signal/slot between two different classes

[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 .

[KroMignon]

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

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

I don't want to be pedantic, but as you seems to be very unexperimented I will try to be as clear as possible.

There is no need of a global variable at all.

For me, there are 2 clean way to do it:

1. create a variable in main() and do all connection there.
2. add a class attribute from type Backend into MainWindow and eventually give access to it through getter.

My preferred way is the second.
But this is up to you.