pointers in the class
            Engine (Engine const &other);
            Engine const &operator= (Engine const &other);
            ~Engine ();

            // interface to get/set stuff
            void setserial (char const *serial_nr);
            char const *getserial () const;
            void setpower (int power);
            int getpower () const;
            void setfueltype (char const *type);
            char const *getfueltype () const;

        private:
            // data
            char const *serial_number, *fuel_type;
            int power;
    };

To represent an Auto but with all information about the engine, a class MotorCar can be derived from Auto and from Engine; as is illustrated in the below listing. By using multiple derivation, the functionality of a Auto and of an Engine are swept into a MotorCar:

    class MotorCar: public Auto, public Engine
    {
        public:
            // constructors
            MotorCar ();
            MotorCar (int wt, int sp, char const *nm,
                      char const *ser, int pow, char const *fuel);
    };                

    MotorCar::MotorCar (int wt, int sp, char const *nm,
                        char const *ser, int pow, char const *fuel)
            : Engine (ser, pow, fuel), Auto (wt, sp, nm)
    {
    }

A few remarks concerning this derivation are:

• The keyword public is present both before the classname Auto and before the classname Engine. This is so because the default derivation in C++ is private: the keyword public must be repeated before each base class specification.
• The multiply derived class MotorCar introduces no `extra' functionality of its own, but only combines two pre-existing types into one aggregate type. Thus, C++ offers the possibility to simply sweep multiple simple types into one more complex type.

  This feature of C++ is very often used. Usually it pays to develop `simple' classes each with its strict well-defined functionality. More functionality can always be achieved by combining several small classes.

• The constructor which expects six arguments contains no code of its own. Its only purpose is to activate the constructors of the base classes. Similarly, the class definition contains no data or interface functions: here it is sufficient that all interface is inherited from the base classes.

Note also the syntax of the constructor: following the argument list, the two base class constructors are called, each supplied with the correct arguments. It is also noteworthy that the order in which the constructors are called is defined by the derivation, and not by the statement in the constructor of the class MotorCar. This means that:

• First, the constructor of Auto is called, since MotorCar is first of all derived from Auto;
• Then, the constructor of Engine is called,
• Last, any actions of the constructor of MotorCar itself are executed (in this example, none).

Lastly, it should be noted that the multiple derivation in this example may feel a bit awkward: the derivation implies that MotorCar is an Auto and at the same time is an Engine. A relationship `a MotorCar has an Engine' would be expressed as composition, by including an Engine object in the data of a MotorCar. But using composition, consider the unnecessary code duplication in the interface functions for an Engine (here we assume that a composed object engine of the class Engine exists in a MotorCar):

    void MotorCar::setpower (int pow)
    {
        engine.setpower (pow);
    }

    int MotorCar::getpower () const
    {
        return (engine.getpower ());
    }

    // etcetera, repeated for set/getserial(),
    // and set/getfueltype()

Clearly, such simple interface functions are better avoided by using derivation. Alternatively, when insisting on the has relationship and hence on composition, the interface functions could be avoided using inline functions.

When inheritance is used in the definition of classes, it can be said that an object of a derived class is at the same time an object of the base class. This has important consequences which shall be discussed in this section.

Conversions in object assignments

We define two objects, one of a base class and one of a derived class:

    Vehicle
        v (900);                // vehicle with weight 900 kg
    Auto
        a (1200, 130, "Ford");  // automobile with weight 1200 kg,
                                // max speed 130 km/h, make Ford

The object a is now initialized with its specific values. However, an Auto is at the same time a Vehicle, which makes the assignment from a derived object to a base object possible:

    v = a;

The effect of this assignment is that the object v now receives the value 1200 as its weight field. A Vehicle has neither a speed nor a name field; these data are therefore not assigned.

The conversion from a base object to a derived object poses however problems: what data should a statement like

    a = v;

substitute for the fields speed and name, which are missing in the right-hand side Vehicle? Such an assignment is therefore not accepted by the compiler.

The following general rule applies: when assigning related objects, an assignment where some data are dropped is legal. An assignment where data would have to be left blank is however not legal. This rule is a syntactic one: it also applies when the classes in question have their overloaded assignment functions.

The conversion of an object of a base class to an object of a derived class can of course be explicitly defined, if needed. E.g., to achieve the correct working of a statement

    a = v;

the class Auto would need an assignment function accepting a Vehicle as its argument. It would then be the programmer's responsibility to decide what to do with the missing data:

    Auto const &Auto::operator= (Vehicle const &veh)
    {
        setweight (veh.getweight ());
        .
        .  code to handle other fields should
        .  be supplied here
        .
    }

Conversions in pointer assignments

We define the following objects and one pointer variable:

    Land
        l (1200, 130);
    Auto
        a (500, 75, "Daf");
    Truck
        t (2600, 120, "Mercedes", 6000);
    Vehicle
        *vp;

Subsequently we can assign vp to the addresses of the three objects of the derived classes:

    vp = &l;
    vp = &a;
    vp = &t;

Each of these assignments is perfectly legal. However, an implicit conversion of the type of the derived class to a Vehicle is made, since vp is defined as a pointer to a Vehicle. Hence, when using vp only the member functions which manipulate the weight can be called; this is the only functionality of a Vehicle and thereby the only functionality which can be accessed by using a pointer to a Vehicle.

The restriction in functionality has furthermore an important effect for the class Truck. After the statement vp = &t, vp points to a Truck; nevertheless, vp->getweight() will return 2600; and not 8600 (i.e., the combined weight of the cabin and of the trailer, 2600+6000) which t.getweight() would return.

When a function is called via a pointer to an object, then the type of the pointer and not the object itself determines which member function is available and executed. In other words, C++ always implicitly converts the object which is pointed to to the type of the pointer.

There is of course a way around the implicit conversion, which is an explicit type cast:

    Truck
        truck;
    Vehicle
        *vp;

    vp = &truck;    // vp now points to a truck object
    .
    .
    .
    Truck
        *trp;

    trp = (Truck *) vp;
    printf ("Make: %s\n", trp->getname ());

The second to last statement of the code fragment above specifically casts a Vehicle* variable to a Truck* in order to assign the value to the pointer trp. This code will only work if vp indeed points to a Truck and hence a function getname() is available; otherwise unexpected behavior of the program may be the result.

The fact that pointers to a base class can be used to address derived classes can be used to develop general-purpose classes which can process objects of the derived types. A typical example of such processing is the storage of objects, be it in an array, a list, a tree or whichever storage method may be appropriate. Classes which are designed to store objects of other classes are therefore often called container classes. The stored objects are then contained in the container class.

As an example we present here the class VStorage, which is used to store pointers to Vehicles. The actual pointers may be addresses of Vehicles themselves, but also may refer to derived types such as Autos.

The definition of the class is the following:

    class VSTorage
    {
        public:
            // constructors, destructor
            VStorage ();
            VSTorage (VStorage const &other);
            ~VStorage ();

            // overloaded assignment
            VStorage const &operator= (VStorage const &other);

            // interface:
            // add Vehicle* to storage
            void add (Vehicle *vp);
            // retrieve first Vehicle*
            Vehicle *getfirst (void) const;
            // retrieve next Vehicle*
            Vehicle *getnext (void) const;

        private:
            // data
            Vehicle **storage;
            int nstored, current;
    };

Concerning this class definition we remark the following:

• The class contains three interface functions: one to add a Vehicle* to the storage, one to retrieve the first Vehicle* from the storage, and one to retrieve next pointers until no more are in the storage.

  The class could therefore be used as is illustrated in the following example:

          Land
              l (200, 20);            // weight 200, speed 20
          Auto
              a (1200, 130, "Ford");  // weight 1200 , speed 130,
                                      // make Ford
          VStorage
              garage;                 // the storage
  
          garage.add (&l);        // add to storage
          garage.add (&a);
  
          Vehicle
              *anyp;
          int
              total_wt = 0;
  
          for (anyp = garage.getfirst (); anyp; anyp = garage.getnext())
              total_wt += anyp->getweight ();
  
          printf ("Total weight: %d\n", total_wt);
      
  

  This example demonstrates how derived types (one Auto and one Land) are implicitly converted to their base type (a Vehicle), so that they can be stored in a VStorage. Base-type objects are then retrieved from the storage; the function getweight(), defined in the base type, is the greatest common denominator and hence can be used to compute the combined weight.

• The class VStorage furthermore contains all the tools to ensure that two VStorage objects can be assigned to one another etc.. These tools are the overloaded assignment function and the copy constructor.
• The actual internal workings of the class only become apparent once the private section is seen. The class VStorage maintains an array of pointers to Vehicles and needs two ints to store how many objects are in the storage and which the `current' index is, to be returned by getnext().

The class VStorage shall not be further elaborated; similar examples shall appear in the next chapters. It is however very noteworthy that by providing class derivation and base/derived conversions, C++ presents a powerful tool: these features of C++ allow the processing of all derived types by one generic class.

The above class VStorage could even be used to store all types which may be derived from a Vehicle in the future. It seems a bit paradoxical that the class should be able to use code which isn't even there yet, but there is no real paradox: VStorage uses a certain protocol, defined by the Vehicle and obligatory for all derived classes.

The above class VStorage has just one disadvantage: when we add a Truck object to a storage, then a code fragment like:

    Vehicle
        *any;
    VStorage
        garage;

    .
    .
    any = garage.getnext ();
    printf ("%d\n", any->getweight ());

will not print the truck's combined weight of the cabin and the trailer. Only the weight stored in the Vehicle portion of the truck will be returned via the function any->getweight().

There is, of course, also a remedy to this slight disadvantage. This will be discussed in the next chapter.