FLTK vs Qt vs wxWidgets: When to Choose FLTK

Getting Started with FLTK — Installation and First WindowFLTK (Fast Light Toolkit) is a lightweight, cross-platform C++ GUI library designed for speed, small binary size, and simplicity. This guide walks through installing FLTK on Windows, macOS, and Linux, building a minimal “Hello, World” window, and explaining core concepts: widgets, layouts, event handling, and simple drawing.

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Why choose FLTK?

• Small and fast: FLTK produces lightweight binaries with fast startup.
• Portable: Runs on Windows, macOS, Linux, and many UNIX variants.
• C++-friendly: Simple C++ API without heavy meta-systems.
• Permissive license: LGPL with modifications (allows static linking).

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Prerequisites

• A C++ compiler: GCC/Clang on Linux/macOS, MSVC or MinGW on Windows.
• CMake (recommended) or a build system (make, Visual Studio).
• Basic familiarity with compiling C++ programs.

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Installing FLTK

Linux (Debian/Ubuntu)

1. Install packages (Debian/Ubuntu):

   
   sudo apt update sudo apt install libfltk1.3-dev build-essential cmake 

2. Alternatively, build from source (useful for latest version):

   
   git clone https://github.com/fltk/fltk.git mkdir fltk/build && cd fltk/build cmake ..  make -j$(nproc) sudo make install 

macOS (Homebrew)

1. Install with Homebrew:

   
   brew install fltk 

2. Or build from source (using CMake):

   
   git clone https://github.com/fltk/fltk.git mkdir fltk/build && cd fltk/build cmake -DCMAKE_BUILD_TYPE=Release .. make -j$(sysctl -n hw.ncpu) sudo make install 

Windows (MSYS2 / MinGW)

1. Using MSYS2 / pacman:
   • Open MSYS2 MinGW shell (mingw64) and run:

     
     pacman -Syu pacman -S mingw-w64-x86_64-toolchain mingw-w64-x86_64-cmake mingw-w64-x86_64-fltk 

2. Or build with Visual Studio:
   • Install CMake and Visual Studio (with C++ tools).
   • From a developer command prompt:

     
     git clone https://github.com/fltk/fltk.git mkdir fltkuild cd fltkuild cmake -G "Visual Studio 17 2022" -A x64 .. cmake --build . --config Release 

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Your first FLTK program

Create a file named main.cpp with the following code:

#include <FL/Fl.H> #include <FL/Fl_Window.H> #include <FL/Fl_Box.H> int main() {     Fl_Window *window = new Fl_Window(340,180, "Hello FLTK");     Fl_Box *box = new Fl_Box(20,40,300,100, "Hello, World!");     box->box(FL_UP_BOX);     box->labelsize(24);     box->labelfont(FL_BOLD+FL_ITALIC);     window->end();     window->show();     return Fl::run(); } 

Explanation:

• Fl_Window creates a top-level window (width, height, title).
• Fl_Box is a simple widget for displaying text or graphics.
• window->end() signals adding widgets is complete.
• window->show() displays the window.
• Fl::run() starts the FLTK event loop.

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Building the example

Using g++ (Linux/macOS with pkg-config)

g++ main.cpp -o hello `fltk-config --cxxflags --ldflags` 

Or explicitly:

g++ main.cpp -o hello -I/usr/local/include -L/usr/local/lib -lfltk 

With CMake

Create CMakeLists.txt:

cmake_minimum_required(VERSION 3.5) project(hello_fltk) find_package(FLTK REQUIRED) include_directories(${FLTK_INCLUDE_DIR}) add_executable(hello main.cpp) target_link_libraries(hello ${FLTK_LIBRARIES}) 

Then:

mkdir build && cd build cmake .. cmake --build . 

Windows (MSVC)

If you built FLTK with Visual Studio, create a project, add main.cpp, and link fltk.lib (and dependencies). If using MSYS2/MinGW, use g++ similarly to Linux but with the appropriate fltk-config or library paths.

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Widgets, Layout, and Callbacks

• Widgets are subclasses of Fl_Widget. Common widgets: Fl_Button, Fl_Input, Fl_Menu_Bar, Fl_Group.
• Layout is manual: you set widget position and size. There are helper containers (Fl_Pack, Fl_Scroll, Fl_Group).
• Callbacks handle events. Example button callback:

#include <FL/Fl_Button.H> void cb_button(Fl_Widget* w, void* data) {     Fl_Button* b = (Fl_Button*)w;     b->label("Clicked!"); } Fl_Button* btn = new Fl_Button(10,10,100,30,"Click"); btn->callback(cb_button); 

Callbacks receive the widget pointer and a user-data pointer.

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Simple drawing with Fl_Box or Fl_Widget

Override draw() in a custom widget:

#include <FL/Fl_Widget.H> class MyWidget : public Fl_Widget { public:     MyWidget(int X,int Y,int W,int H) : Fl_Widget(X,Y,W,H) {}     void draw() override {         fl_color(FL_BLUE);         fl_rectf(x(), y(), w(), h());         fl_color(FL_WHITE);         fl_draw("Custom draw", x()+10, y()+20);     } }; 

Add to a window like any widget: new MyWidget(10,10,200,100);

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Event handling basics

• handle(int event) lets widgets intercept low-level events (FL_PUSH, FL_RELEASE, FL_KEYDOWN, etc.).
• Use Fl::add_timeout for timers, Fl::repeat_timeout to reschedule.

Example timer:

void tick(void*){   // do something periodically   Fl::repeat_timeout(1.0, tick); } Fl::add_timeout(1.0, tick); 

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Tips and best practices

• Keep the UI simple; FLTK shines for compact interfaces and tools.
• Use Fl_Group/Fl_Pack to manage collections of widgets; experiment with resizable() to enable dynamic layout.
• For complex apps, separate UI code from logic and use callbacks to call controller functions.
• Use fltk-config (on Unix) to get correct compiler/linker flags.

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Debugging and common issues

• Linker errors: ensure you link against libfltk and use correct include paths. Use fltk-config --cxxflags --ldflags.
• On macOS, you may need to link Cocoa frameworks if building manually. Using Homebrew-installed FLTK and cmake usually handles this.
• If fonts or icons look odd, confirm FLTK was built with fontconfig and Xft support (on Linux).

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Further resources

• FLTK official docs and API reference in the source repo.
• Examples folder in the FLTK source tree — great to study.
• Community forums and mailing lists for platform-specific quirks.

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FLTK is ideal when you want a small, fast GUI for tools and utilities. The example above gets you a visible window in a few lines of code and provides the foundation to explore widgets, custom drawing, and event-driven programming.