问一下为什么 基类Shape的构造函数在每次声明派生类时都执行了，但是x,y却没有被赋值

Question

1. #include<iostream>
   #define pai 3.14
   using namespace std;
   class Shape
   {	protected:
   	float x,y;
   	public:
   	Shape(float a,float b){x=a;y=b;cout<<x<<endl;} //就是这句话
   	virtual void area()=0;
   
   };
   class T:public Shape
   {
   	public:
   	T(float a,float b):Shape(x,y){}
   	void area()
   	{ float s;
   	s=0.5*x*y; 
   	cout<<"triangleArea="<<s<<endl;}
    }; 
   class R:public Shape
   {public:
   	R(float a,float b):Shape(x,y){x=a;y=b;} //只有在派生类的构造函数中赋值时是有效的
   	void area()
   	{
   		cout<<"rectangleArea="<<x*y<<endl;
   	}
    }; 
   class C:public Shape
   {public:
   	C(float a,float b):Shape(x,y){x=a;y=b;}
   	void area()
   	{ 
   		cout<<"circleArea="<<pai*x*x<<endl;
   	}
    };
    int main()
    {	float x1,y;
    	cin>>x1>>y;
    	T t1(x1,y);
    	t1.area();
    	R r1(x1,y);
    	r1.area();
    	C c1(x1,y);
    	c1.area();
    	return 0;
    }

 

 

Answer 1

改成Shape(a,b)