"class::data member is private" error, but I'm operating on it with a member function?

Question

I'm fairly new to C++, and I don't understand what is triggering this error:

/home/---/Documents/C++/---_lab2/lab2c.cpp||In function ‘int main()’:|
Line 9: error: ‘float circle::x1’ is private
Line 58: error: within this context

I know the data member x1 (x2,y1,y2 as well) is private, but I am operating on the object myObj using functions that are members of the class circle, so shouldn't they still work? Can someone explain to me what's wrong here?

#include <iostream>
#include <cmath>
#define PI 3.14159

using namespace std;

class circle{

private:
float x1,y1,x2,y2;

protected:

float distance(float x1,float y1,float x2, float y2){
    return sqrt(fabs((x2-x1)*(x2-x1))+fabs((y2-y1)*(y2-y1)));
};

public:

float radius(float x1, float y1, float x2, float y2){
    float rad = distance(x1,y1,x2,y2);
    return rad;
};

float circumference(float rad){
    return 2*PI*rad;
};

float area(float rad){
    return PI*rad*rad;
};

float populate_classobj(float x1main,float x2main,float y1main,float y2main){
x1 = x1main;
x2 = x2main;
y1 = y1main;
y2 = y2main;
};

};

int main(){

circle myObj;
float x1main,x2main,y1main,y2main;
cout << "Coordinates of center" << endl;
cout << "X: ";
cin >> x1main;
cout << "Y: ";
cin >> y1main;
cout << "Coordinates of point on circle" << endl;
cout << "X: ";
cin >> x2main;
cout << "Y: ";
cin >> y2main;

myObj.populate_classobj(x1main,x2main,y1main,y2main);

cout << "Radius is " << myObj.radius(myObj.x1,myObj.y1,myObj.x2,myObj.y2) << endl;
cout << "Circumference is " << myObj.circumference(myObj.radius(myObj.x1,myObj.y1,myObj.x2,myObj.y2)) << endl;;
cout << "Area is " << myObj.area(myObj.radius(myObj.x1,myObj.y1,myObj.x2,myObj.y2)) << endl;



return 0;
}

Solution 1

You're attempting to access private members outside the class when you call radius & other methods.

But your real problem is with the logic. Why do you need to pass parameters to, for example, the radius method of your class:

float radius(float x1, float y1, float x2, float y2){
    float rad = distance(x1,y1,x2,y2);
    return rad;
};

The circle is already self-contained, why not just:

float radius(){
    float rad = distance(x1,y1,x2,y2);
    return rad;
};

Same with:

float circumference(){
    return 2*PI*radius();
};

float area(){
    return PI*radius()*radius();
};

Also, note that:

circle myObj;

creates an invalid object. You shouldn't have to call populate_classobj just to make it valid. Instead, have a proper constructor:

circle(float x1main,float x2main,float y1main,float y2main) :
x1(x1main),
x2(x2main),
y1(y1main),
y2(y2main)
{
};

and create the object as:

circle myObj(x1main,x2main,y1main,y2main);

Solution 2

The various insertion statements at the end of main try to use myObj.x1, which tries to use the member x1 of myObj. They can't, because x1 is private. It doesn't matter what the code is doing with that value; private is private. You can access the value from inside a member function or a friend function, but not from outside.

Solution 3

cout << "Radius is " << myObj.radius(myObj.x1,myObj.y1,myObj.x2,myObj.y2) << endl;
cout << "Circumference is " << myObj.circumference(myObj.radius(myObj.x1,myObj.y1,myObj.x2,myObj.y2)) << endl;;
cout << "Area is " << myObj.area(myObj.radius(myObj.x1,myObj.y1,myObj.x2,myObj.y2)) << endl;

You can't access a private variable. Also you shouldn't have to do that. Your method signature should be myObj.radius() or myObj.area() as x1 y1 x2 y2 are already members of the circle myObj. So passing them again as arguments is redundant.