using a group class container to draw checkers board with movable piece [fltk]

Bjarne Stroustrup “Programming Principles and Practice Using C++”
Chapter 14 Exercise 9
Using GUI library called FLTK (Fast Light Tool Kit, “full tick”).

Output:

[code language=”cpp”]
// Philipp Siedler
// Bjarne Stroustrup’s PP
// Chapter 14 Exercise 9

#define _USE_MATH_DEFINES
#include "Simple_window.h"
#include "Graph.h"
#include <cmath>

using namespace Graph_lib;

ostream& operator<<(ostream& out, Point& a) {
out << "(" << a.x << "," << a.y << ")";
return out;
}

struct Group : Shape
{
void add_board_shape(Shape& shape) { b.push_back(shape); }
void add_piece_shape(Shape& shape) { p.push_back(shape); }

void draw_lines() const;
void move_piece(int index, int dx, int dy);

Vector_ref<Shape> shapes() { return b; }

private:
Vector_ref<Shape> b;
Vector_ref<Shape> p;
};

void Group::draw_lines() const
{
for (int i = 0; i < b.size(); ++i) {
b[i].draw();
}
for (int i = 0; i < p.size(); ++i) {
p[i].draw();
}

}

void Group::move_piece(int index, int fieldx, int fieldy)
{
int dx = fieldx * 100;
int dy = fieldy * 100;
p[index].move(dx, dy);
}

void initialize_checkers_board(Group& group, Vector_ref<Graph_lib::Rectangle>& board)
{
for (int i = 0; i < 8; i++) {
for (int j = 0; j < 8; j++) {
// pushback to vector_ref
board.push_back(new Graph_lib::Rectangle(Point(j * 100, i * 100), Point(j * 100 + 100, i * 100 + 100)));

// setup color of checkers_board
if (i % 2 == 0) {
if (((i * 8) + j) % 2 == 0) {
board[i * 8 + j].set_fill_color(Color::black);
}
else {
board[i * 8 + j].set_fill_color(Color::white);
}
}
else {
if (((i * 8) + j) % 2 == 0) {
board[i * 8 + j].set_fill_color(Color::white);
}
else {
board[i * 8 + j].set_fill_color(Color::black);
}
}

// add rectangle to checkers_board Group
group.add_board_shape(board[i * 8 + j]);
}
}
}

int main()
try
{
Point tl(100, 100);
Simple_window win(tl, 800, 800, "Simple Window");
Point center(win.x_max() / 2, win.y_max() / 2);

Group checkers_board;
Vector_ref<Graph_lib::Rectangle> board;
Vector_ref<Graph_lib::Circle> pieces;
initialize_checkers_board(checkers_board, board);

pieces.push_back(new Graph_lib::Circle(Point(50, 50), 40));
pieces[0].set_color(Color::black);
pieces[0].set_fill_color(Color::white);
checkers_board.add_piece_shape(pieces[0]);

win.attach(checkers_board);
win.wait_for_button();

checkers_board.move_piece(0, 3, 4);
win.attach(checkers_board);

win.wait_for_button();
}
catch (exception& e) {
cout << e.what() << endl;
return 1;
}
catch (…) {
cout << "Exiting" << endl;
return 2;
}
[/code]