C++ not modifiable lvalue - c++

Question

When I run this code, I get the an error saying C.getRadius() isn't modifiable lvalue:
#include <iostream>
#include <conio.h>
using namespace std;
class Circle {
double x, y, r;
public:
Circle (double a=1.0, double b=1.0, double c=1.0) { x=a; y=b; r=c; }
~Circle() {}
double Area();
double getRadius();
};
class Conus {
double height;
Circle C;
public:
Conus (double , double , double , double );
~Conus() {};
double Volume();
void setRadius(double );
};
Conus::Conus(double h, double a, double b, double c)
: C (a, b, c)
{
height=h;
}
double Circle::Area() { return r*r*3.14; }
double Conus::Volume() { return C.Area()*height; }
double Circle::getRadius() { return r; }
void Conus::setRadius(double t ) { C.getRadius()=t; }
int main() {
double a=2.4, b=3.5, r=5.4, h=5.9;
Circle Wre (a, b, r) ;
Conus Konusi (h, a, b, r) ;
cout << "Wris centris koordinatebia: " << a << " da " << b << ", radiusi: " << r
<< ". Wris fartobia :" << Wre.Area() << endl;
cout << "Konusis fudzis centris koordinatebia: " << a << " da " << b << ", radiusi: " << r
<<endl<< "konusis moculobaa: " << Konusi.Volume() << endl;
Konusi.setRadius(r+3);
cout << Konusi.Volume() << endl;
_getch();
return 0;
}

Answer 1

getRadius() returns a copy of the object's radius, not a reference to it, so you can't use it for modification. So the following expression:
C.getRadius()=t;
attempts to modify a temporary copy, which isn't allowed. (The reason that its not allowed is that otherwise that code would compile but do nothing useful, giving a subtle bug rather than an easily identified compile error).
There are various options:
return a reference, double & getRadius();
add a void setRadius(double); function, and use that for modification
make the radius public; there's little point in using accessors unless they're necessary to enforce invariants.
If you do want accessor functions, you should keep the existing double getRadius(), but declare it const. That will allow you to get, but not modify, the radius of a constant object.

Answer 2

This is a not a correct assignment, you are attempting to modify a temporary copy of the r not r itself:
C.getRadius()=t;
One way to fix this would be to add this method to Circle:
void setRadius( double t ){ r = t ; }
and call like:
void Conus::setRadius(double t ) { C.setRadius( t ); }
this is a great article Understanding lvalues and rvalues in C and C++ and the first example is similar to the problem you were facing just now.