Bjarne Stroustrup “Programming Principles and Practice Using C++” Chapter 13 Exercise 9
Using GUI library called FLTK (Fast Light Tool Kit, “full tick”).

Output:

// Philipp Siedler
// Bjarne Stroustrup's PP
// Chapter 13 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 Regular_hexagon : Closed_polyline
{
Regular_hexagon(Point origin, int radius)
: o(origin), r(radius)
{
generate_points();
};
void generate_points();
void draw_lines() const;
vector<Point> get_pts() { return pts; };
private:
Point o;
vector<Point> pts;
int r;
};
void Regular_hexagon::generate_points()
{
int n = 6;
for (int i = 0; i < n; i++) {
pts.push_back(Point(o.x + r * cos(2 * M_PI * i / n), o.y + r * sin(2 * M_PI * i / n)));
add(pts[i]);
}
};
void Regular_hexagon::draw_lines() const
{
Closed_polyline::draw_lines();
};
struct rh_tiling : Shape
{
rh_tiling(Point basepoint, double radius, int num_xdir, int num_ydir)
: bp(basepoint), r(radius)
{
nx = (num_xdir == 0) ? 1 : num_xdir;
ny = (num_ydir == 0) ? 1 : num_ydir;
generate_pts();
}
void generate_pts();
void draw_lines() const;
private:
vector<Point> pts;
Point bp;
double r;
int nx;
int ny;
};
void rh_tiling::generate_pts()
{
double triangle_h = sqrt(pow(r, 2) - pow(r / 2, 2));
cout << triangle_h << endl;
for (int i = 0; i < nx; i++) {
for (int j = 0; j < ny; j++) {
if (j % 2 == 0 || j == 0) {
pts.push_back(Point(bp.x + (r * 3) * i, bp.y + triangle_h * j));
}
else {
pts.push_back(Point(bp.x + (r * 3) * i + (1.5 * r), bp.y + j * triangle_h));
}
}
}
};
void rh_tiling::draw_lines() const
{
for (int i = 0; i < pts.size(); i++) {
Regular_hexagon(pts[i], r).draw_lines();
}
};
int main()
try
{
Point tl(100, 100);
Simple_window win(tl, 720, 400, "Simple Window");
Point center(win.x_max() / 2, win.y_max() / 2);
rh_tiling myTiling(Point(50,100),50,5,6);
myTiling.set_color(Color::red);
win.attach(myTiling);
win.wait_for_button();
}
catch (exception& e) {
cout << e.what() << endl;
return 1;
}
catch (...) {
cout << "Exiting" << endl;
return 2;
}