问使用QScrollArea时出错

EN

#include <QtCore>
#include <QtGui>
#include <QLabel>
#include <QScrollArea>
#include <QScrollBar>
#include <QFileDialog>
#include"QDebug"
#include<math.h>
#include<QApplication>
QScrollArea* scrollarea = new QScrollArea;

MainWindow::MainWindow(QWidget *parent) :
QMainWindow(parent),
ui(new Ui::MainWindow)
{
ui->setupUi(this);
ui->label->setBackgroundRole(QPalette::Base);
ui->label->setSizePolicy(QSizePolicy::Ignored, QSizePolicy::Ignored);
ui->label->setScaledContents(true);

scrollarea->setBackgroundRole(QPalette::Dark);
scrollarea->setWidget(ui->label);
setCentralWidget(scrollarea);
QImage img("D:\\picture.jpg");
ui->label->setPixmap(QPixmap::fromImage(img));
}

MainWindow::~MainWindow()
{
delete ui;
}