C/C++ :: How To Make A Shape Outside Of Main Function

I know how to define a shape (here, a rectangle) and attach it to a window in C++ as follows:

#include <Simple_window.h>
void cir() { Circle c(Point(100,100),50); }
int main() {
Simple_window win(Point(100,100),600,400, "test");
Rectangle r(Point(100,100),Point(300,200));
win.attach(r);
win.wait_for_botton();
}

But how to define a shape (say a circle by that cir() function which is) outside of the main() function? And how to attach it on the window win to be visible?