09_c++多继承

c++多继承

1、什么是多继承？

多继承是指一个子类继承多个父类。多继承对父类的个数没有限制，继承方式可以是公共继承、保护继承和私有继承，不写继承方式，默认是private继承。

class Sofa{
public:
    void watchTV(void)
    {
        cout<<"watch TV"<<endl;
    }
};

class Bed{
public:
    void sleep(void)
    {
        cout<<"sleep"<<endl;
    }
};

class Sofabed : public Sofa, public Bed{    /* Sofabed继承于Sofa和Bed */
};

int main(int argc, char **argv)
{
    Sofabed s;
    s.watch();
    s.sleep();
    return 0;
}

2、多继承的二义性

参考文章：https://www.cnblogs.com/sunada2005/archive/2013/10/30/3397090.html

2.1、什么是多继承二义性

多继承有多个父类，在父类中如果出现同名，同参数，同返回值的成员，则在子类的实例化对象中调用同名的方法时则会出错，产生二义性。如下例子：在Sofa类和Bed类中都有setWeight和getWeight成员，Sofabed类继承于Sofa类和Bed类，如果在Sofabed类的实例化对象中调用setWeight或getWeight就会产生二义性，编译出错。

class Sofa{
private:
    int weight;
public:
    void watchTV(void)
    {
        cout<<"watch TV"<<endl;
    }
    void setWeight(int weight)
    {
        this->weight = weight;
    }
    int getWeight(void) const
    {
        return weight;
    }
};

class Bed{
private:
    int weight;
public:
    void sleep(void)
    {
        cout<<"sleep"<<endl;
    }
    void setWeight(int weight)
    {
        this->weight = weight;
    }
    int getWeight(void) const
    {
        return weight;
    }
};

class Sofabed : public Sofa, public Bed {
};

int main(int argc, char **argv)
{
    Sofabed s;
    s.watchTV();
    s.sleep();
    //s.setWeight(100); /* error, 有二义性 */
    return 0;
}

2.2、怎么解决多继承二义性

方法一：类名限定

int main(int argc, char **argv)
{
    Sofabed s;
    s.watchTV();
    s.sleep();
    //s.setWeight(100); /* error, 有二义性 */
    s.Sofa::setWeight(100); //使用类名限定解决多继承二义性
    return 0;
}

方法二： 虚基类（用于有共同基类的场合）

虚拟继承使得类派生出的对象只继承一个基类对象。如下例子：sofabed、sofa、bed共享一个furniture基类对象

class Furniture{
private:
    int weight;
public:
    void setWeight(int weight)
    {
        this->weight = weight;
    }
    int getWeight(void) const
    {
        return weight;
    }
};

class Sofa : virtual public Furniture{/* 虚拟继承Furniture类 */
private:
    int a;
public:
    void watchTV(void)
    {
        cout<<"watch TV"<<endl;
    }
}

class Bed : virtual public Furniture{/* 虚拟继承Furniture类 */
private:
    int b;
public:
    void sleep(void)
    {
        cout<<"sleep"<<endl;
    }
};

class Sofabed : public Sofa, public Bed{
private:
    int c;
};

int main(int argc, char **argv)
{
    Sofabed s;
    s.watchTV();
    s.sleep();
    s.setWeight(100);
}