Qt: Getting the current application palette

I have a class that composes a palette and assigns it to the application using QApplication::instance()->setPalette(QPalette palette).

And it effectively works.

But then I try to use QPalette QApplication::instance()->palette() to extract some colours.

But here it does not work, it just returns the default palette, not the current one.

After I have discovered that it is working as supposed and described in the documentation.

And now I have just 2 questions:

1. Why it is working in such a strange, useless and counter-intuitive mode?
2. How I can retrieve the palette which was set using QApplication::instance()->setPalette(QPalette palette)?

P.S. No, I can't keep that palette elsewhere.

I think it is an issue of your Qt version (you marked the question as Qt 5 but didn't indicate a specific version), or you have something else in your project that is resetting the palette (you mentioned it has a large code base).

This minimum example shows correct behavior, at least with Qt 5.12.3 32bits, Windows, VS 2017:

#include <QApplication>
#include <QPalette>
#include <QDebug>
#include <QTimer>
#include <QWidget>

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

  const auto group = QPalette::Active;
  const auto role = QPalette::Text;

  auto palette = QApplication::palette();
  qDebug() << "palette before:" << palette.color(group, role).name();

  palette.setColor(group, role, "#123456");
  qDebug() << "palette set:" << palette.color(group, role).name();

  QApplication::setPalette(palette);

  const auto palette2 = QApplication::palette();
  qDebug() << "palette after:" << palette2.color(group, role).name();

  QTimer::singleShot(100, [=]() { // check palette after the events loop has started
    const auto palette3 = QApplication::palette();
    qDebug() << "palette after 100ms:" << palette3.color(group, role).name();    
  });

  QWidget w;
  w.show();

  return a.exec();
}

I've used QApplication::palette my self to retrieve custom palettes in different projects and had no issues at all.

QGuiApplication::setPalette is documented to change the default palette, so basically I think default palette means the palette used if a widget doesn't specify the other one; not the default system palette.

PS: I couldn't make it compile when using QApplication::instance()->setPalette since QApplication doesn't defines instance() but it falls to QCoreApplication::instance(), which obviously returns a QCoreApplication. Probably just a typo when you wrote the question, but I thought it deserved some lines. Given that the palette related methods are static, I decided to use those in the example, but I had the same results using the singleton from qApp.