The code is not a substitute for good documentation, which is definitely the case for Qt. But unfortunately, it is not always the case. From hobby projects to

The code is not a substitute for good documentation, which is definitely the case for Qt. But unfortunately, it is not always the case. From hobby projects to internal company software, you'd be lucky to have documentation at all, and when you have, it is often outdated or just wrong. Popular, company-backed projects are usually good, as it is the case for Qt. Because companies have a budget for it, you need a budget, because good documentation is expensive, and generally not something people do "for fun". Prestige projects (GNU, BSD, etc...) may get good documentation "for free" as people who are not developers can get credits, but it is not a given. For example, Linux kernel internal APIs are poorly documented, that they are (deliberately) unstable probably doesn't help. The code is great though. At work, the first think I ask for when starting to work on a project is the source code, because documentation is almost always terrible (if it exists). And in the rare case where it isn't, that's when there is at least one person who is paid full time to do just that.