button changing random location on click [fltk]

Bjarne Stroustrup “Programming Principles and Practice Using C++”
Chapter 16 Exercise 3
Using GUI library called FLTK (Fast Light Tool Kit, “full tick”).

Output:

[code language=”cpp”]
// Philipp Siedler
// Bjarne Stroustrup’s PP
// Chapter 16 Exercise 3

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

inline int rand_int(int min, int max)
{
static default_random_engine ran;
return uniform_int_distribution<>(min, max)(ran);
}

// layout
constexpr int xmax = 400;
constexpr int ymax = 400;

struct My_window : Window {
My_window(Point xy, int w, int h, const string& title)
: Window(xy, w, h, title),
next_button_pushed(false),
next_button(Point(x_max() – 140, 0), 70, 20, "Next", cb_next),
quit_button_pushed(false),
quit_button(Point(x_max() – 70, 0), 70, 20, "Quit", cb_quit)
{
attach(next_button);
attach(quit_button);
}

void wait_for_button()
{
while (!next_button_pushed) Fl::wait();
next_button_pushed = false;
Fl::redraw();
}

Button next_button;
Button quit_button;

private:
bool next_button_pushed;
bool quit_button_pushed;

static void cb_next(Address, Address addr)
{
reference_to<My_window>(addr).next();
}

static void cb_quit(Address, Address addr)
{
reference_to<My_window>(addr).quit();
}

void next() { next_button_pushed = true; }
void quit() { hide(); }
};

struct Random_button_win : My_window
{
Random_button_win(Point xy, int w, int h, const string& title, const string& url)
:My_window(xy, w, h, title),
url(url),
rand_button(Point(xmax/2, ymax/2), 40, 40, "", cb_rand),
button_image(Point(xmax / 2, ymax / 2), url)
{
attach(rand_button);
button_image.set_mask(Point(xmax / 2, ymax / 2), 40, 40);
attach(button_image);
}

Button rand_button;
Image button_image;

private:
string url;

static void cb_rand(Address, Address addr)
{
static_cast<Random_button_win*>(addr)->change_button_pos();
}

void change_button_pos()
{
rand_button.hide();

int current_x = rand_button.loc.x;
int current_y = rand_button.loc.y;

int new_x = rand_int(0, xmax – 40) – current_x;
int new_y = rand_int(20, ymax – 40) – current_y;

rand_button.move(new_x, new_y);
button_image.move(new_x, new_y);
redraw();
}
};

int main()
try
{
Random_button_win win(Point(100, 100), xmax, ymax + 20, "random buttons window", "Data/supercell.jpg");
return gui_main();
}
catch (exception& e) {
cerr << "error: " << e.what() << ‘\n’;
keep_window_open();
return 1;
}
catch (…) {
cerr << "Unknown exception!\n";
keep_window_open();
return 2;
}
[/code]

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