《C++ Primer Plus》14.3 多重继承 学习笔记

多重继承（MI）描述的是有多个直接基类的类。与单继承一样，共有MI表示的也是is-a关系。例如，可以从Awiter类和Singer类派生出SingingWaiter类：
class SingingWaiter : public Waiter, public Singer {...};
MI可能会给程序员带来很多新问题。其中两个主要的问题是：从两个不同的基类继承同名方法；从两个或更多相关基类那里继承同一个类的多个实例。
在下面的例子中，我们将定义一个抽象基类Worker，并使用它派生出Waiter类和Singer类。然后，使用MI从Waiter类和Singer类派生出SingingWaiter类。
程序清单14.7 worker0.h

// worker0.h -- working classes
#ifndef WORKER0_H_
#define WORKER0_H_

#include <string>

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 // WORKER0_H_

程序清单14.7的类声明中包含一些表示声音类型的内部变量。一个枚举类型符号常量alto、contralto等表示声音类型，静态数组pv存储了指向相应C-风格字符串的指针，程序清单14.8初始化了该数组，并提供了方法的定义。
程序清单14.8 worker0.cpp

// worker0.cpp -- working class methods
#include "worker0.h"
#include <iostream>
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;
}

程序清单14.9是一个简短的程序，它使用一个多台指针数组对这些类进行了测试。
程序清单14.9 worktest.cpp

// worktest.cpp -- test worker class hierarchy
#include <iostream>
#include "worker0.h"
const int LIM = 4;
int main()
{
    Waiter bob("Bob Apple", 314L, 5);
    Singer bev("Beverly Hills", 522L, 3);
    Waiter w_temp;
    Singer s_temp;

    Worker * pw[LIM] = {&bob, &bev, &w_temp, &s_temp};

    int i;
    for (i = 2; i < LIM; i ++)
        pw[i]->Set();
    for (i = 0; i < LIM; i ++)
    {
        pw[i]->Show();
        std::cout << std::endl;
    }

    return 0;
}

效果：

Enter worker‘s name: Waldo Dropmaster
Enter worker‘s ID: 442
Enter waiter‘s panache rating: 3
Enter worker‘s name: Sylvis Sirenne
Enter worker‘s ID: 555
Enter number for singer‘s vocal range:
0: other    1: alto    2: contralto    3: soprano
4: bass    5: baritone    6: tenor
3
Category: waiter
Name: Bob Apple
Employee ID: 314
Panache rating: 5

Category: singer
Name: Beverly Hills
Employee ID: 522
Vocal range: soprano

Category: waiter
Name: Waldo Dropmaster
Employee ID: 442
Panache rating: 3

Category: singer
Name: Sylvis Sirenne
Employee ID: 555
Vocal range: soprano

这种设计看起来是可行的：使用Waiter指针来调用Waiter::Show()和Waiter::Set()；使用Singer指针来调用Singer::Show()和Singer::Set()。然后，如果添加一个从Singer和Waiter类派生出的SingingWaiter类后，将带来一些问题。具体地说，将出现以下问题。
* 有多少Worker？
* 哪个方法？