菱形继承

布满荆棘的人生 2022-05-18 10:45 230阅读 0赞

## 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 ##

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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");
    }

[70]: /images/20220518/a28360a335e64f2d9a9c5305c660ba5f.png