Multiple inheritance works fine as long as only one class is derived from QObject, usually it’s a lightweight class with low memory requirements tied with a QObject class for signal and slot functionality.
From: Interest <[email protected]> On Behalf Of NIkolai Marchenko Sent: Monday, January 12, 2026 17:52 To: Qt Interest <[email protected]> Subject: [Interest] How valid is using pure virtual functions in place of signals in base classes? The problem: qt doesn't really allow us to use multiple inheritance, but sometimes it's very desirable to split some logic into a base class, so I had an idea: why not declare "signals" before you mark them as signal? That way base class could directly call pure virtuals and the QObject based subclass will have the "signals:" section that will force moc to implement these pure virtuals and call the signal magic to actually work. That way we both have a base class that emits and can inherit however many bases we'd like. I've verified that the pattern works for a simple test case but then I went digging on the internet and while it is mentioned a few times there'sd a lot of confusion on whether it can work at all(clearly people who say it's just completely not working are mistaken as evidenced from my testing) and whether it is problematic. Personally, I've delved a bit into the source code of how signal dispatching works in such cases, but didn't see anything that could obviously break - we're just figuring out the signal index in the connect() statement and call that later. But I am also not qualified enough template metaprogrammer to say for sure. An example of someone recommending this pattern (and a code example) but it is both too simple and has no discussion on it for me to be sure it is allowed to be used: https://www.man42.net/blog/2012/05/qt-4-qobject-pure-virtual-signal/ So my question is - is it safe, and is it safe both for qt5 and qt6? Will something in qt potentially break in non obvious way if this pattern is used?
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