On 4/22/20 1:44 AM, Ben Haller via Interest wrote:
Hi folks. I’m seeing something weird that I don’t understand. I have a connection defined for the main window of my Qt Widgets app, like so:connect(qApp, &QApplication::focusChanged, [this]() { updateUIEnabling(); }); I’m seeing a 100% reproducible crash as a result of this. When the focusChanged signal fires, I end up in updateUIEnabling() as expected, but I immediately crash because something about the state of things is corrupted; the immediate symptom, according to the debugger, is that the ui pointer for my main window is equal to 0x1 (yuck). I try to access a button, ui->playOneStepButton, and it crashes. As I said, 100% reproducible. The weird thing is that this does*not* happen if I change the connect statement to this: connect(qApp, &QApplication::focusChanged, this, &QtSLiMWindow::updateUIEnabling); Now, 100% of the time, there is no problem, no crash, and my app chugs along happily. No other code changes involved. I can literally flip this connect definition back and forth in my code and go from 100% crashing to 100% fine.
A quick checklist:0) I hope threads are not involved (you're not really allowed to do any GUI work from any other thread).
1) Never use the 3-arguments connect(). If you want to use the lambda, pass "this" as 3rd parameter and lambda as fourth. Why: if "this" has been destroyed, the first version won't disconnect automatically and crash. The second will disconnect and not crash.
More info: https://stackoverflow.com/a/279545022) ASAN, Valgrind and friends will help you locate the source of the crash if it's due to memory corruption. Use them.
HTH, -- Giuseppe D'Angelo | giuseppe.dang...@kdab.com | Senior Software Engineer KDAB (France) S.A.S., a KDAB Group company Tel. France +33 (0)4 90 84 08 53, http://www.kdab.com KDAB - The Qt, C++ and OpenGL Experts
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