Button Widgets

Interactive button widgets including push buttons, checkboxes, and radio buttons

Button widgets generate callbacks when clicked by the user. FLTK provides several button types for different interaction patterns.

Fl_Button

Header: FL/Fl_Button.H

The standard push button widget. Clicking the button triggers a callback.

Constructor

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

Key Methods

callback() - Set Action

Define what happens when the button is clicked.

void button_callback(Fl_Widget *w, void *data) {
  printf("Button clicked!\n");
}

Fl_Button *btn = new Fl_Button(50, 50, 100, 30, "Click Me");
btn->callback(button_callback);

// Lambda syntax (C++11)
btn->callback([](Fl_Widget *w, void *) {
  printf("Clicked!\n");
});

value() - Button State

Get or set button state (pressed/unpressed). For normal buttons, this is only meaningful during callback.

char value() const;           // Get state (0 or 1)
int value(int v);             // Set state
int set();                    // Same as value(1)
int clear();                  // Same as value(0)

shortcut() - Keyboard Shortcut

Define keyboard shortcut to activate button.

btn->shortcut(FL_ALT + 'a');        // Alt+A
btn->shortcut(FL_CTRL + 's');       // Ctrl+S
btn->shortcut(FL_F + 5);            // F5
btn->shortcut('a');                 // Just 'a' key

// Or use '&' in label
btn->label("&Save");  // Alt+S activates

type() - Button Behavior

Control how the button responds to clicks.

• FL_NORMAL_BUTTON (0) - Standard push button
• FL_TOGGLE_BUTTON (1) - Stays pressed/unpressed
• FL_RADIO_BUTTON - Acts like radio button

btn->type(FL_TOGGLE_BUTTON);
if (btn->value()) {
  printf("Button is ON\n");
}

when() - Callback Trigger

Control when callback is invoked.

• FL_WHEN_RELEASE - On mouse release (default)
• FL_WHEN_CHANGED - On every value change
• FL_WHEN_NOT_CHANGED - When released but value didn’t change

btn->when(FL_WHEN_RELEASE);  // Default

Example from Source

test/button.cxx

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

void beepcb(Fl_Widget *, void *) {
  fl_beep();
}

void exitcb(Fl_Widget *, void *) {
  exit(0);
}

int main(int argc, char **argv) {
  Fl_Window *window = new Fl_Window(320, 65);
  
  Fl_Button *b1 = new Fl_Button(20, 20, 80, 25, "&Beep");
  b1->callback(beepcb, 0);
  
  Fl_Button *b2 = new Fl_Button(120, 20, 80, 25, "&no op");
  
  Fl_Button *b3 = new Fl_Button(220, 20, 80, 25, "E&xit");
  b3->callback(exitcb, 0);
  
  window->end();
  window->show(argc, argv);
  return Fl::run();
}

Fl_Check_Button

Header: FL/Fl_Check_Button.H

A button with a checkbox indicator. Inherits from Fl_Light_Button.

Constructor

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

Example

Fl_Check_Button *check = new Fl_Check_Button(50, 50, 150, 30, "Enable Feature");
check->value(1);  // Start checked
check->callback([](Fl_Widget *w, void *) {
  Fl_Check_Button *cb = (Fl_Check_Button*)w;
  printf("Checkbox is %s\n", cb->value() ? "checked" : "unchecked");
});

Visual Description

Displays a small square checkbox to the left of the label text. When checked, an “X” or checkmark appears inside the box.

Fl_Radio_Button

Header: FL/Fl_Radio_Button.H

Radio buttons work in groups where only one can be selected at a time. All radio buttons within the same Fl_Group parent are automatically grouped.

Constructor

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

Example

Fl_Window *win = new Fl_Window(300, 200);

// Radio buttons in same group are mutually exclusive
Fl_Radio_Button *r1 = new Fl_Radio_Button(50, 50, 100, 30, "Option 1");
Fl_Radio_Button *r2 = new Fl_Radio_Button(50, 90, 100, 30, "Option 2");
Fl_Radio_Button *r3 = new Fl_Radio_Button(50, 130, 100, 30, "Option 3");

r1->type(FL_RADIO_BUTTON);
r2->type(FL_RADIO_BUTTON);
r3->type(FL_RADIO_BUTTON);

r1->setonly();  // Select first option initially

win->end();

Visual Description

Displays a circular button to the left of the label. When selected, the circle is filled with a dot.

Fl_Return_Button

Header: FL/Fl_Return_Button.H

A special button that responds to the Enter/Return key. Commonly used for default dialog actions.

Fl_Return_Button *ok = new Fl_Return_Button(200, 150, 80, 30, "OK");

Visual Description

Appears with a thicker border than regular buttons, indicating it’s the default action.

Button Gallery

Standard Button

Fl_Button - Single-click action button

Appears as a raised rectangular button. Darkens when pressed.

Use for: Actions, commands, navigation

Checkbox

Fl_Check_Button - On/off toggle with indicator

Shows a small square box that displays a checkmark when active.

Use for: Boolean settings, multi-select options

Radio Button

Fl_Radio_Button - Mutually exclusive selection

Circular button with filled dot when selected. Only one per group.

Use for: Single choice from multiple options

Return Button

Fl_Return_Button - Enter key activates

Visually emphasized with thicker border.

Use for: Default dialog actions (OK, Save, etc.)

Common Patterns

OK/Cancel Dialog

void make_dialog() {
  Fl_Window *dlg = new Fl_Window(300, 150, "Confirm");
  
  Fl_Box *msg = new Fl_Box(50, 30, 200, 30, "Are you sure?");
  
  Fl_Return_Button *ok = new Fl_Return_Button(80, 100, 80, 30, "OK");
  ok->callback([](Fl_Widget *, void *v) {
    ((Fl_Window*)v)->hide();
    // Do action
  }, dlg);
  
  Fl_Button *cancel = new Fl_Button(170, 100, 80, 30, "Cancel");
  cancel->callback([](Fl_Widget *, void *v) {
    ((Fl_Window*)v)->hide();
  }, dlg);
  
  dlg->end();
  dlg->set_modal();
  dlg->show();
}

Radio Button Group

// Create explicit group for radio buttons
Fl_Group *grp = new Fl_Group(50, 50, 200, 120, "Select Size:");
grp->box(FL_ENGRAVED_BOX);
grp->align(FL_ALIGN_TOP_LEFT);

Fl_Radio_Button *small = new Fl_Radio_Button(60, 70, 80, 30, "Small");
Fl_Radio_Button *med = new Fl_Radio_Button(60, 100, 80, 30, "Medium");
Fl_Radio_Button *large = new Fl_Radio_Button(60, 130, 80, 30, "Large");

small->type(FL_RADIO_BUTTON);
med->type(FL_RADIO_BUTTON);
large->type(FL_RADIO_BUTTON);

med->setonly();  // Default to Medium

grp->end();

Checkbox Settings Panel

Fl_Check_Button *cb1 = new Fl_Check_Button(50, 50, 200, 30, "Auto-save");
Fl_Check_Button *cb2 = new Fl_Check_Button(50, 90, 200, 30, "Show tooltips");
Fl_Check_Button *cb3 = new Fl_Check_Button(50, 130, 200, 30, "Play sounds");

cb1->value(1);  // Enabled by default
cb2->value(1);

// Callback updates configuration
auto config_cb = [](Fl_Widget *w, void *) {
  Fl_Check_Button *cb = (Fl_Check_Button*)w;
  update_config(cb->label(), cb->value());
};

cb1->callback(config_cb);
cb2->callback(config_cb);
cb3->callback(config_cb);

Styling Buttons

Fl_Button *btn = new Fl_Button(50, 50, 100, 40, "Custom");

// Colors
btn->color(FL_BLUE);              // Background
btn->labelcolor(FL_WHITE);        // Text color
btn->selection_color(FL_DARK_BLUE);  // Pressed color

// Typography
btn->labelsize(16);
btn->labelfont(FL_HELVETICA_BOLD);

// Border style
btn->box(FL_ROUND_UP_BOX);
btn->down_box(FL_ROUND_DOWN_BOX);

Important Notes

Radio buttons only work correctly when they share the same parent group. Buttons in different groups won’t be mutually exclusive.

The shortcut character ’&’ in a label creates an Alt+key shortcut on most systems. The character after ’&’ becomes the shortcut key.

Use when(FL_WHEN_CHANGED) on toggle buttons to get callbacks on both press and release events.

Reference

• Source: FL/Fl_Button.H
• Source: FL/Fl_Check_Button.H
• Source: FL/Fl_Radio_Button.H
• Source: FL/Fl_Return_Button.H
• Example: test/button.cxx