Qt is described as 'cross-platform application framework that is widely used for developing application software that can be run on various software and hardware platforms with little or no change in the underlying codebase, while having the power and speed of native applications' and is a very popular app in the development category. There are more than 25 alternatives to Qt for a variety of platforms, including Windows, Linux, Mac, Android and iPhone. The best alternative is GTK+, which is both free and Open Source. Other great apps like Qt are Electron, Flutter, Felgo and React Native.
GTK+ is a multi-platform toolkit for creating graphical user interfaces. Offering a complete set of widgets, GTK+ is suitable for projects ranging from small one-off tools to complete application suites.
GTK+ vs Qt opinions
Most users think GTK+ is a great alternative to Qt.
GTK+ is more portable (including embedded systems) and includes wrappers to more languages than Qt.
Electron vs Qt opinions
Some users think Electron is a great alternative to Qt, some don't.
It is very ineficient compared to qt and not as crossplatform
wxWidgets is a C++ library that lets developers create applications for Windows, OS X, Linux and UNIX on 32-bit and 64-bit architectures as well as several mobile platforms including Windows Mobile, iPhone SDK and embedded GTK+.
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Most users think wxWidgets is a great alternative to Qt.
The .NET Framework is a software framework developed by Microsoft that runs primarily on Microsoft Windows. It includes a large library and provides language interoperability (each language can use code written in other languages) across several programming languages.
Mono is a free and open source project led by Xamarin (formerly by Novell and originally by Ximian) to create an Ecma standard compliant .NET Framework-compatible set of tools including, among others, a C# compiler and a Common Language Runtime.
.NET Core is a cross-platform free and open-source managed software framework similar to .NET Framework. It consists of CoreCLR, a complete cross-platform runtime implementation of CLR, the virtual machine that manages the execution of .NET programs.
Ultimate++, also called U++, is a C++ cross-platform rapid application development framework focused on programmers productivity. It includes a set of libraries (GUI, SQL, etc..), and an integrated development environment.