Signal emitted but Slot won't work

checkers

Hello, I am relatively a new to be using Qt and studying Signals and Slot recently.

My first try is that when QString variable is set via a certain function outside of UI, append it to the QPlainTextEdit on UI.

So I write the code like

customconnectobject.h
class CustomConnectObject : public QObject
{
Q_OBJECT

signals:
void mySignal(QString);

public:
void setString(QString msg)
{
emit mySignal(msg);
}

mainwindow.cpp
MainWindow::MainWindow(QWidget *parent) :
QMainWindow(parent)
ui(new Ui::MainWindow)
{
ui->setupUi(this);
CustomConnectObject * cco =new CustomConnectObject();
connect(cco,&CustomConnectObject::mySignal,ui->plainTextOutput,&QPlainTextEdit::appendPlainText)

otherfunction.cpp
CustomConnectObject * co =new CustomConnectObject();
co->setString("test");

I expected whenever I call co->setString, I could get the text updated in the QPlainTextEdit object but nothing happens.

I checked if connect is made well and it was ok.

Please help me out

kenchan

Just guess, perhaps that signal should have public scope?

Eeli K

@checkers You create two different objects of class CustomConnectObject (badly named btw, because it's a class, not an object). When in the otherfunction.cpp you call co->setString that object's signal isn't connected.

checkers

@kenchan Thank you for a replay, did you mean I should rewrite it

public:
void setString(QString msg)

instead of

signals:
void setString(QString msg)

but it didn't work

dheerendra

Signal shud be emitted from cco object not co object. That is the issue here.

kenchan

No, more like
public signals:
or
public
signals:

kenchan

@dheerendra yes, that too :-)

checkers

@Eeli-K
Thank you for a reply. So apparently the object 'co' in otherfunction.cpp and 'cco' in mainwindow.cpp are different.

Then should I call 'cco' in mainwindow.cpp?

Sorry I know I am asking basic things but I wanted to understand Signals and Slot better.

kenchan

Two different instances of the same type of object.
coo is a pointer so you have to use it where you can "see" it. You connected that objects signal to the slot so yes you must use the same pointer to the object to make it emit the signal. as @Eeli-K and @dheerendra have said.

checkers

@kenchan
Thanks a lot.
It is true that I created cco in mainwindow.cpp and connected it in mainwindow.cpp

but is there a way to 'see' cco in otherfunction.cpp ?

kenchan

where is other function relative to main window and cco?
you could:
pass the pointer to the other function directly
or
make cco a member of mainwindow and make mainwidow visible to other function and have a getter function in mainwindow
... etc.

Eeli K

@checkers This is quite basic thing in C++, you clearly need practice in it and unfortunately there's no other way to learn it than practice, fail first and then succeed :) Eventually you will understand how to govern the lifetime of objects, where and when to create and destroy them and how objects can have access to or reach other objects. In C++ you can for example pass pointers and references as arguments to functions and store objects as member data. Kenchan already gave tips for this situation. It's difficult to say more without seeing the whole code.

checkers

@kenchan
I call some functions in otherfunction.cpp from mainwindow.cpp by including otherfunction.h
I wanted to call cco from those functions in otherfunction.cpp like

otherfunction.cpp
void testfunction()
{
cco->setString("test");
}
mainwindow.cpp
void MainWindow::on_pushButton_clicked()
{
testfunction();
}

I will try passing cco directly to my function in otherfunction.cpp

thank you very much

checkers

@Eeli-K
Thank you. I know I need some practice in here as I am a novice to C++ also.
I will try to solve current situation.

jsulm

@checkers Why does otherfunction.cpp need access to cco if cco is part of main window? Doing it like this is bad design. This testfunction could just return the string and you call cco then in mainwindow:

QString testfunction()
{
    return "test";
}

void MainWindow::on_pushButton_clicked()
{
    cco->setString(testfunction());
}

checkers

@jsulm
Thank you for advice
You sound correct

I may be confused the concept of Signals and Slot that Signals must be called from other place than Slot

jsulm

@checkers Well, slots are usually defined in other classes than signals. That is the whole idea behind signals/slots: that an object can notify other objects via signals. The object which emits a signal usually does not know who is connected to this signal nor does it care - it just emits the signal, who ever wants receives it.