QScrollArea contents not showing - qt

Question

Below I present a full Qt project in which I was expecting to see a label and a button showing in a scrollarea, but it seems I'm not expressing myself as Qt expects. What Qt expects?
project.pro
QT += core gui
greaterThan(QT_MAJOR_VERSION, 4): QT += widgets
TARGET = untitled13
TEMPLATE = app
SOURCES += main.cpp\
widget.cpp
HEADERS += widget.h
main.cpp
#include "widget.h"
#include <QApplication>
int main(int argc, char *argv[])
{
QApplication a(argc, argv);
Widget w;
w.show();
return a.exec();
}
widget.h
#ifndef WIDGET_H
#define WIDGET_H
#include <QWidget>
class Widget : public QWidget
{
Q_OBJECT
public:
Widget(QWidget *parent = 0);
~Widget();
};
#endif // WIDGET_H
widget.cpp
#include "widget.h"
#include <QHBoxLayout>
#include <QScrollArea>
#include <QLabel>
#include <QPushButton>
Widget::Widget(QWidget *parent)
: QWidget(parent)
{
QHBoxLayout *mainLayout = new QHBoxLayout(this);
QScrollArea *scrollArea = new QScrollArea;
mainLayout->addWidget(scrollArea);
QWidget *widget = new QWidget;
scrollArea->setWidget(widget);
QHBoxLayout *hLayout = new QHBoxLayout;
widget->setLayout(hLayout);
QLabel *label = new QLabel("somelable");
hLayout->addWidget(label);
QPushButton *bt = new QPushButton("click");
hLayout->addWidget(bt);
}
Widget::~Widget()
{
}

Answer 1

In this specific code the problem is that you call
scrollArea->setWidget(widget);
before you add layout of the widget. Move the above mentioned call to the end of the constructor and you see the label and the button inside the scrollarea.
It's also mentioned in the QScrollArea::setWidget documentation:
Note that You must add the layout of widget before you call this
function; if you add it later, the widget will not be visible -
regardless of when you show() the scroll area.