Help with connect statement

Pl45m4

@mscottm said in Help with connect statement:

QObject::connect(&w, &MainWindow::canMessageOut, &pMessageProcessor, &pMessageProcessor::pIncomingMessage);

In addition to @J-Hilk :

If pMessageProcessor is already a pointer to your class instance, then you have to remove the &.

MainWindow w is referenced by using &w in your connect-statement because it is NOT a pointer.

Try:

pMessageProcessor * pMProc = new pMessageProcessor();
// [ ... ]
QObject::connect(&w, &MainWindow::canMessageOut, pMProc, &pMessageProcessor::pIncomingMessage);

Sender param != class of sender (you need the actual instance of that class there)

Pradeep P N

@pl45m4
I think it should be
connect(&w, &MainWindow::canMessageOut, mp, &pMessageProcessor::pIncomingMessage);
right ?
from this line pMessageProcessor * mp = new pMessageProcessor();

Please tell if i am missing something.

Pl45m4

@pradeep-p-n

After re-reading my "code" I realized that I messed it up :)
(Realized too late that pMessageProcessor is not the instance, but the class name -> low case first letter?!)
I've edited my answer....

I hope OP gets the point

Pradeep P N

@pl45m4
Cool,

All the best.

MScottM

Error remains the same: "pMessageProcessor does not refer to a value".

I will also say that when I move the connect statement into 'MainWindow.cpp', like so:

QObject::connect(this, processReceivedFrames, &pMessageProcessor, &pMessageProcessor::pIncomingMessage);

the error is the same ..."pMessageProcessor does not refer to a value". And when I move the statement into the pMessageProcessor.cpp file like so:

connect(&MainWindow, &MainWindow::processReceivedFrames, &pMessageProcessor, pMessageProcessor::pIncomingMessage());

the interpreter has three errors: "MainWindow does not refer to a value", "pMessageProcessor does not refer to a value", "cpp:11: error: too few arguments to function call, single argument 'message' was not specified"

Pl45m4

@mscottm

Take a look at my (edited) answer.

Again:

// a and b = no pointer
Sender a;
Receiver b;
// Take address from a and b with &
connect(&a, &Sender::signal, &b, &Receiver::slot);

or

//  a and b = pointers
Sender * a = new Sender();
Receiver * b = new Receiver();
// NO & in connection
connect(a, &Sender::signal, b, &Receiver::slot);

If you mix a pointer with &, your connection cannot work :)

Here's another example :)
https://doc.qt.io/qt-5/signalsandslots.html#a-small-example

Pradeep P N

@pl45m4 said in Help with connect statement:

If you mix a pointer with &, your connection cannot work :)

Are you sure of this ?

I did try a sample, it works.
QObject::connect(&w, &Widget::sendMsg, mtc, &MyTestClass::receiveMsg);

main.cpp

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);

    Widget w;
    MyTestClass *mtc = new MyTestClass;

    QObject::connect(&w, &Widget::sendMsg, mtc, &MyTestClass::receiveMsg);

    return a.exec();
}

Widget Class

// .h File
signals:
    void sendMsg(const QString msg);

// .cpp File
Widget::Widget(QWidget *parent)
    : QWidget(parent)
    , m_Count(0)
    , m_testTimer(new QTimer(this))
{
    m_testTimer->start();
    m_testTimer->setInterval(3000);

    connect(m_testTimer, &QTimer::timeout, [this]{
        m_Count++;
        emit sendMsg(" Number Sent :: " + QString::number(m_Count));
    });
}

MyTestClass

// .h File
public slots:
    void receiveMsg(const QString msg);

//.cpp File
void MyTestClass::receiveMsg(const QString msg)
{
    qDebug() << Q_FUNC_INFO << msg;
}

Output:

void MyTestClass::receiveMsg(QString) " Number Sent :: 1"
void MyTestClass::receiveMsg(QString) " Number Sent :: 2"
void MyTestClass::receiveMsg(QString) " Number Sent :: 3"
void MyTestClass::receiveMsg(QString) " Number Sent :: 4"
void MyTestClass::receiveMsg(QString) " Number Sent :: 5"
void MyTestClass::receiveMsg(QString) " Number Sent :: 6"

SGaist

@pradeep-p-n what @Pl45m4 was warning against is using & when the target is already a pointer to an object.

Pradeep P N

Ah, how did i misunderstood that.
Oops @Pl45m4 i got it.
@SGaist Thanks.

MScottM

I've tried many variations of the above thoughts - no version of the connect statement works. Here's a little of what I've tried (these are not the only variations).

When in main.cpp:

QObject::connect(&w, &MainWindow::canMessageOut, &pMessageProcessor, &pMessageProcessor::pIncomingMessage);

interpreter: error: 'pMessageProcessor' does not refer to a value

QObject::connect(w, &MainWindow::canMessageOut, pMessageProcessor, &pMessageProcessor::pIncomingMessage);

interpreter: error: 'pMessageProcessor' does not refer to a value

QObject::connect(w, MainWindow::canMessageOut, pMessageProcessor, pMessageProcessor::pIncomingMessage);

interpreter:
error: call to non-static member function without an object argument,
error: 'pMessageProcessor' does not refer to a value,
error: call to non-static member function without an object argument

Connect statement in MainWindow.cpp:

QObject::connect(this, processReceivedFrames, &pMessageProcessor, &pMessageProcessor::pIncomingMessage);

interpreter: error: 'pMessageProcessor' does not refer to a value

QObject::connect(this, processReceivedFrames, pMessageProcessor, &pMessageProcessor::pIncomingMessage);

interpreter: error: 'pMessageProcessor' does not refer to a value

QObject::connect(this, processReceivedFrames, pMessageProcessor, pMessageProcessor::pIncomingMessage);

interpreter:
error: 'pMessageProcessor' does not refer to a value
error: call to non-static member function without an object argument

and so on...

connect statement in pMessageProcessor.cpp:

connect(&MainWindow, &MainWindow::processReceivedFrames, &pMessageProcessor, &pMessageProcessor::pIncomingMessage);

interpreter:
error: 'MainWindow' does not refer to a value
error: 'pMessageProcessor' does not refer to a value

connect(MainWindow, &MainWindow::processReceivedFrames, pMessageProcessor, &pMessageProcessor::pIncomingMessage);

interpreter:
error: 'MainWindow' does not refer to a value
error: 'pMessageProcessor' does not refer to a value

and so on...

I don't think that the connect statement is the issue, but I'm too much of a beginner to know what else to look for in the code.

I appreciate the help so far!

Oh - and if it's not clear - I'm trying the connect statement in one place only - not in three places.

aha_1980

Hi @mscottm,

it seems you have a fundamental understanding problem, and that in turn will make it hard for others to understand your problem, because the description is already wrong.

and the interpreter doesn't throw an error, but it fails at runtime with LOTS of red ink in the compile screen.

You probably mean: the code model does not show errors, but the program does not compile and the compile output shows LOTS of errors.

Runtime, however, comes after your program successfully compiled and linked - when your program is actually running.

Regarding your real problem: Why don't you just show your code? We can guess for long here, or simply have a look at it. It can either take long or you can get an answer very quick - it just depends on you.

connect(&MainWindow, &MainWindow::processReceivedFrames, &pMessageProcessor, &pMessageProcessor::pIncomingMessage);

You need to understand that the first and third parameter of connect need to be a pointer to a variable, while the second and the fourth need to be a pointer to a member function (the second: a signal, the fourth: a slot).

In your example the parameters one and three are a class, not a variable, therefore it will not compile.

Example how it could look like:

MainWindow window;
pMessageProcessor processor;
connect(&window, &MainWindow::processReceivedFrames, 
&processor, &pMessageProcessor::pIncomingMessage);

As you can see, for the connect you need to have both classes as variable, and then it will work.

Regards

Pl45m4

@mscottm

Your last reply gives the impression that you didn't read our answers to your problem.
In this thread are at least 2-3 helpful explanations (with given code examples) how the connection statement works and how you can connect your signal with your function in your class.

pMessageProcessor is the name of your class and is NOT a valid first or third parameter in your connect statement, neither as pMessageProcessor nor as &pMessageProcessor.
You need the object / instance of this class there.

If your code is compilable and you use your pMessageProcessor class, then there must be an object of this class.

The same for MainWindow (as first / third parameter).

MScottM

@JKSH @jsulm @J-Hilk @Pl45m4 @Pradeep-P-N @SGaist @aha_1980

My sincere apologies. This is what I get for trying to solve coding problems when in a hurry, and just skimming the very helpful replies. I was able to solve the connect statement issue based on help given in one of the very first posts by @jsulm - no instance of the class, and @aha_1980 and @Pl45m4, you of course nailed the other issue.

Thank you all for your replies and taking the time to help!

Best regards