signal/slot between two different classes

dziko147

@jsulm ok I tried this actually

signals:
    void canDeviceChanged(int value);

in //MainWindow.cpp

connect(this, SIGNAL(canDeviceChanged(int value )),&back, SLOT(statuspican(int value)));


if (m_canDevice)
    emit canDeviceChanged(1);

in backend.h

public slots:
    void statuspican(int value);

in backend.cpp

void Backend::statuspican(int value)
{
qDebug() << "from Mainwindow :" << value;
}

KroMignon

@dziko147 said in signal/slot between two different classes:

connect(this, SIGNAL(canDeviceChanged(int value )),&back, SLOT(statuspican2(int value)));

Have you read the link a give you at least 5 times?
This can not work ==> https://doc.qt.io/qt-5/qobject.html#connect

EDIT
What is statuspican2?
According to your code, the slot name is statuspican

jsulm

@dziko147 said in signal/slot between two different classes:

connect(this, SIGNAL(canDeviceChanged(int value )),&back, SLOT(statuspican2(int value)));

Remove "value". Old style connect does only contain the parameter type, not its name.
But you should really switch to new Qt5 connect syntax - if you then do something wrong you will get a compiler error. With old style connect you only get a runtime warning if connect fails.

dziko147

so logically it should works , there is no syntax error ! ?

jsulm

@dziko147 Did you read what I and @KroMignon wrote?

KroMignon

@dziko147 said in signal/slot between two different classes:

so logically it should works , there is no syntax error ! ?

Are you jocking?

Nothing is correct:

the slot name is wrong
the SIGNAL() syntax is wrong ==> only signal signature means: signal name and parameter types
the SLOT() syntax is wrong ==> only slot signature means: slot name and parameter types

dziko147

@KroMignon ok could you provide a connect implementation to solve this .

i am blocked since 3 days :/

Christian Ehrlicher

@dziko147 said in signal/slot between two different classes:

ok could you provide a connect implementation to solve this .

@jsulm already told you what's wrong and you should switch over to the new signal/slot syntax so you get proper compile time errors instead during runtime.

KroMignon

@dziko147 said in signal/slot between two different classes:

ok could you provide a connect implementation to solve this .
i am blocked since 3 days :/

Can you read?

@jsulm told you what to do:

Remove "value". Old style connect does only contain the parameter type, not its name.

And in Qt documentation:

Note that the signal and slots parameters must not contain any variable names, only the type. E.g. the following would not work and return false:
// WRONG
QObject::connect(scrollBar, SIGNAL(valueChanged(int value)),
             label, SLOT(setNum(int value)));

// CORRECT
QObject::connect(scrollBar, SIGNAL(valueChanged(int)),
            label,  SLOT(setNum(int)));

I cannot believe this take 3 days to understand.

dziko147

@KroMignon even i remove the "value" it doesn't works .
I didn't understand why .
No error but the application spits .

KroMignon

@dziko147 said in signal/slot between two different classes:

No error but the application spits .

Why not using the debugger to find out why your application crash?

And in you connect:

connect(this, SIGNAL(canDeviceChanged(int)),&back, SLOT(statuspican(int)));

What is back?
Where is it defined?

jsulm

@dziko147 Next thing to check: what does connect() call return?

qDebug() << connect(this, SIGNAL(canDeviceChanged(int)),&back, SLOT(statuspican(int)));

Next: did you make sure that the signal is actually emitted?

if (m_canDevice) {
    qDebug() << "Emit signal";
    emit canDeviceChanged(1);
}

dziko147

@jsulm

qDebug() << connect(this, SIGNAL(canDeviceChanged(int)),&back, SLOT(statuspican(int)));

return true .

And the signal is emmited

SGaist

Hi,

@dziko147 said in signal/slot between two different classes:

@jsulm
qDebug() << connect(this, SIGNAL(canDeviceChanged(int)),&back, SLOT(statuspican(int)));
return true .

And the signal is emmited

Where is back defined ?
Any chances that this is a function local variable that is destroyed at the end of it ?

dziko147

@SGaist

Backend is my second class . the class which i want to get data in .
So back is an instance of Backend .

So i defined like this :

Backend back;

SGaist

The question is: where is it defined ?

dziko147

@SGaist

MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent),
    m_ui(new Ui::MainWindow),
{
    m_ui->setupUi(this);
    
    Backend back ;
connect(this, SIGNAL(canDeviceChanged(int)),&back, SLOT(statuspican(int)));

SGaist

That's what I suspected: back is destroyed at the end of the constructor.

KroMignon

@dziko147 said in signal/slot between two different classes:

{

m_ui->setupUi(this);
Backend back ;
connect(this, SIGNAL(canDeviceChanged(int)),&back, SLOT(statuspican(int)));

I know that you are a C++/Qt beginner, but as many times written: please take time to understand C++ basics.

C++ variable scope and life-cycle is one of them:

https://www.tutorialspoint.com/cplusplus/cpp_variable_scope.htm
https://www.geeksforgeeks.org/scope-of-variables-in-c/
there are many more available on internet

If you don't learn, how do you expect programming?

dziko147

@KroMignon Hello
to create a global variable . Can i create Backend back; in MainWindow.h .