C/C++中虚基类详解及其作用介绍

概述

虚基类 (virtual base class) 是用关键字 virtual 声明继承的父类.

C/C++中虚基类详解及其作用介绍

多重继承的问题

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

存在的问题:

A::a 和 B::a 是 N 类成员的拷贝
A::a 和 B::a 占用不同的空间

C/C++中虚基类详解及其作用介绍

虚基类

我们希望继承间接共同基类时只保留一份成员, 所以虚基类就诞生了. 当基类通过多条派生路径被一个派生类继承时, 该派生类只继承该基类一次.

语法:

class 派生类名: virtual 继承方式 基类名

初始化

通过构造函数的初始化表对虚拟类进行初始化. 例如:

N 类:

class N {
public:
  int n;
  N(int n) : n(n) {};
};

A 类:

class A : virtual public N {
public:
  A(int n) : N(n) {};
};

B 类:

class B : virtual public N {
public:
  B(int n) : N(n) {};
};

C 类:

class C: public A, public B{
public:
  C(int n) : N(n), A(n), B(n){};
};

例子

Person 类:

#ifndef PROJECT5_PERSON_H
#define PROJECT5_PERSON_H

#include <iostream>
#include <string>
using namespace std;

class Person {
protected:
  string name;
  char gender;
public:
  Person(string n, char g) : name(n), gender(g) {}
  void display() {
      cout << "name: " << name << endl;
      cout << "gender: " << gender << endl;
  }
};

#endif //PROJECT5_PERSON_H

Student 类:

#ifndef PROJECT5_STUDENT_H
#define PROJECT5_STUDENT_H

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

class Student : virtual public Person {
protected:
  double score;
public:
  Student(string n, char g, double s) : Person(n, g), score(s) {};
};

#endif //PROJECT5_STUDENT_H

Teacher 类:

#ifndef PROJECT5_TEACHER_H
#define PROJECT5_TEACHER_H

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

class Teacher : virtual public Person {
protected:
  string title;
public:
  Teacher(string n, char g, string t) : Person(n, g), title(t) {};
};

#endif //PROJECT5_TEACHER_H

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 n, char g, double s, string t, double w) : Person(n, g), Student(n, g, s), Teacher(n, g, t), wage(w) {};
  void display() {
      Person::display();
      cout << "score: " << score << endl;
      cout << "title: " << title << endl;
      cout << "wages: " << wage << endl;
  };
};

#endif //PROJECT5_GRADUATE_H

main:

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

int main() {
  Graduate grad1("小白",'f',89.5,"教授",1234.5);
  grad1.display();

  return 0;
}

输出结果: