Qt signal and slot internal connection details
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@Circuits said in What does it mean when an entire function is a slot?:
how can an entire function be a slot?
Like @JonB said, a slot is a function that is intended to run when a signal is emitted.
Could you describe what you think a slot should be?
Also, it's a private slot, when would you want a slot to be private?
When you want to implement event-driven logic inside your class, but you don't want the slot function to be called by anyone outside the class.
@JonB said in What does it mean when an entire function is a slot?:
I believe one of the changes in Qt 5 for the new signal/slots syntax was that slots now have to be public, but I could be wrong :)
Signals are now be public; slots are unchanged.
Qt 4:
#define signals protected
Qt 5:#define signals publicSee
- https://code.woboq.org/qt5/qtbase/src/corelib/kernel/qobjectdefs.h.html#_M/Q_SIGNALS
- https://www.kdab.com/wp-content/uploads/stories/slides/DD13/dd13_signalsslots.pdf
[EDIT: Discussion about public/protected/private forked to https://forum.qt.io/topic/107926/qt-signal-and-slot-internal-connection-details --JKSH]
[EDIT: Forked from https://forum.qt.io/topic/107901/what-does-it-mean-when-an-entire-function-is-a-slot --JKSH]
@JKSH said in What does it mean when an entire function is a slot?:
Signals are now be public; slots are unchanged.
Yep, thanks, I had a funny feeling I was getting that the wrong way round :)
I believe from reading the discussion about why the change (to do with the new signal/slot syntax, I believe), they never wanted to make
#define signals public, but had to for compilation, so it's presumably good to regard/pretend as though it were still#define signals protected, i.e. never invoke signal from outside world. -
[EDIT: Forked from https://forum.qt.io/topic/107901/what-does-it-mean-when-an-entire-function-is-a-slot --JKSH]
@JKSH said in What does it mean when an entire function is a slot?:
Signals are now be public; slots are unchanged.
Yep, thanks, I had a funny feeling I was getting that the wrong way round :)
I believe from reading the discussion about why the change (to do with the new signal/slot syntax, I believe), they never wanted to make
#define signals public, but had to for compilation, so it's presumably good to regard/pretend as though it were still#define signals protected, i.e. never invoke signal from outside world.@JonB said in What does it mean when an entire function is a slot?:
so it's presumably good to regard/pretend
All you can do is pretend as it doesn't really matter if a signal is public, protected or private. The meta-system circumvents these language boundaries and you can always call even a private signal of another class via invokeMethod().
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@JonB said in What does it mean when an entire function is a slot?:
so it's presumably good to regard/pretend
All you can do is pretend as it doesn't really matter if a signal is public, protected or private. The meta-system circumvents these language boundaries and you can always call even a private signal of another class via invokeMethod().
@Chris-Kawa
Yes, but the change was for the new connection syntax, not the "magic" of the oldSIGNAL/SLOT()mechanism.I'm not a C++-er (I'm stinky Python/PyQt, where just about everything is public and everything can access everything else, lovely!), but I do like to understand how it's supposed to be from C++. So please correct me if I'm wrong, but:
- In the new syntax, both
signals&slotsmacros resolve topublic. - This is required to allow the new, direct
connect(signal, slot)from the outside world, code not in either the signal or slot classes. - To raise a signal you use
emit(), and theemitpart resolves to nothing, so it's just a straight call to the signal function. - In the old days you could keep
signalsasprotected, so that only the signalling/emitting class couldemit()the signal. - Now that
signalshas to bepublic, anybody could call theemit(), but Qt didn't really want that. So you are "politely requested" not to actually write anemit()unless you are inside the signal class, as that's how it is intended to be used.
Please correct me if I have misunderstood.
- In the new syntax, both
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@Chris-Kawa
Yes, but the change was for the new connection syntax, not the "magic" of the oldSIGNAL/SLOT()mechanism.I'm not a C++-er (I'm stinky Python/PyQt, where just about everything is public and everything can access everything else, lovely!), but I do like to understand how it's supposed to be from C++. So please correct me if I'm wrong, but:
- In the new syntax, both
signals&slotsmacros resolve topublic. - This is required to allow the new, direct
connect(signal, slot)from the outside world, code not in either the signal or slot classes. - To raise a signal you use
emit(), and theemitpart resolves to nothing, so it's just a straight call to the signal function. - In the old days you could keep
signalsasprotected, so that only the signalling/emitting class couldemit()the signal. - Now that
signalshas to bepublic, anybody could call theemit(), but Qt didn't really want that. So you are "politely requested" not to actually write anemit()unless you are inside the signal class, as that's how it is intended to be used.
Please correct me if I have misunderstood.
I'm stinky Python/PyQt, where just about everything is public and everything can access everything else, lovely!
Savages... :P
Please correct me if I have misunderstood.
Sounds right except for the part where previously only the owning class could call protected signal - as I said earlier, you could still call it (indirectly) via
invokeMethod(). Anyway it's history and everything is public now, like in Python... <shivers>... - In the new syntax, both
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I'm stinky Python/PyQt, where just about everything is public and everything can access everything else, lovely!
Savages... :P
Please correct me if I have misunderstood.
Sounds right except for the part where previously only the owning class could call protected signal - as I said earlier, you could still call it (indirectly) via
invokeMethod(). Anyway it's history and everything is public now, like in Python... <shivers>...@Chris-Kawa
Yes, butinvokeMethod()is onQMetaObject, and that's what I refer to asthe "magic" of the old ...(look up a string in a table of functions, rather than using language constructs). You have to pass a test before being allowed to use that, and your programming license will be revoked if you don't know what you are doing and get it wrong... :)
