继承的类和父级的构造函数

Question

#include<iostream>
#define PI 3.14
using  namespace std;

    class ellipse 
    {
          protected:
          float a,b;

          public:
            ellipse(float x, float y) 
                {
                a=x;
                b=y;
                }

                 float area()
                 {
                 return (PI*a*b);
                 }
  };

  class circle : public ellipse 
  {
     public:
        circle(float r) 
         {
            a=r;
            b=r;
         }
  };

main() {
    float x,y,r;
    cout<<"Enter the two axes of the ellipse (eg. 5 4) : ";
    cin>>x>>y;
    cout<<"\nEnter the radius of the circle : ";
    cin>>r;
    ellipse obj(x,y);
    cout<<"\nEllipse Area : "<<obj.area();
    circle obj1(r);
    cout<<"\nCircle Area : "<<obj1.area();
}

When I compiled this program I got the follwing errors:

friendclass4.cpp: In constructor ‘circle::circle(float)’:
friendclass4.cpp:24:1: error: no matching function for call to ‘ellipse::ellipse()’
friendclass4.cpp:24:1: note: candidates are:
friendclass4.cpp:10:1: note: ellipse::ellipse(float, float)
friendclass4.cpp:10:1: note:   candidate expects 2 arguments, 0 provided
friendclass4.cpp:5:7: note: ellipse::ellipse(const ellipse&)
friendclass4.cpp:5:7: note:   candidate expects 1 argument, 0 provided

I added a second constructor for ellipse as shown below (trial and error) and solved the issue

ellipse() {

}

But I am not sure why the errors occurred before adding this. Could anybody explain this to me?

Answer 1

In the constructor circle(float) , it needs to call the default constructor ellipse() with no arguments, because you did not provide any arguments in the "initialization list." To fix it, do this (and remove your ellipse() default constructor):

circle(float r)
  : ellipse(r, r)
{
}

This simply delegates the initialization of a and b to the two-argument ellipse constructor. It offers better encapsulation and more concise code.