Container Widgets

Container widgets hold and organize other widgets. They manage layout, resizing, and interaction between child widgets.

Fl_Group

Header: FL/Fl_Group.H The base container class. Groups maintain an ordered list of child widgets and control their layout and event handling.

Constructor

Fl_Group(int x, int y, int w, int h, const char *label = 0);

Key Methods

begin() / end() - Add Children

Control which group receives newly created widgets.

void begin();  // New widgets go into this group
void end();    // Stop adding to this group

Fl_Group *group = new Fl_Group(10, 10, 200, 100);
group->begin();
  Fl_Button *btn1 = new Fl_Button(20, 20, 80, 30, "Button 1");
  Fl_Button *btn2 = new Fl_Button(20, 60, 80, 30, "Button 2");
group->end();

// Widgets created after end() go to parent

Important: Always call end() after adding children!

add() / remove() - Manage Children

Explicitly add or remove widgets from group.

void add(Fl_Widget &widget);
void add(Fl_Widget *widget);
void remove(Fl_Widget &widget);
void remove(int index);
void clear();  // Remove all children

Fl_Button *btn = new Fl_Button(20, 20, 80, 30, "Button");
group->add(btn);

// Later...
group->remove(btn);

children() / child() - Access Children

Get information about child widgets.

int children() const;              // Number of children
Fl_Widget* child(int n) const;     // Get nth child (0-based)
int find(const Fl_Widget *) const; // Get index of child

for (int i = 0; i < group->children(); i++) {
  Fl_Widget *w = group->child(i);
  printf("Child %d: %s\n", i, w->label());
}

resizable() - Resize Behavior

Control how the group and its children resize.

void resizable(Fl_Widget *widget);
Fl_Widget* resizable() const;

// NULL = fixed size, children don't move
group->resizable(NULL);

// Group itself = children resize proportionally
group->resizable(group);

// Specific widget = defines resize box
Fl_Box *spacer = new Fl_Box(50, 50, 100, 100);
group->resizable(spacer);

The resizable widget defines a “resize box”. Widgets inside this box scale with the group, widgets outside it stay at fixed distance from edges.

clip_children() - Clip Drawing

Prevent children from drawing outside group bounds.

void clip_children(int c);  // 1 = clip, 0 = no clip
unsigned int clip_children() const;

group->clip_children(1);  // Enable clipping

Example

#include <FL/Fl.H>
#include <FL/Fl_Window.H>
#include <FL/Fl_Group.H>
#include <FL/Fl_Button.H>
#include <FL/Fl_Box.H>

int main() {
  Fl_Window *win = new Fl_Window(400, 300, "Group Example");
  
  // Create a group with frame
  Fl_Group *group = new Fl_Group(20, 20, 360, 120);
  group->box(FL_ENGRAVED_BOX);
  group->begin();
    Fl_Box *title = new Fl_Box(30, 30, 340, 30, "Button Group");
    title->labelfont(FL_HELVETICA_BOLD);
    
    Fl_Button *b1 = new Fl_Button(40, 70, 100, 30, "Button 1");
    Fl_Button *b2 = new Fl_Button(150, 70, 100, 30, "Button 2");
    Fl_Button *b3 = new Fl_Button(260, 70, 100, 30, "Button 3");
  group->end();
  
  win->end();
  win->show();
  return Fl::run();
}

Visual Description

An Fl_Group is typically invisible (FL_NO_BOX) but can have any box type. It defines a rectangular region containing child widgets. The group handles events and passes them to appropriate children.

Fl_Window

Header: FL/Fl_Window.H Creates an actual window with title bar and window manager controls. Inherits from Fl_Group.

Constructor

// Window positioned by window manager
Fl_Window(int w, int h, const char *title = 0);

// Window at specific position  
Fl_Window(int x, int y, int w, int h, const char *title = 0);

Key Methods

show() / hide() - Display Window

Show or hide the window.

void show();
void show(int argc, char **argv);  // For main window
void hide();

int shown() const;  // Returns 1 if shown

win->show(argc, argv);  // Main window
dialog->show();         // Other windows

label() - Window Title

Set the window title bar text.

void label(const char *title);
const char* label() const;

win->label("My Application");
win->label("Document.txt - Editor");

size_range() - Set Size Constraints

Limit how the window can be resized.

void size_range(int minw, int minh, 
               int maxw=0, int maxh=0,
               int dw=0, int dh=0, 
               int aspect=0);

// Minimum 400x300, maximum 1920x1080
win->size_range(400, 300, 1920, 1080);

// Fixed size (not resizable)
win->size_range(400, 300, 400, 300);

// Minimum only
win->size_range(400, 300);

position() - Set Window Position

Set window position on screen.

void position(int x, int y);
void hotspot(int x, int y);  // Position relative to mouse

win->position(100, 100);  // Top-left at (100, 100)

Modal Windows

Create modal dialogs that block other windows.

void set_modal();       // Modal to all windows
void set_non_modal();   // On top, but doesn't block
void clear_modal_states();  // Back to normal

Fl_Window *dialog = new Fl_Window(300, 150, "Confirm");
dialog->set_modal();
dialog->show();

fullscreen() - Full Screen Mode

Make window fill entire screen.

void fullscreen();
void fullscreen_off();
void fullscreen_off(int x, int y, int w, int h);

unsigned int fullscreen_active() const;

win->fullscreen();      // Go fullscreen
win->fullscreen_off();  // Restore

border() - Window Decorations

Control window manager border.

void border(int b);  // 1 = with border, 0 = borderless
unsigned int border() const;

win->border(0);  // Borderless window

Example from Source

#include <FL/Fl.H>
#include <FL/Fl_Window.H>
#include <FL/Fl_Button.H>

void close_cb(Fl_Widget *w, void *) {
  ((Fl_Window*)w->parent())->hide();
}

int main(int argc, char **argv) {
  // Main window
  Fl_Window *window = new Fl_Window(320, 240, "My Application");
  window->begin();
    Fl_Button *btn = new Fl_Button(110, 100, 100, 30, "Close");
    btn->callback(close_cb);
  window->end();
  
  window->show(argc, argv);  // Parse command line args
  return Fl::run();
}

Visual Description

Fl_Window appears as a system window with title bar, close/minimize/maximize buttons (platform-dependent), and optional border. Background is typically gray (FL_BACKGROUND_COLOR).

Fl_Scroll

Header: FL/Fl_Scroll.H A container that adds scrollbars when children exceed available space.

Constructor

Fl_Scroll(int x, int y, int w, int h, const char *label = 0);

Scroll Types

enum {
  HORIZONTAL = 1,          // Only horizontal scrollbar
  VERTICAL = 2,            // Only vertical scrollbar  
  BOTH = 3,                // Both scrollbars (default)
  HORIZONTAL_ALWAYS = 5,   // Horizontal always visible
  VERTICAL_ALWAYS = 6,     // Vertical always visible
  BOTH_ALWAYS = 7          // Both always visible
};

scroll->type(Fl_Scroll::VERTICAL);

Key Methods

void scroll_to(int x, int y);  // Scroll to position
int xposition() const;          // Current X scroll
int yposition() const;          // Current Y scroll

scroll->scroll_to(0, 100);  // Scroll down 100 pixels

Example

Fl_Scroll *scroll = new Fl_Scroll(10, 10, 380, 280);
scroll->type(Fl_Scroll::BOTH);
scroll->begin();
  // Add many widgets that exceed scroll area
  for (int i = 0; i < 20; i++) {
    new Fl_Button(20, 20 + i*40, 300, 30, 
                 ("Button " + std::to_string(i+1)).c_str());
  }
scroll->end();

Visual Description

Appears as a rectangular region with scrollbars on the right and/or bottom when content is larger than the visible area. Content can be scrolled by dragging scrollbars or mouse wheel.

Fl_Pack

Header: FL/Fl_Pack.H Automatically arranges children in a row or column.

Fl_Pack *pack = new Fl_Pack(10, 10, 200, 300);
pack->type(Fl_Pack::VERTICAL);  // Stack vertically
pack->spacing(5);               // 5 pixel gap between children

pack->begin();
  new Fl_Button(0, 0, 0, 30, "Button 1");  // Width auto-fills
  new Fl_Button(0, 0, 0, 30, "Button 2");
  new Fl_Button(0, 0, 0, 30, "Button 3");
pack->end();

Fl_Tabs

Header: FL/Fl_Tabs.H Tabbed interface where only one child group is visible at a time.

Fl_Tabs *tabs = new Fl_Tabs(10, 10, 380, 280);

// Tab 1
Fl_Group *tab1 = new Fl_Group(10, 35, 380, 255, "General");
tab1->begin();
  // Add widgets for tab 1
tab1->end();

// Tab 2  
Fl_Group *tab2 = new Fl_Group(10, 35, 380, 255, "Advanced");
tab2->begin();
  // Add widgets for tab 2
tab2->end();

tabs->end();

Container Gallery

Group

Fl_Group - Basic containerHolds widgets, manages events and layout.Use for: Organizing related widgets, grouping radio buttons

Window

Fl_Window - Top-level windowSystem window with title bar and decorations.Use for: Application windows, dialogs

Scroll

Fl_Scroll - Scrolling containerAdds scrollbars when content exceeds bounds.Use for: Long forms, large content areas

Pack

Fl_Pack - Auto-layout containerAutomatically arranges children in rows/columns.Use for: Toolbars, button groups, vertical lists

Common Patterns

Dialog with OK/Cancel

Fl_Window* create_dialog(const char *title) {
  Fl_Window *dlg = new Fl_Window(400, 200, title);
  dlg->set_modal();
  dlg->begin();
    // Content area
    Fl_Box *msg = new Fl_Box(20, 20, 360, 120, "Message here");
    msg->box(FL_FLAT_BOX);
    msg->align(FL_ALIGN_WRAP);
    
    // Button group at bottom
    Fl_Return_Button *ok = new Fl_Return_Button(190, 150, 90, 30, "OK");
    ok->callback([](Fl_Widget *, void *v) {
      ((Fl_Window*)v)->hide();
    }, dlg);
    
    Fl_Button *cancel = new Fl_Button(290, 150, 90, 30, "Cancel");
    cancel->callback([](Fl_Widget *, void *v) {
      ((Fl_Window*)v)->hide();
    }, dlg);
  dlg->end();
  return dlg;
}

Scrollable Content Panel

Fl_Scroll *scroll = new Fl_Scroll(10, 10, 580, 480);
scroll->box(FL_DOWN_BOX);
scroll->type(Fl_Scroll::VERTICAL);

scroll->begin();
  // Create invisible box to define scroll area
  Fl_Box *bounds = new Fl_Box(10, 10, 560, 2000);
  
  // Add many widgets
  for (int i = 0; i < 50; i++) {
    Fl_Button *btn = new Fl_Button(20, 20 + i*45, 540, 35);
    btn->label(("Item " + std::to_string(i+1)).c_str());
  }
scroll->end();

Resizable Window Layout

Fl_Window *win = new Fl_Window(600, 400, "Resizable Layout");

// Toolbar (fixed height)
Fl_Group *toolbar = new Fl_Group(0, 0, 600, 40);
toolbar->box(FL_FLAT_BOX);
toolbar->color(FL_BACKGROUND2_COLOR);
toolbar->begin();
  new Fl_Button(10, 5, 80, 30, "New");
  new Fl_Button(100, 5, 80, 30, "Open");
toolbar->end();

// Content area (resizable)
Fl_Box *content = new Fl_Box(0, 40, 600, 330);
content->box(FL_DOWN_BOX);

// Status bar (fixed height)  
Fl_Box *status = new Fl_Box(0, 370, 600, 30);
status->box(FL_FLAT_BOX);
status->align(FL_ALIGN_LEFT | FL_ALIGN_INSIDE);
status->label(" Ready");

win->end();
win->resizable(content);  // Only content area resizes
win->size_range(400, 300);  // Minimum size

Tabbed Settings Panel

Fl_Window *win = new Fl_Window(500, 400, "Settings");

Fl_Tabs *tabs = new Fl_Tabs(10, 10, 480, 340);

// General tab
Fl_Group *general = new Fl_Group(10, 35, 480, 315, "General");
general->begin();
  new Fl_Check_Button(30, 60, 200, 25, "Auto-save");
  new Fl_Check_Button(30, 90, 200, 25, "Show tooltips");
general->end();

// Appearance tab
Fl_Group *appearance = new Fl_Group(10, 35, 480, 315, "Appearance");
appearance->begin();
  Fl_Choice *theme = new Fl_Choice(150, 60, 150, 25, "Theme:");
  theme->add("Default|Dark|Light");
appearance->end();

tabs->end();

Fl_Return_Button *ok = new Fl_Return_Button(300, 360, 80, 30, "OK");
Fl_Button *cancel = new Fl_Button(390, 360, 80, 30, "Cancel");

win->end();

Important Notes

Always call end() after adding children to a group or window. Forgetting this causes subsequent widgets to be added as children, breaking your layout.

The resizable() widget must be a direct child of the group. Setting a grandchild or unrelated widget as resizable causes undefined behavior.

Use current(0) after show() to prevent accidentally adding widgets to a window: win->show(); Fl_Group::current(0);

Window Types

Fl_Double_Window

Reduces flicker with double-buffering:

Fl_Double_Window *win = new Fl_Double_Window(400, 300);
// Use for windows with complex graphics or animations

Fl_Overlay_Window

Supports overlay planes for temporary graphics:

Fl_Overlay_Window *win = new Fl_Overlay_Window(400, 300);
// Override draw_overlay() for rubber-band selection, etc.

Reference

Source: FL/Fl_Group.H (lines 33-283)
Source: FL/Fl_Window.H (lines 35-649)
Source: FL/Fl_Scroll.H (lines 26-224)
Source: FL/Fl_Pack.H
Source: FL/Fl_Tabs.H

Getting Started

Core Concepts

Widgets

Layout Management

Advanced Topics

FLUID Designer

Platform Guides

​Fl_Group

​Constructor

​Key Methods

​Example

​Visual Description

​Fl_Window

​Constructor

​Key Methods

​Example from Source

​Visual Description

​Fl_Scroll

​Constructor

​Scroll Types

​Key Methods

​Example

​Visual Description

​Fl_Pack

​Fl_Tabs

​Container Gallery

Group

Window

Scroll

Pack

​Common Patterns

​Dialog with OK/Cancel

​Scrollable Content Panel

​Resizable Window Layout

​Tabbed Settings Panel

​Important Notes

​Window Types

​Fl_Double_Window

​Fl_Overlay_Window

​Reference

Build docs developers (and LLMs) love

Fl_Group

Constructor

Key Methods

Example

Visual Description

Fl_Window

Constructor

Key Methods

Example from Source

Visual Description

Fl_Scroll

Constructor

Scroll Types

Key Methods

Example

Visual Description

Fl_Pack

Fl_Tabs

Container Gallery

Common Patterns

Dialog with OK/Cancel

Scrollable Content Panel

Resizable Window Layout

Tabbed Settings Panel

Important Notes

Window Types

Fl_Double_Window

Fl_Overlay_Window

Reference