第53课被遗弃的多重继承（上）

时间：2018-09-04 00:13:35 阅读：143 评论：0 收藏：0 [点我收藏+]

问题：

C++中是否允许一个类继承自多个父类？

这就是多重继承。

技术分享图片

多重继承问题1：

 1 #include <iostream>
 2 #include <string>
 3 
 4 using namespace std;
 5 
 6 class BaseA
 7 {
 8     int ma;
 9 public:
10     BaseA(int a)
11     {
12         ma = a;
13     }
14     int getA()
15     {
16         return ma;
17     }
18 };
19 
20 class BaseB
21 {
22     int mb;
23 public:
24     BaseB(int b)
25     {
26         mb = b;
27     }
28     int getB()
29     {
30         return mb;
31     }
32 };
33 
34 class Derived : public BaseA, public BaseB
35 {
36     int mc;
37 public:
38     Derived(int a, int b, int c) : BaseA(a), BaseB(b)
39     {
40         mc = c;
41     }
42     int getC()
43     {
44         return mc;
45     }
46     void print()
47     {
48         cout << "ma = " << getA() << ", "
49              << "mb = " << getB() << ", "
50              << "mc = " << mc << endl;
51     }
52 };
53 
54 int main()
55 {
56     cout << "sizeof(Derived) = " << sizeof(Derived) << endl;    // 12
57     
58     Derived d(1, 2, 3);
59     
60     d.print();
61     
62     cout << "d.getA() = " << d.getA() << endl;
63     cout << "d.getB() = " << d.getB() << endl;
64     cout << "d.getC() = " << d.getC() << endl;
65     
66     cout << endl;
67     
68     BaseA* pa = &d;
69     BaseB* pb = &d;
70     
71     cout << "pa->getA() = " << pa->getA() << endl;
72     cout << "pb->getB() = " << pb->getB() << endl;
73     
74     cout << endl;
75     
76     void* paa = pa;
77     void* pbb = pb;
78     
79     
80     if( paa == pbb )
81     {
82         cout << "Pointer to the same object!" << endl; 
83     }
84     else
85     {
86         cout << "Error" << endl;
87     }
88     
89     cout << "pa = " << pa << endl;
90     cout << "pb = " << pb << endl;
91     cout << "paa = " << paa << endl;
92     cout << "pbb = " << pbb << endl; 
93     
94     return 0;
95 }

结果如下：

技术分享图片

pa和pb地址是不一样的，这就是多重继承的问题。

分析：

技术分享图片

两个指针地址值不一样时，它们也有可能指向的是同一个对象，这就是多重继承引入的问题。

多重继承的问题2：

技术分享图片

示例程序：

 1 #include <iostream>
 2 #include <string>
 3 
 4 using namespace std;
 5 
 6 class People
 7 {
 8     string m_name;
 9     int m_age;
10 public:
11     People(string name, int age)
12     {
13         m_name = name;
14         m_age = age;
15     }
16     void print()
17     {
18         cout << "Name = " << m_name << ", "
19              << "Age = " << m_age << endl;
20     }
21 };
22 
23 class Teacher : virtual public People
24 {
25 public:
26     Teacher(string name, int age) : People(name, age)
27     {
28     }
29 };
30 
31 class Student : virtual public People
32 {
33 public:
34     Student(string name, int age) : People(name, age)
35     {
36     }
37 };
38 
39 class Doctor : public Teacher, public Student
40 {
41 public:
42     Doctor(string name, int age) : Teacher(name, age), Student(name, age), People(name, age)
43     {
44     }
45 };
46 
47 int main()
48 {
49     Doctor d("Delphi", 33);
50     
51     d.print();
52     
53     return 0;
54 }