connect doesn't call a slot but invokes lambda

SeeLook

Hi,

Could someone enlighten me what I'm doing wrong that basic connect syntax:

connect(myObj, &MyObjClass::signal1, this, &ThisClass::signal1Slot);

doesn't work.

(No matter ThisClass::signal1Slot() method is ordinary method or marked as slots)
but

connect(myObj, &MyObjClass::signal1, [=]{ signal1Slot(); });

is invoked as intended

but

connect(myObj, &MyObjClass::signal1, this, [=]{ signal1Slot(); });

doesn't.

Also 'old syntax' with SIGNAL and SLOT keywords doesn't work.

One thing could be a key here: signal1 is called from another thread (and not QThread subclass) than the receiver thread.

mranger90

You'll have to provide a little more information. What do you mean by "doesn't work". It doesn't compile ? Does it compile but the slot is not being called ?
Are there any debug messages being printed about the connect call ?
Can you show the signatures of the signal and the slot ?

SeeLook

To clarify more:
Signatures are as I wrote:

connect(myObj, &MyObjClass::signal1, this, &ThisClass::signal1Slot); // doesn't work
connect(myObj, &MyObjClass::signal1, [=]{ signal1Slot(); }); // only one that works
connect(myObj, &MyObjClass::signal1, this, [=]{ signal1Slot(); }); // doesn't

All options above compiles fine, any message about that connections either during compilation or runtime. Any troublesome message at all :-)

With old syntax also there is no any runtime warning and of course in this case `slots' keyword is used above slot method declaration.

SGaist

Hi,

Maybe a silly question but, how do you know it's not called ?
Can you show the complete code that triggers this ?
What version of Qt are you using ?
On what platform ?
What compiler ?

SeeLook

I'm on Arch Linux with Qt 5.13. My app is compilled with GCC 9.1.
And simply I set debug message inside the slot method.

This "other thread" signal is emitted from audio callback function. (RtAudio actually) and by debug message before emit signal1(); I can detect it is indeed invoked.

In general all works as intended, but just for my knowledge I'm wondering why only that particular way of connection is respected.

Shall I write even more?

JKSH

@seelook said in connect doesn't call a slot but invokes lambda:

connect(myObj, &MyObjClass::signal1, [=]{ signal1Slot(); });

is invoked as intended

This connection runs the lambda in myObj's thread.

All your other connections try to run the lambda/slot in this's thread.

One thing could be a key here: signal1 is called from another thread (and not QThread subclass) than the receiver thread.

Does this live in a thread that has a running, non-blocked event loop?

SeeLook

I forgot to explain one thing, but I forgot it for myself as well:
Year ago I moved this to its own thread (audio data preparing heavy load), but I just forgot. Sorry.
But this explains everything.

connect(myObj, &MyObjClass::signal1, [=]{ signal1Slot(); });
This connection runs the lambda in myObj's thread.

This is so obvious that I even didn't think about it. It expalins how it works.
@JKSH Thank You a lot!
But this is first conclusion.
Another one is:

               main thread (event loop)
                        ^        \
                       /          \
                      /            \         
         thread_A(sender)      thread_B(receiver)

Signal sent by sender in thread A has to pass trough main thread to be delivered to receiver in thread B, in other words somewhere in main thread has to be receiver of thread A signal which will invoke slot in thread B (or will forward signal to it). Makes sense.
So I have to manage that.

JKSH

@seelook said in connect doesn't call a slot but invokes lambda:

               main thread (event loop)
                        ^        \
                       /          \
                      /            \         
         thread_A(sender)      thread_B(receiver)
Signal sent by sender in thread A has to pass trough main thread to be delivered to receiver in thread B, in other words somewhere in main thread has to be receiver of thread A signal which will invoke slot in thread B (or will forward signal to it).

In your diagram, Thread B must still have a running event loop. If it doesn't have an event loop, then Thread B can't receive any signals and can't have any slot invoked.

A signal can go straight from Thread A to Thread B without going through the main thread. Thread B just needs a running event loop.