Hello World Example

Your first FLTK application - a simple hello world program

What This Example Demonstrates

This example demonstrates the absolute basics of creating an FLTK application:

• Creating a window (Fl_Window)
• Adding a widget to display text (Fl_Box)
• Customizing appearance with fonts, sizes, and box styles
• Running the FLTK event loop

This is the traditional “Hello, World!” program adapted for GUI applications. It creates a window with a styled text label.

Complete Source Code

Source file: test/hello.cxx

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

int main(int argc, char **argv) {
  Fl_Window *window = new Fl_Window(340, 180);
  Fl_Box *box = new Fl_Box(20, 40, 300, 100, "Hello, World!");
  box->box(FL_UP_BOX);
  box->labelfont(FL_BOLD + FL_ITALIC);
  box->labelsize(36);
  box->labeltype(FL_SHADOW_LABEL);
  window->end();
  window->show(argc, argv);
  return Fl::run();
}

Compilation Command

# Using fltk-config (recommended)
fltk-config --compile hello.cxx

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

# Run the program
./hello

Expected Output

When you run this program, you’ll see:

• A 340x180 pixel window
• Text reading “Hello, World!” in bold italic font
• Font size of 36 points
• Shadow effect on the text
• A raised box style (FL_UP_BOX) around the text

Key Concepts

Window Creation

Fl_Window *window = new Fl_Window(340, 180);

Creates a window with width=340, height=180. The window is the top-level container.

Widget Positioning

Fl_Box *box = new Fl_Box(20, 40, 300, 100, "Hello, World!");

FLTK uses absolute positioning: Fl_Box(x, y, width, height, label)

• x=20, y=40: position from top-left corner
• width=300, height=100: size of the box

Box Styles

box->box(FL_UP_BOX) sets the visual style. Other options include:

• FL_DOWN_BOX - recessed appearance
• FL_FLAT_BOX - no border
• FL_BORDER_BOX - simple border
• FL_NO_BOX - invisible

Font Properties

box->labelfont(FL_BOLD + FL_ITALIC);
box->labelsize(36);
box->labeltype(FL_SHADOW_LABEL);

Combine font styles with + and set size in points.

Event Loop

window->show(argc, argv);
return Fl::run();

• show() displays the window and parses command-line arguments
• Fl::run() starts the event loop and handles user interaction

Variations and Extensions

Centered Text

Fl_Box *box = new Fl_Box(0, 0, 340, 180, "Hello, World!");
box->align(FL_ALIGN_CENTER | FL_ALIGN_INSIDE);

Different Colors

box->color(FL_WHITE);
box->labelcolor(FL_BLUE);

Window Title

window->label("My First FLTK App");

Minimum Window Size

window->size_range(200, 100);  // min width, min height

Next Steps

• Learn about buttons and callbacks
• Explore input fields
• Understand layout management