Find QWidget of single instance Qt application [duplicate]

Question

I'm trying to create a single instance Qt application and I'm at the point this works, but now I want to focus the already started instance when a second is started. QWidget::find(g_hWnd) should return the widget but it fails and crashes on w->show();

Any thoughts?

#pragma data_seg("Shared")
HWND g_hWnd = NULL;
#pragma data_seg()
#pragma comment(linker,"/section:Shared,rws")

int main(int argc, char *argv[])
{
    if (g_hWnd)
    {
        QWidget* w = QWidget::find(g_hWnd);
        w->show();
        return 0;
    }
    else
    {
        QApplication a(argc, argv);
        mainWindow w;
        w.show();
        g_hWnd = a.topLevelWidgets().at(0)->winId(); //or w.winId()?

        return a.exec();
    }
}

edit: I now see Trolltech released the QtSingleApplication class under LGPL.

Answer 1

You should use the qtsingleapplication API

edit- It's a separate download see here for both LGPL and Commercial editions

Answer 2

#include <QtGui/QApplication>
#include "mainwindow.h"
#include <QMessageBox>
#include <QSharedMemory>


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

    QSharedMemory shared("61BB200D-3579-453e-9044-");
    if(shared.create(512,QSharedMemory::ReadWrite)==true)
    {
        QMessageBox msgBox;
        msgBox.setText("I am first.");
        msgBox.exec();
    }
    else
    {
        QMessageBox msgBox;
        msgBox.setText("i am already running.");
        msgBox.exec();
        exit(0);
    }
    //shared.AlreadyExists()

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