Move semantic between objects in different threads
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wrote on 1 Jun 2020, 12:18 last edited by
Hello there!
I have the following situation: I have to get information from serial port parse it into QVector<float> and pass in the other object for the further processing. This processing object is on other thread, so I was going to use signal-slot mechanism. I could not find a definetely correct answer for the following question: what is a correct way to use move semantics to pass data between objects in differenth threads with usage of a signal-slot mechanism. I would be appreciated for any information and thank you in advance. -
wrote on 2 Jun 2020, 12:53 last edited by
Passing by pointer/reference is infinitesimally more efficient in the same thread but if you pass them across threads using signal/slot it's going to be the same as passing by value
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Hi,
There's no move semantics involved especially between threads. Qt copies the data you pass as parameter(s) of your signal.
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Hi,
There's no move semantics involved especially between threads. Qt copies the data you pass as parameter(s) of your signal.
wrote on 2 Jun 2020, 10:31 last edited by@SGaist Thank you for the answer, that's why I found nothing special about this :) So the closest way that I could use (and have already tried successfully) to "emulate" move semantic with QSharedPointer, right?
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@SGaist Thank you for the answer, that's why I found nothing special about this :) So the closest way that I could use (and have already tried successfully) to "emulate" move semantic with QSharedPointer, right?
wrote on 2 Jun 2020, 10:55 last edited by VRonin 6 Feb 2020, 10:56@St-Stanislav said in Move semantic between objects in different threads:
"emulate" move semantic with QSharedPointer, right?
No,
QSharedPointeris just a smart pointer, not thread safeQVector<float> and pass in the other object
So here is the thing, you don't need to implement move semantic for Qt types. Basically all the Qt types did move semantic before it was cool (i.e. before it was included in the standard) so the copy constructor is already as inexpensive as it gets (a pointer assignment and an integer increment). This is normally referred to as implicit sharing. The moral of the story here is that you shouldn't worry about move semantic at all for Qt types (either in the same thread or a different thread). Just copy them around and let Qt do the hard work.
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@St-Stanislav said in Move semantic between objects in different threads:
"emulate" move semantic with QSharedPointer, right?
No,
QSharedPointeris just a smart pointer, not thread safeQVector<float> and pass in the other object
So here is the thing, you don't need to implement move semantic for Qt types. Basically all the Qt types did move semantic before it was cool (i.e. before it was included in the standard) so the copy constructor is already as inexpensive as it gets (a pointer assignment and an integer increment). This is normally referred to as implicit sharing. The moral of the story here is that you shouldn't worry about move semantic at all for Qt types (either in the same thread or a different thread). Just copy them around and let Qt do the hard work.
wrote on 2 Jun 2020, 11:58 last edited by@VRonin Thank you for the detailed answer, I have last point that want to decide and understand: should I use passing by reference or coping process will be with the same efficiency? I have found old topic with the same discussion and answers from you, but what it correct way now, in 5.15.0? Is it the same?
Thank you in advance for helping me!
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wrote on 2 Jun 2020, 12:53 last edited by
Passing by pointer/reference is infinitesimally more efficient in the same thread but if you pass them across threads using signal/slot it's going to be the same as passing by value
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Passing by pointer/reference is infinitesimally more efficient in the same thread but if you pass them across threads using signal/slot it's going to be the same as passing by value
wrote on 2 Jun 2020, 13:00 last edited by@VRonin Yes, I have mentioned passing between threads, thank you!
4/7