Qt GUI Easiest way to access MainWindow from another class

Question

I am doing a blackjack program and I am keeping track of the cards in the players hand in another class ("hand.h") than the main window class.

In the hand class, for every card that I collect, I am also creating a QLabel that grabs the proper card image for the card and also sets the coordinates for where the card should appear on the main window.

The problem is that I am not able to create the QLabel based on the MainWindows object that is originally created at the main function. Is there any easy way that I am able to get that information fairly easily? Thanks for your help!

I have tried using QGuiApplication::topLevelWindows(), but haven't came to luck with using that. Here is my function that I am using.

    #include <QRect>
    #include <QApplication>
    #include <iostream>
    #include <QLabel>
    #include "mainwindow.h"
    #include <QMainWindow>
    #include <QWindowList>
    #include <QWidgetList>
    #include "ui_mainwindow.h"

    void Test() {

    QList<QWindow*> Main_Window = QGuiApplication::topLevelWindows();
     for (int i = 0; i < Main_Window.size(); ++i) {
        if(Main_Window.objectName() == "mainWindow") // name is OK
                break;
        }
    QMainWindow* mainWindow = static_cast<QMainWindow*>(Main_Window);


    QLabel* temp;
    temp = new QLabel(Main_Window);
    temp->setPixmap(QString("Ten of Clubs.png"));
    temp->setGeometry(290, 300, 350, 390);
    temp->show();

    }

Here is the main.cpp file that creates the mainwindow

    int main(int argc, char *argv[])
    {
      srand(time(NULL));
      QApplication a(argc, argv);
      MainWindow w;

      w.show();
      return a.exec();
    }

I found the iterating code online and have been having issues from it. I am having issues while trying to iterate through the list, but I have no idea how to identify the list and the error says that there is no objectName() function. Also, in the static cast line, there is an error that says that I cannot convert an QList to type QMainWindow. Any help would be greatly appreciated.

Answer 1

No way in general, because some applications may have several (toplevel) QMainWindow-s (and their list could change with time). So for that case you'll better pass the pointer to it (the particular QMainWindow you want to deal with) explicitly....

A possible way might be to have your specific subclass of QApplication (which is a singleton class, see QCoreApplication::instance to get its sole instance) and in your application subclass put, as fields, the explicit windows you want to deal with (maybe you even want to add some new signal or slot to your application class).

However, you could use QGuiApplication::topLevelWindows() or QGuiApplication::allWindows() to get the list of all such windows. Notice that a QWindowList is just a QList<QWindow *>. So see QList for how to traverse or iterate on that list.

Once you have found which QMainWindow you want, adding a QLabel into it is usual practice (but again, signals & slots could be helpful).

BTW, each (displayed) widget has its window, see QWidget::window()

---

About your code:

Your Main_Window is really poorly named (and the name is so confusing that I cannot use that). It is a list not a window. So code first:

QMainWindow* mainWindow = nullptr;
{
  QList<QWindow*> topwinlist = QGuiApplication::topLevelWindows();
  int nbtopwin = topwinlist.size();
  for (int ix=0; ix<nbtopwin; ix++) {
    QWindow*curwin = topwinlist.at(ix);
    if (curwin->objectName() == "mainWindow")
      mainWindow = dynamic_cast<QMainWindow*>(curwin);
  }
} 

I did not test the above code and I am not sure it is correct or even can compile. But why don't you just have a global pointer to your main window:

 MainWindow*mymainwinp = nullptr;

and initialize it appropriately in your main body:

int main(int argc, char *argv[]) {
  srand(time(NULL));
  QApplication a(argc, argv);
  MainWindow w;
  mymainwinp = &w;
  w.show();
  int r = a.exec();
  mymainwinp = nullptr;
  return r;
}

then use mymainwinp elsewhere (e.g. in your Test)? If you want more elegant code, define your own subclass of QApplication and have mymainwinp be a field in it.