Application GUI state saving in Qt - qt

Question

What is an optimal and an appropriate way to save the state of a Qt GUI so I get the same state I had back when I closed the application ?
By state I mean : current indexes (for combo box ...), color palette, widgets positions... right before closing the application

Answer 1

You can use the QSettings class.
Simple use of QSettings class (code inspired from Qt's documentation):
In the main-window of your application code member functions that saves and restore the settings:
void MainWindow::writeSettings()
{
QSettings settings("reaffer Soft", "reafferApp");
settings.beginGroup("MainWindow");
settings.setValue("size", size());
settings.setValue("pos", pos());
settings.endGroup();
}
void MainWindow::readSettings()
{
QSettings settings("reaffer Soft", "reafferApp");
settings.beginGroup("MainWindow");
resize(settings.value("size", QSize(400, 400)).toSize());
move(settings.value("pos", QPoint(200, 200)).toPoint());
settings.endGroup();
}
Call those 2 functions from the MainWindow constructor and from the closeEvent override, like this:
MainWindow::MainWindow()
{
// code from constructor
//...
readSettings();
}
void MainWindow::closeEvent(QCloseEvent *event)
{
//optional check if the user really want to quit
// and/or if the user want to save settings
writeSettings();
event->accept();
}

Answer 2

The direct answer requires specific elaborated design for your code and not really a short Qt question or even the question specific to Qt. That is about C++ which is not the VM-based language that assists with serializing the state of program code to data. Having all objects serializable we can then attempt to apply certain C++/Qt classes/techniques.
This task is much easier to accomplish with languages like Java, though. And with C++/Qt you have to routinely make serialize-able / serialize / restore everything that is running in your code and still no guarantee that works as long as the context is not fully captured. This task is not easy for sure and makes sense only in specific application.
The most you can get directly from Qt is to save/restore QMainWindow and other independent widgets geometry (position/size):
saveGeometry
restoreGeometry
... and that solution is still somewhat incomplete or you may/not use QSettings for the storage.

Answer 3

I use QSettings for this. With routines similar to Zlatomir's.
For each window I have in the project I use a different section in QSettings and have readSettings() and writeSettings() in the source for each window.
Anything on the form that I want to persist I have to explicitly save and recall. In the case of a QComboBox it would be something like:
QSettings settings("Organisation", "MySoftware");
settings.beginGroup("WindowNumberTwo");
settings.setValue("ComboIndex", combobox->currentIndex());
// save more values here
// ...
settings.endGroup();
I don't know of a built in way to persist your window states - it has to be don't value by value.