Windows - FLTK

Windows are the container widgets that provide the actual operating system windows for your FLTK applications. The Fl_Window class manages both top-level windows (with borders and title bars) and embedded subwindows.

Window Types

FLTK provides two main window types defined in FL/Fl_Window.H:27-28:

FL_WINDOW (0xF0): Standard single-buffered window
FL_DOUBLE_WINDOW (0xF1): Double-buffered window (flicker-free)

#include <FL/Fl_Window.H>
#include <FL/Fl_Double_Window.H>

// Single-buffered window
Fl_Window *win = new Fl_Window(400, 300);

// Double-buffered window (recommended for animation)
Fl_Double_Window *dbl_win = new Fl_Double_Window(400, 300);

Creating Windows

Constructor Forms

Windows can be created with or without explicit position (FL/Fl_Window.H:139-190):

Auto-positioned
Explicit Position
Subwindow

// Window manager chooses position
Fl_Window *win = new Fl_Window(640, 480);
Fl_Window *titled = new Fl_Window(640, 480, "My Application");

// Position at specific screen coordinates
Fl_Window *win = new Fl_Window(100, 100, 640, 480);
Fl_Window *titled = new Fl_Window(100, 100, 640, 480, "My App");

// Create parent window
Fl_Window *parent = new Fl_Window(800, 600);
parent->begin();

// Subwindow positioned relative to parent
Fl_Window *child = new Fl_Window(50, 50, 300, 200);
child->end();

parent->end();

Window Properties

Labels and Icons

// Set window title
win->label("Application Title");

// Set icon label (for taskbar/iconified state)
win->iconlabel("App");

// Set both at once
win->label("Window Title", "Icon Title");

// Copy label (widget manages memory)
win->copy_label(temp_string);

Window Icons

Fl_PNG_Image *icon = new Fl_PNG_Image("icon.png");
win->icon(icon);

// Multiple sizes for best quality
Fl_RGB_Image *icons[] = {icon16, icon32, icon48};
win->icons(icons, 3);

// Set default icon for all windows
Fl_Window::default_icon(icon);

Border and Decoration

// Show/hide window border
win->border(1);        // Show border
win->border(0);        // Hide border
win->clear_border();   // Hide border (faster)

if (win->border()) {
  // Window has border
}

// Override window manager (no border + always on top)
win->set_override();

// Get decorated window size (including title bar)
int total_width = win->decorated_w();
int total_height = win->decorated_h();

Window Modality

From FL/Fl_Window.H:239-296, FLTK supports three window states:

Normal Windows

Fl_Window *win = new Fl_Window(400, 300);
// Normal window - allows interaction with other windows

Modal Windows

Fl_Window *dialog = new Fl_Window(300, 200, "Dialog");
dialog->set_modal();
dialog->show();
// Blocks all other windows until closed

if (dialog->modal()) {
  // Window is modal
}

Non-Modal Windows

Fl_Window *toolbar = new Fl_Window(400, 60, "Tools");
toolbar->set_non_modal();
toolbar->show();
// Stays on top but doesn't block other windows

if (toolbar->non_modal()) {
  // Window is non-modal
}

Changing Modality

// Must hide window first to change modality
win->hide();
win->clear_modal_states();  // Clear modal/non-modal
win->set_non_modal();       // Set new state
win->show();

Showing and Hiding Windows

Basic Display

// Show window
win->show();

// Show with command-line arguments (for main window)
win->show(argc, argv);

// Hide window
win->hide();

// Check if shown
if (win->shown()) {
  // Window has been show() called
}

// Wait for window to be fully displayed
win->wait_for_expose();

Window Positioning

Manual Position
Hotspot Positioning
Multi-screen

win->position(100, 100);  // Position before show()
win->show();

// Free position (let window manager decide)
win->free_position();

// Center on mouse cursor
win->hotspot(widget);
win->show();

// Position relative to coordinates
win->hotspot(x, y);
win->show();

// Get screen dimensions
int X, Y, W, H;
Fl::screen_xywh(X, Y, W, H, 1);  // Screen #1

// Position on specific screen
win->position(X + 100, Y + 100);
win->screen_num(1);
win->show();

Window States

Fullscreen

// Enter fullscreen mode
win->fullscreen();

if (win->fullscreen_active()) {
  // Window is fullscreen
}

// Exit fullscreen
win->fullscreen_off();

// Exit fullscreen and resize
win->fullscreen_off(100, 100, 800, 600);

// Fullscreen across multiple monitors
win->fullscreen_screens(0, 0, 0, 1);  // top, bottom, left, right screen

Maximize and Iconify

// Maximize window
win->maximize();

if (win->maximize_active()) {
  // Window is maximized
}

// Restore from maximized
win->un_maximize();

// Iconify (minimize)
win->iconize();

Size Constraints

From FL/Fl_Window.H:362, control how users can resize windows:

// Set minimum and maximum size
win->size_range(
  400, 300,      // minimum width, height
  1920, 1080     // maximum width, height (0 = no limit)
);

// Allow resize in fixed aspect ratio
win->size_range(
  200, 150,      // min size
  0, 0,          // no max size
  0, 0,          // width/height step (for grid snapping)
  1              // keep aspect ratio
);

// Get current size constraints
int minw, minh, maxw, maxh;
win->get_size_range(&minw, &minh, &maxw, &maxh);

Special Window Types

For internal use, but available for custom implementations:

// Create popup menu window
Fl_Window *menu = new Fl_Window(200, 150);
menu->set_menu_window();
menu->border(0);

if (menu->menu_window()) {
  // Special menu window handling
}

// Create tooltip window
Fl_Window *tip = new Fl_Window(100, 30);
tip->set_tooltip_window();
tip->border(0);

if (tip->tooltip_window()) {
  // Tooltip-specific behavior
}

Window Graphics Context

Making a Window Current

// Set as current drawing context
win->make_current();

// Get current window
Fl_Window *current = Fl_Window::current();

// Flush buffered graphics
win->flush();

Drawing to Windows

class My_Window : public Fl_Window {
public:
  My_Window(int W, int H, const char *L = 0) 
    : Fl_Window(W, H, L) {}
  
  void draw() override {
    // Clear background
    fl_color(FL_WHITE);
    fl_rectf(0, 0, w(), h());
    
    // Draw content
    fl_color(FL_BLACK);
    fl_line(0, 0, w(), h());
    fl_line(0, h(), w(), 0);
  }
};

Mouse Cursors

// Set window cursor
win->cursor(FL_CURSOR_DEFAULT);
win->cursor(FL_CURSOR_WAIT);
win->cursor(FL_CURSOR_CROSS);
win->cursor(FL_CURSOR_HAND);
win->cursor(FL_CURSOR_MOVE);

// Custom cursor from image
Fl_RGB_Image *cursor_img = new Fl_RGB_Image(...);
win->cursor(cursor_img, hot_x, hot_y);

// Set default cursor (used when mouse is over window)
win->default_cursor(FL_CURSOR_ARROW);

Window Shapes

Create non-rectangular windows:

// Create shaped window from image alpha channel
Fl_PNG_Image *shape = new Fl_PNG_Image("shape.png");
win->border(0);
win->shape(shape);

// Get current shape
const Fl_Image *s = win->shape();

Window Callbacks

Windows have a default callback for the close button:

void my_close_callback(Fl_Widget *w, void *data) {
  Fl_Window *win = (Fl_Window*)w;
  
  // Check why callback was triggered
  if (Fl::callback_reason() == FL_REASON_CLOSED) {
    // User clicked close button
    if (confirm_exit()) {
      win->hide();
    }
  }
}

win->callback(my_close_callback);

Platform-Specific Features

X11 Window Class

// Set X11 window class (for window manager hints)
Fl_Window::default_xclass("MyApplication");
win->xclass("SpecialWindow");

const char *wm_class = win->xclass();

Window System ID

// Get native window handle
fl_uintptr_t id = win->os_id();
// Use for platform-specific operations

Best Practices

Use Double Buffering for Smooth Graphics

// Prefer Fl_Double_Window to avoid flicker
Fl_Double_Window *win = new Fl_Double_Window(800, 600);

Set Size Constraints

// Prevent windows from becoming too small
win->size_range(400, 300);

Handle Close Events

void close_cb(Fl_Widget *w, void *data) {
  if (Fl::callback_reason() == FL_REASON_CLOSED) {
    // Save state before closing
    if (confirm_exit()) {
      ((Fl_Window*)w)->hide();
    }
  }
}
win->callback(close_cb);

Use show(argc, argv) for Main Window

int main(int argc, char **argv) {
  Fl_Window *win = new Fl_Window(640, 480);
  // ... add widgets ...
  win->show(argc, argv);  // Parses command-line args
  return Fl::run();
}

Window Events

Windows receive special events defined in FL/Enumerations.H:

Event	When Triggered
`FL_SHOW`	Window becomes visible
`FL_HIDE`	Window becomes hidden/minimized
`FL_FULLSCREEN`	Fullscreen state changes
`FL_CLOSE`	User clicks close button

int My_Window::handle(int event) {
  switch(event) {
    case FL_SHOW:
      printf("Window shown\n");
      break;
    case FL_HIDE:
      printf("Window hidden\n");
      break;
    case FL_FULLSCREEN:
      printf("Fullscreen toggled\n");
      break;
  }
  return Fl_Window::handle(event);
}

Next Steps

Widgets

Add widgets to your windows

Layout

Arrange widgets with layout managers

Events

Handle window and widget events

Drawing

Custom drawing in windows

Getting Started

Core Concepts

Widgets

Layout Management

Advanced Topics

FLUID Designer

Platform Guides

Documentation Index

​Window Types

​Creating Windows

​Constructor Forms

​Window Properties

​Labels and Icons

​Window Icons

​Border and Decoration

​Window Modality

​Changing Modality

​Showing and Hiding Windows

​Basic Display

​Window Positioning

​Window States

​Fullscreen

​Maximize and Iconify

​Size Constraints

​Special Window Types

​Menu and Tooltip Windows

​Window Graphics Context

​Making a Window Current

​Drawing to Windows

​Mouse Cursors

​Window Shapes

​Window Callbacks

​Platform-Specific Features

​X11 Window Class

​Window System ID

​Best Practices

​Window Events

​Next Steps

Widgets

Layout

Events

Drawing

Window Types

Creating Windows

Constructor Forms

Window Properties

Labels and Icons

Window Icons

Border and Decoration

Window Modality

Changing Modality

Showing and Hiding Windows

Basic Display

Window Positioning

Window States

Fullscreen

Maximize and Iconify

Size Constraints

Special Window Types

Menu and Tooltip Windows

Window Graphics Context

Making a Window Current

Drawing to Windows

Mouse Cursors

Window Shapes

Window Callbacks

Platform-Specific Features

X11 Window Class

Window System ID

Best Practices

Window Events

Next Steps