Qt: Best Practice for a Single Instance App Protection

Single instance application in Qt

For single instance, read Run only one instance of a Qt application.

Disregarding portability, if you are running on Windows, another common approach is to create a named pipe (or a named mutex) when the application starts, and destroy it before exiting. If the named pipe already exists, another instance is already running. You could even write to the named pipe asking the other instance to bring its window to front, altough a console application might not benefit from this specifically.

Another way to focus the existing instance is to find the window of the already running instance, then call SetFocus on its HWND, or the corresponding function on your platform.

How to manage single instance of a Qt app on crash?

You can use a QSetting:

int main(int argc, char *argv[])
{
    QSettings settings;
    QApplication a(argc, argv);

    if(!settings.exitedNormaly()) {
        // In case of crash
    }

    // set the flag to false
    settings.setExitedNormaly(false);

    MainWindow w(&settings);
    w.processArg(argc, argv);
    w.show();

    int result = a.exec();
    settings.setExitedNormaly(result == 0);

    return result;
}

Combined with your shared memory, you'll be able to unlock the block in case of application crash.

Qt5: Preventing another instance of the application doesn't work anymore...!

I finally discovered where the problem was... and it's not the antivirus to be blamed :)

When I upgraded the Qt (Creator v3.6.0) to the newest version (5.5.1), there is a setting in Tools->Options->Build&Run named [Stop app before building]... that was set to Current project or something. Hence, the Qt Creator was killing the old instance before launching a new one(!).

Setting this option to None solved the issue.

So, it seems the code was just fine, antivirus was fine, yet launching the app from within Qt Creator was somehow restricted to only one instance :)

I decided to share this, maybe it will be helpful for other folks as well.

Remark : I checked again and now I can confirm: That setting didn't exist before, at least not in Qt Creator v3.3.2.

Restoring or bringing to front Qt desktop application

For some reason setActivationWindow() and activateWindow() don't work for me. This is my workaround:

#include <QWidget>
#include <qtsingleapplication.h>

class Window : public QWidget
{
    Q_OBJECT
public:
    Window(QWidget *parent = 0) : QWidget(parent) {}

public slots:
    void readMessage(const QString &str) { showNormal(); }
};

int main(int argc, char *argv[])
{
    QtSingleApplication instance(argc, argv);
    Window *window = new Window;
    window->show();

    QObject::connect(&instance, SIGNAL(messageReceived(const QString &)), window, SLOT(readMessage(const QString &)));

    if (instance.sendMessage(""))
        return 0;

    return instance.exec();
}

#include "main.moc"

Find QWidget of single instance Qt application

You should use the qtsingleapplication API

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

Best practice when setting a string to (possibly) its own value?

It doesn't matter in these cases. Pick what looks clearest to you and your team. Don't sweat over it.

There's no general rule of thumb either. There are specific cases where checking before setting is a must because of some side effects in the setter and there are specific cases where settings regardless is a must because of performance criticality / checking is expensive / multithreading, this case is neither.

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