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20 ways to debug Qt signals and slots

Sam Dutton’s blog

Below are some suggestions for troubleshooting signals and slots in the Qt C++ library.

1. Check for compiler warnings about non-existent signals and/or slots.

2. Use break points or qDebug to check that signal and slot code is definitely reached:
– the connect statement
– code where the signal is fired
– the slot code.

3. Check that the parameter types of the signal and slot are exactly correct and, as appropriate, that they match.

4. Make sure you haven’t added a name to the signal or slot argument: for example, use textChanged(const QString &) not textChanged(const QString &text).

5. Check that the connect argument types and syntax are correct. The connect statement should look like this:

connect(senderObject, SIGNAL(mySignal(const QString&)), receiverObject, SLOT(mySlot(const QString&)));

Check brackets, check that SIGNAL and SLOT are capitalised and that the sender and receiver are both objects, not class names.

6. Check that the signal is being fired…

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