Make QMainWindow scrollable

[longWorm]

hello everybody.
I can't add scrollbar to my widget. I've tried everything, here is the latest version:
@
#include "mainwindow.h"

MainWindow::MainWindow(QWidget *parent)
: QMainWindow(parent)
{
setFixedSize(640,640);

addOrderButton = new QPushButton(QString::fromLocal8Bit("Добавить заказ"),this);
connect(addOrderButton, SIGNAL(clicked()),this, SLOT(addOrder()));
orders = new QVector<QWidget*>();
layout = new QVBoxLayout;

scrollArea = new QScrollArea;
setCentralWidget(scrollArea);
addOrder();
layout->addWidget(addOrderButton);
scrollArea->setLayout(layout);

}

MainWindow::~MainWindow()
{

}

void MainWindow::addOrder()
{
orderWidget* w = new orderWidget(this);
orders->append(w);
layout->insertWidget(orders->count() - 1, w);
}
@

Sorry for the stupid question but i really stuck with it. What am i doing wrong?

[noregister]

@layout->insertWidget(orders->count() - 1, w);@

here:
@orders->count() - 1 == -1@

You should
@layout->addWidget(addOrderButton);
orders->append(addOrderButton);@

[longWorm]

After i make orders->append(w); count() already can't be equal to 0.
And even if it was so, i don't understand how your advise can solve my problem