[Interest] Signal/Slot connection fails, even though QMetaObject::activate is called

Scott Bloom scott at towel42.com
Sat Feb 8 00:03:26 CET 2020

Are you sure the function is getting called (via a break point)?

-----Original Message-----
From: Interest [mailto:interest-bounces at qt-project.org] On Behalf Of Jonathan Purol
Sent: Friday, February 7, 2020 2:44 PM
To: Qt Project <interest at qt-project.org>
Subject: [Interest] Signal/Slot connection fails, even though QMetaObject::activate is called

Hello everyone,

I have a QMainWindow subclass called `text_editor` with a function `save` which looks as follows:
void text_editor::save() {


     emit saved();


I now connect to that slot in another class like this:
_information_editor = new text_editor{this};

     [=]() {
         std::cout << "hello\n";


I have verified that the signal is emitted, in fact, the method generated by the moc:
void text_editor::saved()
     QMetaObject::activate(this, &staticMetaObject, 0, nullptr); } ``` is definitely called (verified with gdb and some other debugging shenenigans).
In addition, the connection object returned by `connect` is valid, as verified by its implicit bool-cast operator.
However, "hello" is never printed. I suspected this could be because `text_editor` inherits from `QMainWindow` and could have a different thread affinity, so I tried the following things:

1. Move the editor to the same thread as the object which establishes the connection 2. Use a QueuedConnection 3. Connect to the slot from WITHIN the text editor itself, making sure that we are 100% on the same slot

and none of them worked.
Just for clarification, the print of "hello" is only an example, so even if there was some issue with that, I would have detected it, the actual code is of course different.

I'm really out of luck here, and would appreciate any help.


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