Free open source alternative to IntelliJ IDEA and CLion?

It depends on what you are trying to do. Are you trying to look at the code and/or modify it to fit your needs? Are you simply looking for a free IDE?

Here is what I used 10 years ago when I was still doing Java.

Open Source IDE --------------- Netbeans IDE -> Loved the UI Eclipse -> My team at Amazon used this and it was great

Free to Use (Closed-Source) --------------------------- IntelliJ IDEA Community Edition

Open Source Editor ------------------ Visual Studio Code - If you don't care about IDE related features and just want an Editor

I don't have any background in C/C++ so I don't have any recommendations.

Hope this helps!

There’s still Eclipse. I prefer IntelliJ by a lot but it actually does work for Java although my experience is that installing a plug in is like Russian Roulette and that anything that isn’t the stock configuration could die of pluginitis.