Issue with QMainWindow instance access

MichaelM

Hi,

I'm having an OO-programming issue when trying to access functions contained in the class derived from QMainWindow.

Here are the simplified corresponding parts of my code :

In mainwindow.h :

class MainWindow : public QMainWindow
{
public:
explicit MainWindow(QWidget *parent = 0);
~MainWindow();

public:
void theFunctionIWantToCallOnWindowInstance();

private:
Ui::MainWindow *ui;

In mainwindow.cpp :

void MainWindow::theFunctionIWantToCallOnWindowInstance()
{
QMessageBox::information(this, "[DEBUG]", "theFunctionIWantToCallOnWindowInstance() called");
}

In main.cpp file :

#include "mainwindow.h"
#include <QApplication>

int main(int argc, char *argv[])
{
QApplication a(argc, argv);
MainWindow w;
w.show();
return a.exec();
}

In external.cpp :

Basically, I simply want to call a function from external code on the window instance created in main.cpp, something like :

#include "mainwindow.h"
...
MainWindow::getInstanceCreatedInMain().theFunctionIWantToCallOnWindowInstance();

I tried many different approaches using singletons and static (and even global) variables but I cannot solve my problem i.e. get a reference to my window from the external code.

Thanks in advance for your help.

Michael

Hamed.Masafi

@MichaelM said:

getInstanceCreatedInMain

Where is the getInstanceCreatedInMain defined?

Chris Kawa

Hi and welcome,

Using globals or calling a method of a main window deep in your code are signs of flawed design. There's usually no (good) reason to do that. You could, for example, emit a signal and connect that to the main window. This way you won't create a coupling, where a child needs to know about its parent.

To answer your question and tell you how to shoot yourself in the foot - here's how you would make your single instance of main window accessible from anywhere in the code:

//mainwindow.h
class MainWindow : public QMainWindow
{
   ...
public:
   static MainWindow* instance();
private:
   static MainWindow* s_instance;
};

//mainwindow.cpp
static MainWindow* MainWindow::s_instance = nullptr;

MainWindow* MainWindow::instance() { return s_instance; }

MainWindow::MainWindow(QWidget* parent) : QMainWindow(parent)
{
   assert(!s_instance); //another instance already exists!
   s_instance = this;
}

Then you can reference the main window from anywhere like this:

MainWindow::instance()->doWhatever();

Just to repeat - I advise against using this. You have a case of inverted relation in your design and it would be better to look into that.

MichaelM

Thanks a lot for your help.
I finally managed to solve my problem by using your method. I know about coupling but for some reasons I cannot change the structure / design of external.cpp.

Just a detail, in the code you provided I had to replace :

//mainwindow.cpp
static MainWindow* MainWindow::s_instance = nullptr;

By :

//mainwindow.cpp
MainWindow* MainWindow::s_instance = nullptr;

as it throws the compilation error :
error: 'static' may not be used when defining (as opposed to declaring) a static data member [-fpermissive]