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Multiple Inheritance

MI describes a class that has more than one immediate base class. As with single inheritance, public MI should express an is-a relationship. For example, if you have a Waiter class and a Singer class, you could derive a SingingWaiter class from the two:

class SingingWaiter : public Waiter, public Singer {...};

Note that you must qualify each base class with the keyword public. That’s because the compiler assumes private derivation unless instructed otherwise:

class SingingWaiter : public Waiter, Singer {...}; // Singer is a private base

As discussed earlier in this chapter, private and protected MI can express a has-a relationship; the studenti.h implementation of the Student class is an example. We’ll concentrate on public inheritance now.

MI can introduce new problems for programmers. The two chief problems are inheriting different methods with the same name from two different base classes and inheriting multiple instances of a class via two or more related immediate base classes. Solving these problems involves introducing a few new rules and syntax variations. Thus, using MI can be more difficult and problem-prone than using single inheritance. For this reason, many in the C++ community object strongly to MI; some want it removed from the language. Others love MI and argue that it’s very useful, even necessary, for particular projects. Still others suggest using MI cautiously and in moderation.

Let’s explore a particular example and see what the problems and solutions are. You need several classes to create an MI situation. For this example, we’ll define an abstract Worker base class and derive a Waiter class and a Singer class from it. Then we can use MI to derive a SingingWaiter class from the Waiter and Singer classes (see Figure 14.3). This is a case in which a base class (Worker) is inherited via two separate derivations, which is the circumstance that causes the most difficulties with MI. We start with declarations for the Worker, Waiter, and Singer classes, as shown in Listing 14.7.

Figure 14.3. MI with a shared ancestor.

Listing 14.7. worker0.h

// worker0.h  -- working classes

#ifndef WORKER0_H_

#define WORKER0_H_

#include

class Worker   // an abstract base class

{

private:

    std::string fullname;

    long id;

public:

    Worker() : fullname("no one"), id(0L) {}

    Worker(const std::string & s, long n)

            : fullname(s), id(n) {}

    virtual ~Worker() = 0;   // pure virtual destructor

    virtual void Set();

    virtual void Show() const;

};

class Waiter : public Worker

{

private:

    int panache;

public:

    Waiter() : Worker(), panache(0) {}

    Waiter(const std::string & s, long n, int p = 0)

            : Worker(s, n), panache(p) {}

    Waiter(const Worker & wk, int p = 0)

            : Worker(wk), panache(p) {}

    void Set();

    void Show() const;

};

class Singer : public Worker

{

protected:

    enum {other, alto, contralto, soprano,

                    bass, baritone, tenor};

    enum {Vtypes = 7};

private:

    static char *pv[Vtypes];    // string equivs of voice types

    int voice;

public:

    Singer() : Worker(), voice(other) {}

    Singer(const std::string & s, long n, int v = other)

            : Worker(s, n), voice(v) {}

    Singer(const Worker & wk, int v = other)

            : Worker(wk), voice(v) {}

    void Set();

    void Show() const;

};

#endif

The class declarations in Listing 14.7 include some internal constants that represent voice types. An enumeration makes alto, contralto, and so on symbolic constants for voice types, and the static array pv holds pointers to the C-style string equivalents. The implementation file, shown in Listing 14.8, initializes this array and provides method definitions.

Listing 14.8. worker0.cpp

// worker0.cpp -- working class methods

#include "worker0.h"

#include

using std::cout;

using std::cin;

using std::endl;

// Worker methods

// must implement virtual destructor, even if pure

Worker::~Worker() {}

void Worker::Set()

{

    cout << "Enter worker's name: ";

    getline(cin, fullname);

    cout << "Enter worker's ID: ";

    cin >> id;

    while (cin.get() != '\n')

        continue;

}

void Worker::Show() const

{

    cout << "Name: " << fullname << "\n";

    cout << "Employee ID: " << id << "\n";

}

// Waiter methods

void Waiter::Set()

{

    Worker::Set();

    cout << "Enter waiter's panache rating: ";

    cin >> panache;

    while (cin.get() != '\n')

        continue;

}

void Waiter::Show() const

{

    cout << "Category: waiter\n";

    Worker::Show();

    cout << "Panache rating: " << panache << "\n";

}

// Singer methods

char * Singer::pv[] = {"other", "alto", "contralto",

            "soprano", "bass", "baritone", "tenor"};

void Singer::Set()

{

    Worker::Set();

    cout << "Enter number for singer's vocal range:\n";

    int i;

    for (i = 0; i < Vtypes; i++)

    {

        cout << i << ": " << pv[i] << "   ";

        if ( i % 4 == 3)

            cout << endl;

    }

    if (i % 4 != 0)

        cout << endl;

    while (cin >>  voice && (voice < 0 || voice >= Vtypes) )

        cout << "Please enter a value >= 0 and < " << Vtypes << endl;

    while (cin.get() != '\n')

        continue;

}

void Singer::Show() const

{

    cout << "Category: singer\n";

    Worker::Show();

    cout << "Vocal range: " << pv[voice] << endl;

}