The "desired" behavior that you see is in fact an error, and I will file a bug report for it. Thanks for spotting it - nice corner case.
You need to set the size policy of the button to expanding in both directions. Buttons normally don't want to expand vertically, so if you tried a variant that toggles the alignment, you'd see that it works only horizontally, and that's correct.
This is a simple demonstration that shows the correct behavior that also fulfills your needs.
#include <QPushButton>
#include <QGridLayout>
#include <QApplication>
class AlignButton : public QPushButton {
Q_OBJECT
Qt::Alignment m_alignment;
Q_SLOT void clicked() {
m_alignment ^= Qt::AlignCenter;
parentWidget()->layout()->setAlignment(this, m_alignment);
label();
}
void label() {
setText(QString("Alignment = %1").arg(m_alignment));
}
public:
AlignButton(Qt::Alignment alignment, QWidget * parent = 0) :
QPushButton(parent),
m_alignment(alignment)
{
connect(this, SIGNAL(clicked()), SLOT(clicked()));
setSizePolicy(QSizePolicy::Expanding, QSizePolicy::Expanding);
label();
}
};
int main(int argc, char *argv[]) {
QApplication app(argc, argv);
QWidget window;
QGridLayout layout(&window);
layout.addWidget(new AlignButton(0), 0, 0, 0);
layout.addWidget(new AlignButton(Qt::AlignCenter), 1, 0, Qt::AlignCenter);
window.setMinimumSize(500, 200);
window.show();
return app.exec();
}
#include "main.moc"