Sending static signals, is it possible?

  • Is it possible to send a static signal?

    I have a list of entry widgets inside a list and I want the container to be notified if one of them gets selected.
    I don't want to connect all the entries idividually so I tought sending a static signal might work

    public signals:
            static void SelectedStateChanged();

    I connect it to the QCheckBox

    connect(selectedCheckBox, &QCheckBox::stateChanged,
            emit SelectedStateChanged();

    Then in the list widget I connect to the static signal.
    I have tried multiple ways:

    • Qt5 syntax:
    // Connect the static signal from a dummy entry
        DeckScrollListEntry *dummyEntry = new DeckScrollListEntry(this);
        connect(dummyEntry, &DeckScrollListEntry::SelectedStateChanged,
                this, &DeckScrollList::refreshSelections);
        delete dummyEntry;

    Compiler error:

    error: no matching function for call to 'DeckScrollList::connect(DeckScrollListEntry*&, void (*)(), DeckScrollList*, void (DeckScrollList::*)())'
                 this, &DeckScrollList::refreshSelections);
    • Macro syntax:
    DeckScrollListEntry *dummyEntry = new DeckScrollListEntry(this);
        connect(dummyEntry, SIGNAL(SelectedStateChanged()),
                this, SLOT(refreshSelections));
        delete dummyEntry;

    Compiler error:

    error: 'this' is unavailable for static member functions
         QMetaObject::activate(this, &staticMetaObject, 1, nullptr);

    Is there any way or should I redesign and make connections for every entry.

  • Moderators

    Connections always relay on concrete object instances. It makes no sense for signal to be static, because signal has to come from a "living" object.

    You are creating the DeckScrollListEntry objects somewhere, so connect the signals there, too. Alternatively, use automated connections: connectSlotsByName(). I don't recommend it, though, because it is very easy to make a mistake here. The new connection syntax is definitely the best.

  • I'm not sure if you understood what I was trying to do. It wasn't really clear from what I posted.
    Tough I have since realized that it's a horrible design that would lead to all kinds of bugs down the road.
    What I was trying to do is something like this

    connect(static signal &DeckScrollListEntry::SelectedStateChanged,
                        this, &DeckScrollList::refreshSelections)

    So it would connect to a static signal that is sent by any DeckScrollListEntry.
    So in a way it would connect to every DeckScrollListEntry.
    And I also realized this is not possible of course, and it would be horrible if it were possible.

  • Moderators

    @plaktos said in Sending static signals, is it possible?:

    So it would connect to a static signal that is sent by any DeckScrollListEntry.

    I understood it exactly like that. It's not possible to do.

  • Disclaimer

    I'm not saying this is something you'd want to do

    You can use a static/singleton QObject as the source of your signal

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