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09-虚表、抽象类和多继承

09-虚表、抽象类和多继承

作者: 一亩三分甜 | 来源:发表于2021-07-23 10:24 被阅读0次

    《C++文章汇总》
    上一篇介绍了引用和汇编《08-初始化列表、父类构造函数、虚函数和多态》,本文介绍虚表、抽象类和多继承。

    1.虚表

    ◼ 虚函数的实现原理是虚表，这个虚表里面存储着最终需要调用的虚函数地址，这个虚表也叫虚函数表

    struct Animal{
    int m_age;
    virtual void speak(){
        cout << "Animal::speak()" << endl;
    }
    virtual void run(){
        cout << "Animal::run()" << endl;
    }
};
struct Dog:Animal{
    void speak(){
        cout << "Dog::speak()" << endl;
    }
    void run(){
        cout << "Dog::run()" << endl;
    }
};
struct Cat:Animal{
    int m_life;
    void speak(){
        cout << "Cat::speak()" << endl;
    }
    void run(){
        cout << "Cat::run()" << endl;
    }
};
struct Pig:Animal{
    void speak(){
        cout << "Pig::speak()" << endl;
    }
    void run(){
        cout << "Pig::run()" << endl;
    }
};
void liu(Animal *p){
    p->speak();
    p->run();
}
int main(){
    Animal *cat = new Cat();
    cat->m_age = 20;
    cat->speak();
    cat->run();
    
    cat = new Pig();
    cat->speak();
    cat->run();
    return 0;
}
//输出
Cat::speak()
Cat::run()
Pig::speak()
Pig::run()

    x86环境的虚表图


    image

    虚表汇编分析

    //调用speak
Animal *cat = new Cat();
cat->speak();
//ebp-8是指针变量cat的地址
//eax存储的是cat对象的地址
mov eax,dword ptr [ebp-8]
//取出Cat对象的最前面的4个字节(虚表地址)给edx
//取出虚表的地址值给edx
mov edx,dword ptr [eax]
//取出虚表的最前面的4个字节给eax
//取出Cat::speak的函数调用地址给eax
mov eax,dword ptr [edx]

//call Cat::speak
call eax

//调用run
Animal *cat = new Cat();
cat->run();
//ebp-8是指针变量cat的地址
//eax存储的是Cat对象的地址
mov eax,dword ptr [ebp-8]

//取出Cat对象的最前面的4个字节(虚表地址)给edx
mov edx,dword ptr [eax]

//跳过虚表的最前面4个字节，取出4个字节赋值给eax
//取出Cat::run的函数调用地址给eax
mov eax,dword ptr [edx+4]
//call Cat::run
call eax

    虚表内存分析

    //Cat对象的地址
0x016DDB98
//虚表地址
0x00259b34
//虚表的内容
0x00251569
0x00251550

    ◼ 所有的Cat对象(不管在全局区、栈、堆)共用同一份虚表
    子类中没有方法时，会调用父类中virtual修饰的方法，放入子类对象(cat对象)的前4个字节内存地址中

    struct Animal{
    int m_age;
    virtual void speak(){
        cout << "Animal::speak()" << endl;
    }
    virtual void run(){
        cout << "Animal::run()" << endl;
    }
};
struct Dog:Animal{
    void speak(){
        cout << "Dog::speak()" << endl;
    }
    void run(){
        cout << "Dog::run()" << endl;
    }
};
struct Cat:Animal{
    int m_life;
//    void speak(){
//        cout << "Cat::speak()" << endl;
//    }
    void run(){
        cout << "Cat::run()" << endl;
    }
};
struct Pig:Animal{
//    void speak(){
//        cout << "Pig::speak()" << endl;
//    }
    void run(){
        cout << "Pig::run()" << endl;
    }
};
void liu(Animal *p){
    p->speak();
    p->run();
}
int main(){
    Animal *cat = new Cat();
    cat->m_age = 20;
    cat->speak();
    cat->run();
    
    cat = new Pig();
    cat->speak();
    cat->run();
    return 0;
}
//输出
Animal::speak()
Cat::run()
Animal::speak()
Pig::run()
    图片.png

    父类没有子类继承，有virtual修饰的方法函数，则也存在虚表，调用时会从虚表中查找调用函数的地址

    struct Animal{
    int m_age;
    virtual void speak(){
        cout << "Animal::speak()" << endl;
    }
    virtual void run(){
        cout << "Animal::run()" << endl;
    }
};
int main(){
    Animal *ani = new Animal();
    ani->m_age = 20;
    ani->speak();
    ani->run();
}

    子类中不存在virtual修饰的函数，仍存在虚表

    struct Animal{
    int m_age;
    virtual void speak(){
        cout << "Animal::speak()" << endl;
    }
    virtual void run(){
        cout << "Animal::run()" << endl;
    }
};
struct Cat:Animal{
    int m_life;
    void speak(){
        cout << "Cat::speak()" << endl;
    }
};
struct WhiteCat:Cat{
    
};
int main(){
    Animal *ani = new WhiteCat();
    ani->speak();
    ani->run();
    
    return 0;
}
//输出
Cat::speak()
Animal::run()

    若虚函数写在子类中，则从子类开始有虚表

    struct Animal{
    int m_age;
    void speak(){
        cout << "Animal::speak()" << endl;
    }
    void run(){
        cout << "Animal::run()" << endl;
    }
};
struct Cat:Animal{
    int m_life;
    virtual void speak(){
        cout << "Cat::speak()" << endl;
    }
    virtual void run(){
        cout << "Cat::run()" << endl;
    }
};
struct WhiteCat:Cat{
    void speak(){
        cout << "WhiteCat::speak()" << endl;
    }
    void run(){
        cout << "WhiteCat::run()" << endl;
    }
};
int main(){
    Animal *ani = new WhiteCat();
    ani->speak();
    ani->run();
    
    Cat *cat = new WhiteCat();
    cat->speak();
    cat->run();
    return 0;
}
//输出
Animal::speak()
Animal::run()
WhiteCat::speak()
WhiteCat::run()

    子类的函数是虚函数，要么是从父类重写，要么是自己在函数名前加virtual

    2.调用父类的成员函数

    先执行父类的重写的虚函数，C++没有super关键字，用父类的类名调用

    struct Animal{
    int m_age;
    virtual void speak(){
        cout << "Animal::speak()" << endl;
    }
    virtual void run(){
        cout << "Animal::run()" << endl;
    }
};
struct Cat:Animal{
    int m_life;
    void speak(){
        Animal::speak();
        cout << "Cat::speak()" << endl;
    }
    void run(){
        cout << "Cat::run()" << endl;
    }
};
int main(){
    Animal *ani = new Cat();
    ani->speak();
    ani->run();
    return 0;
}
//输出
Animal::speak()
Cat::speak()
Cat::run()

    3.虚析构函数

    ◼ 如果存在父类指针指向子类对象的情况，应该将析构函数声明为虚函数(虚析构函数)
    delete父类指针时，才会调用子类的析构函数，保证析构的完整性,先调用子类析构函数后调用父类析构函数

    struct Animal{
    int m_age;
    virtual void speak(){
        cout << "Animal::speak()" << endl;
    }
    virtual void run(){
        cout << "Animal::run()" << endl;
    }
    Animal(){
        cout << "Animal::Animal()" << endl;
    }
    virtual ~Animal(){
        cout << "Animal::~Animal()" << endl;
    }
};

struct Cat:Animal{
    int m_life;
    void speak(){
        cout << "Cat::speak()" << endl;
    }
    void run(){
        cout << "Cat::run()" << endl;
    }
    Cat(){
        cout << "Cat::Cat()" << endl;
    }
    ~Cat(){
        cout << "Cat::~Cat()" << endl;
    }
};
int main(){
    Animal *ani = new Cat();
    delete ani;
    return 0;
}
//输出
Animal::Animal()
Cat::Cat()
Cat::~Cat()
Animal::~Animal()

    4.纯虚函数

    ◼ 纯虚函数:没有函数体且初始化为0的虚函数，用来定义接口规范

    struct Animal{
    virtual void speak() = 0;
    virtual void run() = 0;
};
struct Dog:Animal{
    void speak(){
        cout << "Dog::speak()" << endl;
    }
    void run(){
        cout << "Dog::run()" << endl;
    }
};
struct Cat:Animal{
    void speak(){
        Animal::speak();
        cout << "Cat::speak()" << endl;
    }
    void run(){
        cout << "Cat::run()" << endl;
    }
};
struct Pig:Animal{
    void speak(){
        cout << "Pig::speak()" << endl;
    }
    void run(){
        cout << "Pig::run()" << endl;
    }
};
int main(){
    Animal *ani = new Pig();
    ani->speak();
    ani->run();
    
    getchar();
    return 0;
}
//输出
Pig::speak()
Pig::run()

    5.抽象类(Abstract Class)

    含有纯虚函数的类，不可以实例化(不可以创建对象)
    抽象类也可以包含非纯虚函数、成员变量

    struct Animal{
    int m_age;
    virtual void speak() = 0;
    virtual void run() = 0;
    void eat(){}
};

    如果父类是抽象类，子类没有完全重写纯虚函数，那么这个子类依然是抽象类

    //抽象类
struct Animal{
    int m_age;
    virtual void speak() = 0;
    virtual void run() = 0;
    void eat(){
        
    }
};
//抽象类
struct Dog:Animal{
    void speak(){
        cout << "Dog::speak()" << endl;
    }
};
//非抽象类
struct HaShiQi:Dog{
    void speak(){
        cout << "HaShiQi::speak()" << endl;
    }
    void run(){
        cout << "HaShiQi::run()" << endl;
    }
};

    6.多继承

    ◼ C++允许一个类可以有多个父类(不建议使用，会增加程序设计复杂度)


    image 
    struct Student{
    int m_score;
    void study(){
        cout << "Student::study() - score = " << m_score << endl;
    }
};
struct Worker{
    int m_salary;
    void work(){
        cout << "Worker::work() - salary = " << m_salary << endl;
    }
};
struct Undergraduate:Student,Worker{
    int m_grade;
    void play(){
        cout << "Undergraduate::play() - m_grade = " << m_grade << endl;
    }
};
int main(){
    Undergraduate *u = new Undergraduate();
    u->m_score = 100;
    u->m_salary = 2000;
    u->m_grade = 4;
    u->work();
    u->study();
    u->play();
    
    return 0;
}
//输出
Worker::work() - salary = 2000
Student::study() - score = 100
Undergraduate::play() - m_grade = 4

    7.多继承体系下的构造函数调用

    struct Student{
    int m_score;
    Student(int score):m_score(score){}
    void study(){
        cout << "Student::study() - score = " << m_score << endl;
    }
};
struct Worker{
    int m_salary;
    Worker(int salary):m_salary(salary){}
    void work(){
        cout << "Worker::work() - salary = " << m_salary << endl;
    }
};
struct Undergraduate:Student,Worker{
    int m_grade;
    Undergraduate(int grade,int score,int salary):m_grade(grade),Student(score),Worker(salary){}
    void play(){
        cout << "Undergraduate::play() - m_grade = " << m_grade << endl;
    }
};
int main(){
    Undergraduate *u = new Undergraduate(4,100,2000);
    cout << u->m_grade << endl;
    cout << u->m_score << endl;
    cout << u->m_salary << endl;
    return 0;
}
//输出
4
100
2000

    8.多继承-虚函数

    ◼ 如果子类继承的多个父类都有虚函数，那么子类对象就会产生对应的多张虚表


    图片.png

    9.同名函数

    struct Student{
    int m_score;
    void eat(){
        cout << "Student::eat()" << endl;
    }
};
struct Worker{
    int m_salary;
    void eat(){
        cout << "Worker::eat()" << endl;
    }
};
struct Undergraduate:Student,Worker{
    int m_grade;
    void eat(){
        cout << "Undergraduate::eat()" << endl;
    }
};
int main(){
    Undergraduate ug;
    ug.eat();
    ug.Student::eat();
    ug.Worker::eat();
    ug.Undergraduate::eat();
    
    return 0;
}
//输出
Undergraduate::eat()
Student::eat()
Worker::eat()
Undergraduate::eat()

    若子类没有eat()，直接调用eat()会产生二义性报错

    10.同名成员变量

    struct Student{
    int m_age;

};
struct Worker{
    int m_age;
};
struct Undergraduate:Student,Worker{
    int m_age;
};
int main(){
    cout << sizeof(Undergraduate) << endl;
    Undergraduate ug;
    ug.m_age = 10;
    cout << ug.m_age << endl;
    ug.Student::m_age = 20;
    ug.Worker::m_age = 23;
    cout << ug.Student::m_age << endl;
    cout << ug.Worker::m_age << endl;
    
    return 0;
}
//输出
12
10
20
23
    image

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