Centering widgets in a dynamic layout? - qt

Question

I'm using the answer to this SO question to make a custom image widget which automatically scales correctly. It works fine but now I am trying to center the image widget instance at the center of my main window.
My idea was to create a QHBoxLayout, add the image widget to that and then add the hBox instance to the ui->verticalLayout.
Doesn't work. The image still displays flush left with the error message: QLayout: Attempting to add QLayout "" to MainWindow "MainWindow", which already has a layout
I then tried a few variations on 'setAlignment` but then the image doesn't appear at all. My simple test code is below.
What am I missing here?
MainWindow::MainWindow(QWidget *parent) :
QMainWindow(parent),
ui(new Ui::MainWindow)
{
ui->setupUi(this);
QPixmap pix;
pix.load("/Users/home/Desktop/test.jpg");
ImageLabel2* image = new ImageLabel2(this);
image->setPixmap(pix);
QHBoxLayout* hbox = new QHBoxLayout(this);
hbox->addWidget(image);
// hbox->setAlignment(image,Qt::AlignCenter);
hbox->setAlignment(Qt::AlignHCenter);
// ui->verticalLayout->addLayout(hbox);
ui->verticalLayout->addLayout(hbox);
// ui->verticalLayout->addWidget(image);
// ui->verticalLayout->setAlignment(image,Qt::AlignCenter);
// ui->verticalLayout->setAlignment(Qt::AlignHCenter);
}

Answer 1

Try this:
QHBoxLayout* hbox = new QHBoxLayout(this);
hbox->addStretch();
hbox->addWidget(image);
hbox->addStretch();

Answer 2

None of the suggestions here worked for me. Don't know why and it seems hard to debug layout problems
However a reply to my question on the Qt Project.org site works perfectly. So not my solution but I am posting it here since this "centering/resizing image" issue seems a common problem.
class CustomWidget : public QWidget {
public:
CustomWidget(const QPixmap &p, QWidget* parent = 0)
: QWidget(parent), pixmap(p) {}
void paintEvent(QPaintEvent * e)
{
QRect srcRect(QPoint(), pixmap.size());
QSize dstSize = srcRect.size().scaled(
e->rect().size(), Qt::KeepAspectRatio);
QRect dstRect(QPoint((width() - dstSize.width())/2,
(height() - dstSize.height())/2), dstSize);
QPainter p(this);
p.setRenderHint(QPainter::Antialiasing);
p.drawPixmap(dstRect, pixmap, srcRect);
}
private:
QPixmap pixmap;
};
and then in the main window:
MainWindow::MainWindow(QWidget *parent) :
QMainWindow(parent),
ui(new Ui::MainWindow)
{
ui->setupUi(this);
setCentralWidget(new CustomWidget(QPixmap("test.png")));
}

Answer 3

main window(QMainwindow) API setCentralWidget to align the widget on center.
Note:Concept of centrewidget, is to differentiate the docking area( Left, Right, Bottom, Top ).
with out a centre how some one know who is where. when we are developing with QMainWindow, you can see in UI_MainWindow.h, setting Centrewidget with dummy QWidget
Below code will work
#include "mainwindow.h"
#include <QLabel>
#include <QHBoxLayout>
MainWindow::MainWindow(QWidget *parent) :
QMainWindow(parent),
ui(new Ui::MainWindowClass)
{
ui->setupUi(this);
QPixmap pix;
pix.load("C:\\Users\\user\\Desktop\\Uninstallation failure2.png");
//Replace with ImageLabel2
QLabel* image = new QLabel(this);
image->setPixmap(pix);
QHBoxLayout* hbox = new QHBoxLayout(this);
hbox->addWidget(image);
QWidget* centreWidget = new QWidget();
//QMainwindow, having a feature called centreWidget, to set the layout.
centreWidget->setLayout( hbox );
setCentralWidget( centreWidget );
}
MainWindow::~MainWindow()
{
}