I have started a project to create a piece of software that generates algebra equations and answers for use in studying that subject. I had intended to use it for personal use but have decided that I wouldn't mind developing it to be multi-platform so I can improve my coding skills, not to mention scratch an itch of doing something more in-depth than basic scripting.

To do this, I have started on C libraries which contains all the functionality to create these equations which can then be compiled to respective platforms. platform specific GUI's would then be created which used these libraries. The two targets that I am interested in is the desktop and Android. I was hopeful that when I get the project up and running, the code is in a state to attract developers who just needs to create GUI's on the platform of their choosing.

I've managed so far to write a library that generates simple equations and a program which uses the library to display the output on the CLI. Before I work any more on the library side of project, I have been looking at the GUI side and frankly I've realised that with the time I have to devote on this project, I can't write separate platform GUI's and will need a toolkit that can compile on multiple systems with little or no additional code. The most promising one I can find is QT.

Before I take the project any further, I wonder if the following questions can be answered.

From what I've read, QT is cross-platform but can I really write GUI code and it will compile with little or no change on say Linux and Android?

I've read that QT has issues with platform native look and feel. The interface for my project is a basic one where a few options are selected, the equations are displayed and a button is used to display the answers to equations. Look and feel doesn't matter but would it be totally different on say Android to native applications?

Although this project is primarily for my own use, I intend to release it as an non-profit, open source educational project. Am I right in saying that I don't need to pay any fees for using QT?

I had been looking at a number of toolkits and all of them are in C++. I have written C++ code that is a CLI interface which calls the C libraries and displays the details on the command line. If I use QT, would it be more beneficial to convert these libraries to C++ and use QT specific data-types and functions?

These are the most obvious questions I can think of at the moment. If I have any more, I'll post them as replies.

Thanks in advance.

Use Qt. It has all the advantages you want and none of the issues you're asking about.

Honestly, I'd recommend just using one of the Qt Quick Controls 2 styles, which don't really attempt to blend in.

https://doc.qt.io/qt-5.11/qtquickcontrols2-styles.html

Yes.

And the answer would be yes even if you were selling your application for profit.

No.

This should be fairly current:

https://youtu.be/uZ68dX1-sVc

Many thanks Dugan for the reply. The information given is much appreciated.

The interface will only require a few drop down boxes, radio and ordinary buttons as well as a frame to display information. The Qt Quick Controls 2 that you mentioned is what I'm after.

I like the thought of separating the underlying code from the GUI code by having it in another language so that's good that it doesn't need to converted. It may not be a good idea in the future but I've converted C code to C++ so it shouldn't be that much of an issue to do it to the libraries if need be.