菱形继承-蒲公英云

菱形继承

两个子类继承同一个父类，而又有子类又分别继承这两个子类,就称作菱形继承

多重继承产生的二义性

假设有一个基类,他派生了两个子类分别继承于它,比如说下面这个例子:

class A {
public:
    A(int d) {
        cout << "A()" << endl;
        _data = d;
    }
protected:
    int _data;
};
class X:public A
{
public:
    X(int x = 0):A{x}{}
    void setdata(int d) {
        _data = d;
    }
};
class Y:public A
{
public:
    Y(int x = 0):A{x}{}
    int getdata() {
        return _data;
    }
};

基类中有一份公共数据_data,两个子类分别对它进行了继承,如果我们调用类X中的setdata修改数据,再调用class Y中的getdata(),得到的数据是不一致的,原因是重名数据存在于各自的作用域中,产生了二义性

为了解决这个问题(产生数据冗余),我们可以使用菱形继承:我们对上述代码按下面步骤进行修改

1 提取各父类中相同的成员构成祖父类

2 让各父类继承祖父类

3 虚继承:是一种继承的扩展:virtual

虚继承实现了在多继承中只保留一份共同成员,防止产生数据冗余

class A {
public:
    A(int d) {
        cout << "A()" << endl;
        _data = d;
    }
protected:
    int _data;
};
class X:virtual public A
{
public:
    X(int x = 0):A{x}{}
    void setdata(int d) {
        _data = d;
    }
};
class Y:virtual public A
{
public:
    Y(int x = 0):A{x}{}
    int getdata() {
        return _data;
    }
};
class Z:public X,public Y
{
public:
    Z(int x) :A{ x } {}
    void dis() {
        cout << _data << endl;
    }
};

这样就实现了了菱形继承

菱形继承的构造:菱形继承的构造由孙子类来完成对祖先类的构造,两个子类中使用默认参数构造即可(注:子类还是要对父类完成构造,只不过参数改为默认参数即可,传参由孙子类来完成)

实例:

class furniture {
public:
    furniture(string color,int weight):_color{color},_weight{weight}{}
    void descripte() {
        cout << _color << endl;
        cout << _weight << endl;
    }
protected:
    string _color;
    int _weight;
};
class sofa:virtual public furniture{
public:
    sofa(string color = "black", int weight = 100):furniture(color, weight){}
    void sit() {
        cout << "take a sit..." << endl;
    }
};
class Bed:virtual public furniture{
public:
    Bed(string color = "black",int weight = 100):furniture(color,weight){}
    void sleep() {
        cout << "have a sleep..." << endl;
    }
};
class SofaBed :public sofa, public Bed {
public:
    SofaBed(string color, int weight) :furniture(color, weight) {}
};
int main() {
    sofa s1("blue", 123);
    s1.sit();
    s1.descripte();
    Bed s2("yellow", 456);
    s2.sleep();
    s2.descripte();
    SofaBed s3("green", 3241);
    s3.sleep();
    s3.sit();
    s3.descripte();
    system("PAUSE");
}