Fltk 1 4 5

FLTK 1.4.5

Overview

Fast Light Toolkit (FLTK) is a cross-platform C++ GUI toolkit for UNIX/Linux (X11 and Wayland), Windows, and macOS. It provides modern GUI functionality with minimal footprint — suitable for static linking. FLTK includes ~80 widget classes, OpenGL/GLUT emulation, drawing primitives, image support (PNG, JPEG, SVG, GIF, BMP, ICO, XPM, PNM), printing to PDF/PostScript/EPS/SVG, and the FLUID visual UI builder.

Key features:

• Small footprint: ~1MB library, designed for static linking
• HiDPI support: Per-screen scale factors (1.0–2.5+), FLTK_SCALING_FACTOR env var
• Wayland + X11 hybrid: Automatic backend selection on Linux (FLTK_BACKEND env var)
• Event-driven model: Fl::run() main loop with callbacks, idle functions, timers
• OpenGL integration: Fl_Gl_Window for embedded OpenGL, GLUT compatibility via <FL/glut.H>

When to Use

• Building desktop GUI applications in C++ across Linux, Windows, macOS
• Creating custom widgets by subclassing Fl_Widget or Fl_Group
• Embedding OpenGL rendering with Fl_Gl_Window
• Migrating from GLUT/FreeGLUT using <FL/glut.H> compatibility header
• Needing a lightweight toolkit suitable for static linking
• Building applications requiring printing (PDF, PostScript, EPS, SVG)
• Using FLUID for visual UI design

Quick Start

Minimal "Hello World" program:

#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();
}

Compile with CMake (recommended):

cmake_minimum_required(VERSION 3.12)
project(myapp)
find_package(FLTK REQUIRED)
add_executable(myapp main.cxx)
target_link_libraries(myapp FLTK::Fltk)

Or directly: g++ main.cxx -lfltk -o myapp

Core Concepts

Widget hierarchy: All widgets derive from Fl_Widget. Container widgets derive from Fl_Group. Windows derive from Fl_Window (which derives from Fl_Group). Widgets created between group->begin() and group->end() are automatically added to the group.

Event loop: Fl::run() processes events until all windows close. For custom loops, call Fl::wait() repeatedly. Events flow through widget handle() methods; return non-zero to claim an event.

Coordinates: Top-left origin (0,0). All positions relative to the enclosing window or group. FLTK 1.4+ uses scale-independent units (multiplied by screen scale factor for actual pixels).

Resizing: Set group->resizable(widget) on a child widget. That widget absorbs all size change; other widgets resize proportionally based on overlap with imaginary cross-lines from the resizable widget's edges.

Advanced Topics

Installation & Build: CMake and autotools build, dependencies, platform-specific notes → Installation & Build

Core Concepts: Widget hierarchy, event loop, coordinates, resizing, show/hide lifecycle → Core Concepts

Widget Reference: Buttons, inputs/outputs, valuators, browsers, groups, text widgets, tables, trees, charts → Widget Reference

Events & Callbacks: Event types, Fl::event_* queries, callback signatures, shortcut handling, propagation → Events & Callbacks

Drawing & Images: Drawing primitives, colors, fonts, box types, image hierarchy (PNG/JPEG/SVG/GIF/BMP/ICO/XPM/PNM), offscreen drawing → Drawing & Images

OpenGL & GLUT: Fl_Gl_Window, GLUT emulation via <FL/glut.H>, HiDPI OpenGL, context management → OpenGL & GLUT

Custom Widgets: Subclassing Fl_Widget/Fl_Group, draw() and handle() overrides, damage bits, FLUID integration → Custom Widgets

Advanced Topics: Multithreading (Fl::lock/unlock/awake), runtime options, Unicode/UTF-8, X11/Wayland backend, printing, Fl_Preferences → Advanced Topics

FLUID & Migration: FLUID UI builder, .fl file format, integrating generated code, FLTK 1.3 → 1.4 migration guide → FLUID & Migration