Qt: When QTimer actually started?

Question

When application calls QTimer::start() is it started immediately or will be started after current event processed ? In other words, should I use single-shot timer with time correction in case of long-time processing in its timeout() slot ?

Answer 1

To answer with certainty would require inspecting platform-specific code within Qt. That's a good sign that this is not something you should be depending on. Moreover, QTimer doesn't promise much in terms of accuracy:

Timers will never time out earlier than the specified timeout value and they are not guaranteed to time out at the exact value specified. In many situations, they may time out late by a period of time that depends on the accuracy of the system timers.

The accuracy of timers depends on the underlying operating system and hardware. Most platforms support a resolution of 1 millisecond, though the accuracy of the timer will not equal this resolution in many real-world situations.

If Qt is unable to deliver the requested number of timer clicks, it will silently discard some.

If you need to know precisely how much time has passed between timeout signals, use your QTimer in conjunction with a QElapsedTimer.