Chapter 15: Inheritance

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When programming in C, it is common to view problem solutions from a top-down approach: functions and actions of the program are defined in terms of sub-functions, which again are defined in sub-sub-functions, etc.. This yields a hierarchy of code: main() at the top, followed by a level of functions which are called from main(), etc..

In C++ the dependencies between code and data can also be defined in terms of classes which are related to other classes. This looks like composition (see section 5.5), where objects of a class contain objects of another class as their data. But the relation which is described here is of a different kind: a class can be defined by means of an older, pre-existing, class. This leads to a situation in which a new class has all the functionality of the older class, and additionally introduces its own specific functionality. Instead of composition, where a given class contains another class, we mean here derivation, where a given class is another class.

Another term for derivation is inheritance: the new class inherits the functionality of an existing class, while the existing class does not appear as a data member in the definition of the new class. When speaking of inheritance the existing class is called the base class, while the new class is called the derived class.

Derivation of classes is often used when the methodology of C++ program development is fully exploited. In this chapter we will first address the syntactical possibilities which C++ offers to derive classes from other classes. Then we will address the peculiar extension to C which is thus offered by C++.

As we have seen the object-oriented approach to problem solving in the introductory chapter (see section 2.4), classes are identified during the problem analysis, after which objects of the defined classes can be declared to represent entities of the problem at hand. The classes are placed in a hierarchy, where the top-level class contains the least functionality. Each derivation and hence descent in the hierarchy adds functionality in the class definition.

In this chapter we shall use a simple vehicle classification system to build a hierarchy of classes. The first class is Vehicle, which implements as its functionality the possibility to set or retrieve the weight of a vehicle. The next level in the object hierarchy are land-, water- and air vehicles.

The initial object hierarchy is illustrated in figure 12.

figure 12: Initial object hierarchy of vehicles.

15.1: Related types

The relationship between the proposed classes representing different kinds of vehicles is further illustrated here. 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 use of the derived class Land consider the following example:


    Land
        veh(1200, 145);

    int main()
    {
        cout << "Vehicle weighs " << veh.getweight() << endl
             << "Speed is " << veh.getspeed() << endl;

        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 to enforce the principle of data hiding. The class Vehicle could, e.g., be recoded and recompiled, after which the program could be relinked. The class Land itself could remain unchanged.

Actually, the previous remark is not quite right: If the internal organization of the Vehicle changes, then the internal organization of the Land objects, containing the data of Vehicle, changes as well. This means that objects of the Land class, after changing Vehicle, might require more (or less) memory than before the modification. However, in such a situation we still don't have to worry about the use of member functions of the parent class Vehicle in the class Land. We might have to recompile the Land sources, though, as the relative locations of the data members within the Land objects will have changed due to the modification of the Vehicle class.

To play it safe, classes which are derived from other classes must be fully recompiled (but don't have to be modified) after changing the data organization of their base class(es). As adding new member functions to the base class doesn't alter the data organization, no such recompilation is needed after adding new member functions. (A subtle point to note, however, is that adding a new member function that happens to be the first virtual member function of a class results in a hidden pointer to a table of pointers to virtual functions. This topic is discussed further in chapter 16).

In the following example we assume that the class Auto, representing automobiles, should be able to contain 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 turn is derived from Vehicle. This is called nested derivation: Land is called Auto's direct base class, while Vehicle is called 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 reach allocated memory, these tools are needed.

15.2: The constructor of a derived class

As mentioned earlier, a derived class inherits the functionality from 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);
    }

This implementation has the following disadvantage. The C++ compiler will generate code to call the default constructor of a base class from each constructor in the derived class, unless explicitly instructed otherwise. This can be compared to the situation which arises in composed objects (see section 5.5).

Consequently, in the above implementation (a) the default constructor of a Vehicle is called, which probably initializes the weight of the vehicle, and (b) subsequently the weight is redefined by calling setweight().

A better solution is of course to call directly the constructor of Vehicle expecting an int argument. The syntax to achieve this is to mention the constructor to be called (supplied with an argument) immediately following the argument list of the constructor of the derived class itself:


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

15.3: The destructor of a derived class

Destructors of classes are called automatically when an object is destroyed. This rule also holds true for objects of classes that are derived from other classes. Assume we have the following situation:


    class Base
    {
        public:
            ...         // members
            ~Base();    // destructor
    };

    class Derived
    {
        public:
            ...         // members
            ~Derived(); // destructor
    }

    ...                 // other code

    int main()
    {
        Derived
            derived;

        ...
        return (0);
    }

At the end of the main() function, the derived object ceases to exists. Hence, its destructor Derived::~Derived() is called. However, since derived is also a Base object, the Base::~Base() destructor is called as well.

It is not necessary to call the Base::~Base() destructor explicitly from the Derived::~Derived() destructor.

Constructors and destructors are called in a stack-like fashion: when derived is constructed, the appropriate Base constructor is called first, then the appropriate Derived constructor is called. When derived is destroyed, the Derived destructor is called first, and then the Base destructor is called for that object. In general, a derived class destructor is called before a base class destructor is called.

15.4: Redefining member functions

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 engine and the trailer have their own weights, but the getweight() function should return the combined weight.

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 in an 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 Auto. 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 Auto (which is the version defined in Vehicle: for a Truck only a setweight() function with two int arguments can be used.
However, note that the Vehicle's setweight() function remains available. But, as the Auto::setweight() function is hidden it must be called explicitly when needed (e.g., inside Truck::setweight(). This is required even though Auto::setweight() has only one int argument, and one could argue that Auto::setweight() and Truck::setweight() are merely overloaded functions within the class Truck. So, the implementation of the function Truck::setweight() could be:
```
    void Truck::setweight(int engine_wt, int trailer_wt)
    {
        trailer_weight = trailer_wt;
        Auto::setweight(engine_wt);     // note: Auto:: is required
    }   
 
```
Outside of the class the Auto-version of setweight() is accessed through the scope resolution operator. So, if a Truck t needs to set its Auto weight, it must use t.Auto::setweight(x)
An alternative to using the scope resolution operator is to include the base-class functions in the class interface as inline functions. This might be an elegant solution for the occasional function. E.g., if the interface of the class Truck contains
```
    void setweight(int engine_wt)
    {
        Auto::setweight(engine_wt);
    }
        
```
then the single argument setweight() function can be used by Truck objects without using the scope resolution operator. As the function is defined inline, no overhead of an extra function call is involved.

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 next code fragment presents the redefined function Truck::getweight():


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

The following example shows the actual usage of the member functions of the class Truck to display several of its weights:


    int main()
    {
        Land
            veh(1200, 145);
   
        Truck 
            lorry(3000, 120, "Juggernaut", 2500);
   
        lorry.Vehicle::setweight(4000);
   
        cout << endl << "Truck weighs " << lorry.Vehicle::getweight() << endl
            << "Truck + trailer weighs " << lorry.getweight() << endl
            << "Speed is " << lorry.getspeed() << endl
            << "Name is " << lorry.getname() << endl;
        return (0);
    }

Note the explicit call to Vehicle::setweight(4000): in order to reach the hidden member function Vehicle::setweight(), which is part of the set of member functions available to the class Vehicle, is must be called explicitly, using the Vehicle:: scope resolution. As said, this is remarkable, because Vehicle::setweight() can very well be considered an overloaded version of Truck::setweight(). The situation with Vehicle::getweight() and Truck::getweight() is a different one: here the function Truck::getweight() is a redefinition of Vehicle::getweight(), so in order to reach Vehicle::getweight() a scope resolution operation (Vehicle::) is required.

15.5: Multiple inheritance

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 from 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 as 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);
            void setpower(int power);
            void setfueltype(char const *type);

            char const *getserial() const;
            int getpower() const;
            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 illustrated in the below listing. By using multiple derivation, the functionality of an Auto and of an Engine are combined 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 interface, and not by the implementation (i.e., by the statement in the constructor of the class MotorCar. This implies 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 it 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, unnecessary code duplication occurs 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 avoided completely by using derivation. Alternatively, when insisting on the has relationship and hence on composition, the interface functions could have been avoided by using inline functions.

15.6: Conversions between base classes and derived classes

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 for the assignment of objects, and for the situation where pointers or references to such objects are used. Both situations will be discussed next.

15.6.1: 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, however, is problematic: In a statement like


    a = v;

it isn't clear what data to enter into the fields speed and name of the Auto object a, as they are missing in the Vehicle object v. Such an assignment is therefore not accepted by the compiler.

The following general rule applies: when assigning related objects, an assignment in which some data are dropped is legal. However, an assignment where data would have to be left blank is 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 could of course be explicitly defined using a dedicated constructor. E.g., to achieve compilability of a statement


    a = v;

the class Auto would need an assignment function accepting a Vehicle as its argument. It would 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
        .
    }

15.6.2: Conversions in pointer assignments

We define the following objects and one pointer variable:


    Land
        land(1200, 130);
    Auto
        auto(500, 75, "Daf");
    Truck
        truck(2600, 120, "Mercedes", 6000);
    Vehicle
        *vp;

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


    vp = &land;
    vp = &auto;
    vp = &truck;

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, as this is the only functionality of a Vehicle and thus it is the only functionality which is available when a pointer to a Vehicle is used.

The same reasoning holds true for references to Vehicles. If, e.g., a function is defined with a Vehicle reference parameter, the function may be passed an object of a class that is derived from Vehicle. Inside the function, the specific Vehicle members of the object of the derived class remain accessible. This analogy between pointers and references holds true in all cases. Remember that a reference is nothing but a pointer in disguise: it mimics a plain variable, but is actually a pointer.

This restriction in functionality has furthermore an important effect for the class Truck. After the statement vp = &truck, vp points to a Truck object. Nevertheless, vp->getweight() will return 2600; and not 8600 (the combined weight of the cabin and of the trailer: 2600 + 6000), which would have been returned by t.getweight().

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 functions are available and executed. In other words, C++ implicitly converts the type of an object reached via a pointer to the type of the pointer pointing to the object.

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 the program may show some unexpected behavior.

15.7: Storing base class pointers

The fact that pointers to a base class can be used to reach 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 contained in the container class.

As an example we present 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:
            VStorage();
            VSTorage(VStorage const &other);
            ~VStorage();
            VStorage const &operator=(VStorage const &other);

                                // add Vehicle& to storage
            void add(Vehicle const &vehicle);
                                // retrieve first Vehicle *
            Vehicle const *getfirst() const;
                                // retrieve next Vehicle *
            Vehicle const *getnext() const;

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

Concerning this class definition we note:

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.
An illustration of the use of this class is given in the next example:
```
        Land
            land(200, 20);          // weight 200, speed 20
        Auto
            auto(1200, 130, "Ford");// weight 1200 , speed 130,
                                    // make Ford
        VStorage
            garage;                 // the storage

        garage.add(land);           // add to storage
        garage.add(auto);

        Vehicle const
            *anyp;
        int
            total_wt = 0;

        for (anyp = garage.getfirst(); anyp; anyp = garage.getnext())
            total_wt += anyp->getweight();

        cout << "Total weight: " << total_wt << endl;
    
```
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 class and the derived classes, is therupon used to compute the total weight.
Furthermore, the class VStorage 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 const
        *any;
    VStorage
        garage;

    any = garage.getnext();
    cout << any->getweight() << endl;

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(). Fortunately, there is a remedy against this slight disadvantage. This remedy will be discussed in the next chapter.