Apps similar to FLTK include Qt, which is open source with a free version. Other options are Qt Creator, GTK, wxWidgets and PyQt. There's no shortage of Software Frameworks like FLTK, with more than 25 alternatives for Linux, Windows, Mac, Android and self-hosting.
Dear ImGui is a bloat-free graphical user interface library for C++. It outputs optimized vertex buffers that you can render anytime in your 3D-pipeline enabled application. It is fast, portable, renderer agnostic and self-contained (no external dependencies).
Crazy Eddie's GUI (CEGUI) system is a graphical user interface C++ library. It was designed particularly for the needs of video games, but the library is usable for non-game tasks, such as applications and tools.
Agar is a modern open-source, cross-platform toolkit for graphical applications implemented in C, C++ and Ada (with bindings to other languages in development). Designed for ease of integration, it follows the philosophy of building the GUI around the application and not the...
gtkmm is the official C++ interface for the popular GUI library GTK+. Highlights include typesafe callbacks, and a comprehensive set of widgets that are easily extensible via inheritance. You can create user interfaces either in code or with the Glade User Interface designer...
Nana C++ Library takes aim at easy-to-use and portable library, it provides a GUI framework and threads for easy programming with modern C++ methods, such as traits, metaprogramming and other template technologies. .
Nevron Open Vision for .NET is a suite of advanced UI components that helps you develop feature-rich Web (Blazor WebAssenbly) and Desktop (WinForms, WPF, Xamarin.Mac) applications from a single code base.
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IUP is a multi-platform toolkit for building graphical user interfaces. It offers a simple API in three basic languages: C, Lua and LED. IUP's purpose is to allow a program source code to be compiled in different systems without any modification.
PyGTK lets you to easily create programs with a graphical user interface using the Python programming language. The underlying GTK+ library provides all kind of visual elements and utilities for it and, if needed, you can develop full featured applications for the GNOME Desktop.
The FOX toolkit is an open source, cross-platform widget toolkit, that is, a library of basic elements for building a graphical user interface (GUI).
Gio is a library for writing cross-platform immediate mode GUI-s in Go. Gio supports all the major platforms: Linux, macOS, Windows, Android, iOS, FreeBSD, OpenBSD and WebAssembly.
Repose is a Rust UI framework with a Compose-like programming model. It's organized as a small core (repose-core) plus higher-level UI/layout (repose-ui) and platform runners (repose-platform) that currently target desktop (winit) and Android, with web support.
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