如何使用具有纯虚函数的父类访问 C++ 中的父类的构造函数

Question

I am trying to create Circle and Rectangle from Class Shape. I want y to be assigned pi if I call Shape() constructor with a parameter (from circle class). Since Shape has a purely virtual function the compiler is showing error. How can I overcome this error. And why is default parameter running correctly then? Also I tried this->Shape(0) from Circle class. Compiler is saying "Invalid use of this"

#include<iostream>
using namespace std;

class Shape
{public:
double x,y;

   Shape()
   {x=0;y=0;}

   Shape(int p,int t=3.14159)
   {x=p;y=t;}

   virtual void display_area()=0;
   virtual void get_data()=0;
};



class Circle: public Shape
{public:

    Circle()
    {Shape(0);}    //ERROR HERE

    void get_data()
    {cout<<"\nRadius: ";cin>>x;}

    void display_area()
    {cout<<"\nArea: "<<y*x*x;}
};

Answer 1

To call the base constructor you need to use the member initialization list .

Change:

Circle()
{
    Shape(0);
}    //ERROR HERE

To

Circle() : Shape(0)
{

}

Answer 2

base classes are always initialized before the constructor's block runs, so you do it in the constructor's member initialization list ..

I also fixed another bug in your code.... You are doing some narrowing conversions, which wouldn't work as you want...

#include<iostream>
using namespace std;

class Shape
{
public:
    double x,y;

Shape()
{  
     x=0;
     y=0;
}

Shape(double p, double t=3.14159)   //changed from Shape(int p, int t=3.14159)
{  
     x=p;
     y=t;
}

virtual void display_area()=0;

virtual void get_data()=0;
};

class Circle: public Shape
{
public:
    Circle() : Shape(0)
{  /*Shape(0); */ }    //Not HERE

void get_data()
{   
     cout<<"\nRadius: ";
     cin>>x;
}

void display_area()
{
     cout<<"\nArea: "<<y*x*x;}
};