QT Signal from static method - c++

Question

I need to send a signal from static callback function. I do the following:
.h
class MainWindow : public QMainWindow {
Q_OBJECT
public:
MainWindow (QWidget* parent=0);
~MainWindow();
static MainWindow* getInstance();
private:
void emitSignal();
static MainWindow* m_instance;
signals:
void mysignal();
slots:
void print();
...
}
.cpp
MainWindow* MainWindow::m_instance = 0;
MainWindow::MainWindow(QWidget* parent):
QMainWindow(parent) {
...
}
MainWindow* MainWindow::getInstance() {
if (m_instance == 0)
m_instance = new MainWindow;
return m_instance;
}
void MainWindow::emitSignal() {
emit mysignal();
}
inside of callback function:
getInstance()->emitSignal();
and somewhere in code:
connect(MainWindow::getInstance(), SIGNAL(mysignal()), this, SLOT(print()));
But the slot print() is not executed. And no errors are printed. Where am I wrong?

Answer 1

The callback uses getInstance() and thus can definitely create a new widget if one doesn't exist yet. Then it's a matter of how well you understand the code that executes the callback. If the callback is made before you make the connection, then the slot won't be invoked. That's likely your problem. It is invalid to invoke the callback without an instance already in place, and you should assert that it is so.
Most likely, you do not need to create an instance of the main window on the fly. You're likely facing an XY problem where you think you need to do something, but you're so invested in a solution that doesn't work that you don't see the bigger problem you're trying to solve. Please clearly expose what exactly is your application - why are you creating the window on the fly?
It is also unnecessary to create a method that forwards to the signal, unless you really somehow need the signal to be private. I doubt that you do - don't make it private.
The code below demonstrates that what you're trying to do definitely works if you do it correctly:
// https://github.com/KubaO/stackoverflown/tree/master/questions/static-signal-48540601
#include <QtCore>
class Main : public QObject {
Q_OBJECT
static Main * m_instance;
public:
Main(QObject * parent = {}) : QObject(parent) {
Q_ASSERT(! hasInstance());
m_instance = this;
}
static Main * instance() {
if (! m_instance) m_instance = new Main;
return m_instance;
}
static bool hasInstance() { return m_instance; }
Q_SIGNAL void theSignal();
};
Main * Main::m_instance;
void callback() {
Q_ASSERT(Main::hasInstance());
// If the instance didn't exist here, nothing can receive the signal.
Main::instance()->theSignal();
}
int main()
{
int slotCalls = {};
Main object;
QObject::connect(&object, &Main::theSignal, [&]{ slotCalls ++; });
Q_ASSERT(slotCalls == 0);
callback();
Q_ASSERT(slotCalls == 1);
}
#include "main.moc"

Answer 2

Are you sure the signal connected to the slot?
To avoid typos you could use this syntax (as long as you dont overload signals/slots you are fine with that ):
connect(MainWindow::getInstance(), &MainWindow::mysignal, this, &MainWindow::print);
another hint/styleguide: name your signals like signal_mySignal and slots like slot_print to avoid typos and clear things up.
You can also call this->dumpObjectInfo(); from somewhere in your main (after the connects of course) and take a look at the output