C/C++中多重继承详解及其作用介绍

概述

多重继承 (multiple inheritance): 一个派生类有两个或多个基类, 派生类从两个或多个基类中继承所需的属性. C++ 为了适应这种情况, 允许一个派生类同时继承多个基类. 这种行为称为多重继承.

优缺点

优点

• 自然地做到了对单继承的扩展
• 可以继承多个类的功能

缺点

• 结构复杂化
• 优先顺序模糊
• 功能冲突

声明多重继承的方法

格式

多重继承的格式:

派生类构造函数名(总形式参数表列): 
    基类1构造函数(实际参数表列),
    基类2构造函数(实际参数表列),
    基类3构造函数(实际参数表列)
{
    派生类中新增数成员据成员初始化语句
}

例子

Teacher 类:

#ifndef PROJECT5_TEACHER_H
#define PROJECT5_TEACHER_H

#include <string>
using namespace std;

class Teacher {
protected:
    string name;
    int age;
    string title;
public:
    Teacher(string n, int a, string t);
    void display_teacher();
};

#endif //PROJECT5_TEACHER_H

Teacher.cpp:

#include <iostream>
#include "Teacher.h"
using namespace std;

Teacher::Teacher(string n, int a, string t) : name(n), age(a), title(t) {}

void Teacher::display_teacher() {
    cout << "Teacher name: " << name << endl;
    cout << "age: " << age << endl;
    cout << "title: " << title << endl;
}

Student 类:

#ifndef PROJECT5_STUDENT_H
#define PROJECT5_STUDENT_H

#include <string>
using namespace std;

class Student {
protected:
    string name;
    char gender;
    double score;
public:
    Student(string n, char g, double s);
    void display_student();
};

#endif //PROJECT5_STUDENT_H

Student.cpp:

#include <iostream>
#include "Student.h"
using namespace std;

Student::Student(string n, char g, double s) : name(n), gender(g), score(s) {}

void Student::display_student() {
    cout << "Student name: " << name << endl;
    cout << "gender: " << gender << endl;
    cout << "score: " << score << endl;
}

Graduate 类：

#ifndef PROJECT5_GRADUATE_H
#define PROJECT5_GRADUATE_H

#include "Teacher.h"
#include "Student.h"
#include <string>
using namespace std;

class Graduate : public Teacher, public Student{
private:
    double wage;
public:
    Graduate(string t_n, int t_a, string t_t, string s_n, char s_g, double s_s);
    void display_graduate();
};

#endif //PROJECT5_GRADUATE_H

Graduate.cpp:

#include "Graduate.h"

Graduate::Graduate(string t_n, int t_a, string t_t, string s_n, char s_g, double s_s) :
    Teacher(t_n, t_a, t_t),
    Student(s_n, s_g, s_s) {}

void Graduate::display_graduate() {
    display_teacher();
    display_student();
}

main:

#include <iostream>
#include "Graduate.h"
using namespace std;

int main() {
    Graduate graduate1("王叔叔", 18, "隔壁老王", "我是小白呀", 'f', 99);
    graduate1.display_graduate();

    return 0;
}

输出结果：

Teacher name: 王叔叔
age: 18
title: 隔壁老王
Student name: 我是小白呀
gender: f
score: 99

二义性

二义性 (Ambiguity) 指在多重继承中, 两个基类中的数据成员名相同.

二义性在派生类中的解决方法:

• 在标识符前用类名做前缀: Teacher::name 和 Student::name
• 基类和派生类需要有一个完整的设计, 不能随意而为

两个基类有同名成员

A 类:

#ifndef PROJECT5_A_H
#define PROJECT5_A_H

#include <iostream>
using namespace std;

class A {
public:
    int num;
    void display() {cout << "A's num:" << num << endl;};
};

#endif //PROJECT5_A_H

B 类:

#ifndef PROJECT5_B_H
#define PROJECT5_B_H

#include <iostream>
using namespace std;

class B {
public:
    int num;
    void display() {cout << "B's num:" << num << endl;};
};

#endif //PROJECT5_B_H

C 类:

#ifndef PROJECT5_C_H
#define PROJECT5_C_H

#include <iostream>
#include "A.h"
#include "B.h"
using namespace std;

class C: public A, public B{
public:
    int c;
    void display() {cout << c << endl;};
};

#endif //PROJECT5_C_H

main:

#include <iostream>
#include "C.h"
using namespace std;

int main() {
    C c1;
    c1.A::num = 1;  // 用基类名限定
    c1.B::num = 2;  // 用基类名限定
    c1.A::display();
    c1.B::display();

    return 0;
}

输出结果:

A's num:1
B's num:2

错误的写法

#include <iostream>
#include "C.h"
using namespace std;

int main() {
    C c1;
    c1.num = 1;
    c1.display();
    
    return 0;
}

基类和派生类有同名成员

A 类:

class A {
public:
    int num;
    void display() {cout << "A's num:" << num << endl;};
};

B 类:

class B {
public:
    int num;
    void display() {cout << "B's num:" << num << endl;};
};

C 类:

class C: public A, public B{
public:
    int num;
    void display() {cout << "C's num:" << num << endl;};
};

main:

int main() {
    C c1;
    c1.num = 3;
    c1.A::num = 1;
    c1.B::num = 2;
    c1.display();
    c1.A::display();
    c1.B::display();

    return 0;
}

输出结果:

C's num:3
A's num:1
B's num:2

同名覆盖:

• 基类的同名成员在派生类中被屏蔽, 成为 "不可见"的
• 对成员函数, 限于函数名和参数个数相同, 类型相匹配. 若只有函数名相同而参数不同, 属于函数重载

两个基类从同一个基类派生

N 类:

class N {
public:
    int a;
    void display(){
        cout << "A::a=" << a <<endl;
    }
};

A 类:

class A : public N {
public:
    int a1;
};

B 类:

class B : public N {
public:
    int a2;
};

C 类:

class C: public A, public B{
public:
    int a3;
    void display() {cout << "a3=" << a3 << endl;};
};

main:

int main() {
    C c1;
    // 合法访问
    c1.A::a = 3;
    c1.A::display();

    return 0;
}

输出结果:

A::a=3

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