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What This Example Demonstrates

This example shows how to:
  • Create different types of buttons
  • Attach callback functions to handle button clicks
  • Use FLTK’s built-in dialogs and alerts
  • Exit an application gracefully
  • Trigger system sounds (beep)

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Complete Source Code

Source file: test/button.cxx 
#include <stdlib.h>
#include <stdio.h>
#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();
  fflush(stdout);
}

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();
}

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Compilation Command

# Using fltk-config
fltk-config --compile button.cxx

# Or manually
g++ -o button button.cxx `fltk-config --cxxflags --ldflags`

# Run
./button

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Expected Behavior

The program displays a window with three buttons:
  1. Beep - Triggers a system beep sound when clicked
  2. no op - Does nothing (no callback attached)
  3. Exit - Closes the application
Notice the & character in button labels creates keyboard shortcuts:
  • Alt+B for Beep
  • Alt+N for “no op”
  • Alt+X for Exit

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Key Concepts

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Button Creation

Fl_Button *b1 = new Fl_Button(20, 20, 80, 25, "&Beep");
Syntax: Fl_Button(x, y, width, height, label)

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Callback Functions

Callbacks have this signature: 
void callback_name(Fl_Widget *widget, void *userdata) {
  // Handle the event
}
Attach callbacks with: 
b1->callback(beepcb, 0);
The second parameter (0 here) is optional user data passed to the callback.

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Keyboard Shortcuts

The & character in a label creates an Alt+key shortcut:
  • "&Beep" → Alt+B
  • "E&xit" → Alt+X
The underlined character shows which key to press.

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System Functions

fl_beep();        // System beep
fflush(stdout);   // Flush output buffer
exit(0);          // Exit program

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Variations and Extensions

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Using Different Button Types

#include <FL/Fl_Return_Button.H>
#include <FL/Fl_Light_Button.H>
#include <FL/Fl_Toggle_Button.H>
#include <FL/Fl_Radio_Button.H>

// Return button (default action, highlighted)
Fl_Return_Button *ok = new Fl_Return_Button(10, 10, 80, 30, "OK");

// Toggle button (on/off state)
Fl_Toggle_Button *toggle = new Fl_Toggle_Button(10, 50, 80, 30, "Toggle");

// Light button (with indicator light)
Fl_Light_Button *light = new Fl_Light_Button(10, 90, 80, 30, "Light");

// Radio button (mutually exclusive in group)
Fl_Radio_Button *radio1 = new Fl_Radio_Button(10, 130, 80, 30, "Option 1");
Fl_Radio_Button *radio2 = new Fl_Radio_Button(10, 160, 80, 30, "Option 2");

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Passing User Data to Callbacks

void counter_cb(Fl_Widget *w, void *data) {
  int *count = (int*)data;
  (*count)++;
  printf("Button clicked %d times\n", *count);
}

int main() {
  int click_count = 0;
  Fl_Window *win = new Fl_Window(300, 100);
  Fl_Button *btn = new Fl_Button(100, 35, 100, 30, "Click Me");
  btn->callback(counter_cb, &click_count);
  win->end();
  win->show();
  return Fl::run();
}

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Getting Button State

void toggle_cb(Fl_Widget *w, void *) {
  Fl_Toggle_Button *btn = (Fl_Toggle_Button*)w;
  if (btn->value()) {
    printf("Button is ON\n");
  } else {
    printf("Button is OFF\n");
  }
}

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Button Colors and Appearance

button->color(FL_RED);                    // Background color
button->labelcolor(FL_WHITE);             // Text color
button->selection_color(FL_DARK_RED);     // Color when pressed
button->box(FL_ROUND_UP_BOX);             // Rounded style
button->down_box(FL_ROUND_DOWN_BOX);      // Style when pressed

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Using Dialogs

#include <FL/fl_ask.H>

void show_message_cb(Fl_Widget*, void*) {
  fl_message("This is a message dialog");
}

void show_alert_cb(Fl_Widget*, void*) {
  fl_alert("Warning: This is an alert!");
}

void ask_question_cb(Fl_Widget*, void*) {
  int choice = fl_choice("Save changes?", "No", "Yes", "Cancel");
  // Returns: 0=No, 1=Yes, 2=Cancel
}

void get_input_cb(Fl_Widget*, void*) {
  const char *name = fl_input("Enter your name:");
  if (name) {
    printf("Hello, %s!\n", name);
  }
}

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Next Steps

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