":connect' error - "multiple definition..." ?

Chris Kawa

@AnneRanch Are you sure you're connecting the right instances? You can print out the pointer and compare the addresses i.e. qDebug() << server; where you make that connection and qDebug() << this; inside BT_ChatServer where you say it works. It should be the same address printed out. If not then you mixed your instances somewhere.

Anonymous_Banned275

@Chris-Kawa Here is a snippet of app output
The pointers match

void BT_SPP_CA_MainWindow::Connect_MainServer()
BT_ChatServer::BT_ChatServer(QObject *)
"\n\t TASK Run \nBT_ChatServer::BT_ChatServer(QObject *)\n"
!!!!!!!!!!!!!!!!!!! this BT_ChatServer(0x5653f9b74550)
conncet OK
TASK Construct BT_ChatServer
BT_ChatServer::BT_ChatServer(QObject *)
& line "84"
!!!!!!!!!!!!!!!!!!!!server BT_ChatServer(0x5653f9b74550)
!!!!!!!!!!!!!!!!!!! this BT_ChatServer(0x5653f9b74550)
"emit NewMessage(text)"
void BT_ChatServer::PostMessage(const QString)
"\n\t TASK Run \nvoid BT_ChatServer::PostMessage(const QString)\n !!!! REMOTE !!!! TEST MESSAGE "

Chris Kawa

@AnneRanch It's very difficult for me to decipher that stream of random text without the matching code, but you said earlier that you had this:

Q_ASSUME(connect(server,&BT_ChatServer::NewMessage,this,&BT_SPP_CA_MainWindow::PostMessage));

but the output says this:

!!!!!!!!!!!!!!!!!!!!server BT_ChatServer(0x5653f9b74550)
!!!!!!!!!!!!!!!!!!! this BT_ChatServer(0x5653f9b74550)

so which is it, because it's physically impossible that this output matches that connect. this can't be both BT_SPP_CA_MainWindow and BT_ChatServer at the same time, so what's the real code that produces this output?.

SimonSchroeder

@Axel-Spoerl said in ":connect' error - "multiple definition..." ?:

so wrapping it with Q_ASSUME(connect(...))will tell right away, if the connection was successful.

Will it, though? I had to quickly look up Q_ASSUME. From my understanding it just tells the compiler that you assert that the return value of the connect is always true. The compiler is allowed to use this assumption for optimization. If the assumption is wrong you have undefined behavior. Or does it actually do a check? I have seen people suggest Q_ASSERT, but this will only do the connect in the debug build, not in the release build. Is there a macro you could actually use here? Or am I mistaken about Q_ASSUME?

With the new connect syntax rarely if ever will the connect fail if it compiles. I have to admit that I never check the return value. Qt will write out warning to the console, though, when the connect fails. This is how you can figure this out at runtime as well.

JonB

@SimonSchroeder said in ":connect' error - "multiple definition..." ?:

If the assumption is wrong you have undefined behavior. Or does it actually do a check?

I believe you are quite correct here. It does not do a check, unlike Q_ASSERT, rather it tells the compiler what is going to happen.

The OP actually asked about this in Usage of Q_ASSUME in connect ?.

Axel Spoerl

@SimonSchroeder said in ":connect' error - "multiple definition..." ?:

Q_ASSUME?

Maybe I wasn't clear in my proposal to wrap QObject::connect() into Q_ASSUME().
This is for debugging and analyzing, if a connection was successful at all.
It's not recommended for production code, however.
Q_ASSUME() instructs the compiler to assume, that the wrapped condition is true. That's for optimizing purposes. The condition itself can be optimized out, even if that's not always the case. It remains the compiler's decision.
Q_ASSERT() will always kick in debug builds and is completely optimized out on production. In a debug build, Q_ASSUME becomes a Q_ASSERT, so they are treated identically.

Having said that, the safe way to assert a connection is to read it's return value into a variable, debug it and act on it. Connections should be designed such, that they always work. Once the plumbing is done, all the wrappers can go out. They create unnecessary overhead in stable code. That's why QObject::connect() is not marked nodiscard, like other functions that do not allow their return value to be ignored.

Anonymous_Banned275

I have added some"B@W" so we can see the compiler / linker actual output.
It still shows that the pointers match.

BT_ChatServer::BT_ChatServer(QObject *)
& line "84"
DEBUG connect void BT_SPP_CA_MainWindow::Connect_MainServer() @ line "1399" this BT_SPP_CA_MainWindow(0x558ce48df8f0, name="BT_SPP_CA_MainWindow")
DEBUG connect void BT_SPP_CA_MainWindow::Connect_MainServer() @ line "1400" server BT_ChatServer(0x558ce4df7170)
!!!!!!!!!!!!!!!!!!!!server BT_ChatServer(0x558ce4df7170)
DEBUG connect void BT_ChatServer::startServer(const QBluetoothAddress &) @ line "109" this BT_ChatServer(0x558ce4df7170)
"emit NewMessage(text)"
void BT_ChatServer::PostMessage(const QString)

As far as Q_ASSUME goes - it does check the syntax , not the actual function of the "connect" .
This is helpful.

I do understated there may be alternatives to accomplish passing messages between objects, but my goal is to find out WHY this does not work.
( The BT_ChatServer is a member of BT_SPP_CA_MainWindow )

If I have a multiple and WRONG "connect" would it still do something , even if incorrect ?
It is (remotely) possible that the "new" syntax does not works BETWEEN objects ?
( I just cannot believe this "passing messages", was never tried by some other coder ....)

Let's keep in focus, it does not matter what / where the error is, I just want to resolve it,
putting the blame on something / somebody IS NOT my goal.

.

Anonymous_Banned275

Maybe Q_ASSUME will do more...

I went back to the "old style" too,
The old style complains about missing parameter.

Then my usage of Q_ASSUME is also wrong - I need to fix that and see if it will help to locate the problem...

..and yes,, I hand two instances of "server" , but either one should pass the test message....

QObject::connect: No such signal BT_ChatServer::&BT_ChatServer().NewMessage() in bt_spp_ca_mainwindow.cpp:1466
QObject::connect:  (receiver name: 'BT_SPP_CA_MainWindow')
ASSERT failure in Q_ASSUME(): "Assumption in Q_ASSUME("connect (server,SIGNAL(&BT_ChatServer().NewMessage()), this,SLOT(&BT_SPP_CA_MainWindow().PostMessage()) );") was not correct", file bt_spp_ca_mainwindow.cpp, line 1470
11:16:58: /mnt/07b7c3f8-0efb-45ab-8df8-2a468771de1f/PROJECTS/BT_JAN13_/Qt-5.15.2/widgets/mainwindows/mdi/mdi crashed.


            Q_ASSUME(
                    connect
                            (server,SIGNAL(&BT_ChatServer().NewMessage()),
                             this,SLOT(&BT_SPP_CA_MainWindow().PostMessage())
                             );

                    );


           //Q_ASSUME(connect(this, &BT_SPP_CA_MainWindow::NewMessage, this, &BT_SPP_CA_MainWindow::PostMessage));

            Q_ASSUME(connect(server,&BT_ChatServer::NewMessage,this,&BT_SPP_CA_MainWindow::PostMessage));

            qDebug() <<"DEBUG connect " << Q_FUNC_INFO << "@ line " << QString::number(__LINE__) << " server  " << server;

Anonymous_Banned275

more debugging....
Added another class member , "standard" Qt Designer Form class with single TEST button emitting test message.

SUCCESS The message gets passed to the "parent" using "connect"....

What is puzzling - the debug outputs are in reverse sequence...

So - I must have a wrong "connect " somewhere.... in my original code....

Anonymous_Banned275

SOLVED

Eureka !!
The original QT example code has multiple instances of ChatServer and no "delete".

Now a minor curiosity question remains:
if the test code emits TWO messages , why do I get only ONE "postmessage " ?

void BT_ChatServer::PostMessage(const QString)
"\n\t TASK Run \nvoid BT_ChatServer::PostMessage(const QString)\n !!!! REMOTE !!!! TEST MESSAGE "
void BT_ChatServer::PostMessage(const QString)
"\n\t TASK Run \nvoid BT_ChatServer::PostMessage(const QString)\n !!!! REMOTE !!!! TEST MESSAGE "
void BT_SPP_CA_MainWindow::PostMessage(const QString)
"\n\t TASK Run \nvoid BT_SPP_CA_MainWindow::PostMessage(const QString)\n !!!! REMOTE !!!! TEST MESSAGE "