How to use an existing class for a new duty - c++

Question

The problem says:
Define a class Arc, which draws a part of an ellipse. Hint: fl_arc().
Ellipse is predefined class which draws an ellipse on the window by a statement, e.g., Ellipse e1(Point(200,200),50,50);
and fl_arc() is part of FLTK which I've previously installed it on my Windows machine.
Since the problem has wanted to draw a part of an ellipse by creating a new class named Arc, I think I should make a new class with that name and also use the definition of the class Ellipse and modify it so that it shows a part of an ellipse instead of whole of an ellipse as wanted. This problem is in Programming Principle and practice using C++ book and the only definition that I found in that book about the Ellipse class was this:
struct Ellipse :Shape {
Ellipse(Point p, int w, int h); //center, max and min distance from center
void draw_lines()const;
Point center()const;
Point focus1()const;
Point focus2()const;
void set_major(int ww) {w=ww;}
int major() const {return w;}
void set_minor(int hh) {h=hh;}
int minor() const {return h;}
private:
int w;
int h;
};
and also I found a case of using the fl_arc() in the book which was used for drawing a Circle like this:
fl_arc(point(0).x,point(0).y,r+r,r+r,0,360);
Here r is radius.
So on my think, I changed the parameters of the fl_arc() and wrote the bellow code to give me what the problem has wanted:
#include <Simple_window.h>
struct arc : Shape {
arc(Point p, int w, int h) // center, min, and max distance from center
: w(w), h(h)
{
add(Point(p.x-w,p.y-h));
}
void draw_lines() const {fl_arc(point(0).x,point(0).y,w+w,h+h,0,120);}
Point center() const { return Point(point(0).x+w,point(0).y+h); }
Point focus1() const { return Point(center().x+int(sqrt(double(w*w-h*h))),center().y); }
Point focus2() const { return Point(center().x-int(sqrt(double(w*w-h*h))),center().y); }
void set_major(int ww) { w=ww; }
int major() const { return w; }
void set_minor(int hh) { h=hh; }
int minor() const { return h; }
private:
int w;
int h;
};
int main()
{
using namespace Graph_lib;
Point t(100,100);
Simple_window win(t,600,400, "semi-ellipse");
arc a(Point(200,200),150,50);
a.draw_lines();
win.wait_for_button();
}
The code runs successfully fortunately but it doesn't show any part of an ellipse.
Question:
Does anyone know why?
PS: If we can find some way for modifying a class we can tell that new class to does some new work for us.

Answer 1

Here is one possible implementation, where you use the already existing facilities of class Ellipse, overwriting the function draw_lines(), to define the class Arc:
#include"Simple_window.h"
#include"Graph.h"
#include<iostream>
using namespace Graph_lib;
//---------------------------------------------------------------------------
//Class Arc
namespace Graph_lib{
class Arc : public Ellipse {
public:
Arc(Point p, int w, int h, int arc_n, int arc_x) : Ellipse(p,w,h), arc_min(arc_n), arc_max(arc_x) {}
void draw_lines() const;
private:
int arc_min;
int arc_max;
};
void Arc::draw_lines() const
{
if(color().visibility())
fl_arc(point(0).x,point(0).y, major()*2, minor()*2, arc_min, arc_max);
}
}
//---------------------------------------------------------------------------
int main()
try
{
Simple_window win(Point(100,100), 800, 600, "Exercise #1");
Graph_lib::Arc arc1(Point(100,100),50,50,0,90);
win.attach(arc1);
win.wait_for_button();
}
catch(exception& e)
{
std::cout << e.what() << std::endl;
return 1;
}
catch(...)
{
std::cout << "unknown error..." << std::endl;
return 2;
}
Programming - Principles and Practice Using C++ (pg.477, exercise 1)
solution by Francisco Tavares

Answer 2

Although I could to find the bug of that code but still I have some issues about that exercise that I like to mention them among you professional guys.
If the line a.draw_lines(); will be replaced by this line win.attach(a); the problem runs successfully.
The remained problems are these:
1- Now when I use the name "Arc" instead of the "arc" in above code I get this error.
Error 8 error C2146: syntax error : missing ';' before identifier 'a' c:\users\cs\documents\visual studio 2012\projects\test2\test2\test2.cpp 25
Error 10
error C3861: 'a': identifier not found c:\users\cs\documents\visual studio 2012\projects\test2\test2\test2.cpp 25
2- The problem has wanted us to define a class (not a struct) so when I replace struct with class and put keyword public just after class arc : Shape {, I get this error.
*Error 8 error C2243: 'type cast' : conversion from 'arc *' to 'Graph_lib::Shape &' exists, but is inaccessible c:\users\cs\documents\visual studio 2012\projects\test2\test2\test2.cpp 29
Error 9 error C2243: 'type cast' : conversion from 'arc ' to 'Graph_lib::Shape &' exists, but is inaccessible c:\users\cs\documents\visual studio 2012\projects\test2\test2\test2.cpp 30
Any idea?