What does it mean when an entire function is a slot?

[Circuits]

I stumbled across a function which is defined as a slot in the header file.. how can an entire function be a slot? Also, it's a private slot, when would you want a slot to be private?

[JonB]

@Circuits
What do you mean "how can an entire function be a slot?", a slot is a function (or a lambda)? As for private, is the example Qt 4? 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 :)

[JKSH]

@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 public

See

• 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]

[Circuits]

This post is deleted!

[JonB]

@Circuits
Hmm, so half the time you have been talking about slots you might mean signals!

If you say you found it in a header file, maybe you mean you're looking at the declaration but not the definition, which would be empty. I don't know now.

[fcarney]

Public slots are also visible to QML code without having to add Q_INVOKABLE.

[Circuits]

@JonB No I am being a fool it was a slot. I was confusing updateStatus() with statusUpdate() which is another signal. Sorry bare with me I am new to signals/slots and Qt in general. In general, I would like to be able to send this slot a signal from outside of this class, will it need to be a public slot for that to happen? If I can call it directly from the QML then great but if I have to call it from within the c++ code using this Q_INVOKABLE fcarney referred to than that's fine too, so long as I can call something like:

onUpdateStatus:

from the QML, eventually, one way or another.

[JonB]

@Circuits
OK :) I'm afraid I don't know QML. It won't matter what the slot access is, that's an issue wherever signal & slot are connected. Your job is just to emit the signal.

[Circuits]

@JonB I am just a bit confused. It would seem that I have a signal, which is connect to a signal which is connected to a normal method?? The method these signals seem to be connect to is not a slot. For instance:

The header:

signals:
void gnssStatusChanged(QStringList);

It's corresponding c++ file:

QObject::connect(m_a, &Application::statusChanged,         this, &GnssPresenter::gnssStatusChanged);        Q_ASSERT(rc); }

statusChanged leads to another header:

signals:
void statusChanged       (QStringList const&);

also inside that header this statusChanged seems to be hooked up to a variable called status:

class ApplicationInterface : public QObject
{
    Q_OBJECT
    Q_PROPERTY(QStringList const& status      READ status      NOTIFY statusChanged)

and when I right-click status and click find references it shows two of them in another c++ file and corresponding header:
from the header:

 QStringList const& status() const override;

from the c++ file:

QStringList const& MockGnssApplication::status() const { return m_status;               }

the header and c++ file above is where the updateStatus() method I was referring to earlier is located. So like I said, it would seem I have a signal hooked to a signal which is hooked to a normal method which is located in the file where the updateStatus() method (slot) is located.

What is confusing me:

1. Must a signal hook up to a slot or can it hook up to a normal method?
2. What is happening in this line:

Q_PROPERTY(QStringList const& status      READ status      NOTIFY statusChanged)

this seems to be a signal/slot syntax without actually having a slot.

[SGaist]

Hi,

It is allowed and has even a name: signal chaining.

This allows to propagate a signal upper while not making the internals public.

[Circuits]

@SGaist Thanks~ Still confused about what emit does. I found lots of signals declared in headers with no corresponding emit call in the c++ file. Perhaps this emit isn't necessary?

In any case, I think what I need to do is generate a new signal and connect it too the updateStatus() slot. Can I just do that directly from the QML, in this case?

[SGaist]

Technically speaking, emit is replaced by nothing (take a look at the macro). However, it does make the code more understandable with regard to what should happen at that point. It make also clear that you are calling a signal.

[JKSH]

@SGaist said in What does it mean when an entire function is a slot?:

Technically speaking, emit is replaced by nothing (take a look at the macro). However, it does make the code more understandable with regard to what should happen at that point. It make also clear that you are calling a signal.

+1

In other words...

void MyClass::func()
{
    emit mySignal(); // [1]

    mySignal(); // [2]
}

... [1] and [2] are exactly the same from a compiler's point of view. However, [2] is clearer to a human reader.