QPixmap load many images in Qt - c++

Question

I am trying to load images from a folder and view the previous and next images by clicking a button on the GUI (similar to Windows Image Viewer). The names of the images in this folder are xxxx_00.jpg to xxxx_99.jpg, so I use index++ and index-- to change the filename when clicking the buttons.
My codes work well for displaying the first image, but when I click a button to view the previous or the next images, it always shows
QPixmap::scaled: Pixmap is a null pixmap
and returns an empty image (the first image disappeared but new image didn't display).
Here's my code:
In mainwindow.cpp
void MainWindow::on_pushButton_4_clicked() //previous image
{
if (index < 1)
{
index = 99;
QLabel label;
label.setText("Go back");
label.show();
}
else
{
index--;
}
RefreshFilename();
loadimage();
}
void MainWindow::on_pushButton_5_clicked() //next image
{
if (index > 98)
{
index = 0;
QLabel label;
label.setText("ALL SET");
label.show();
}
else
{
index = index + 1;
}
RefreshFilename();
loadimage();
}
void MainWindow::loadimage()
{
// image.load(filename);
// im = image.scaled(500,500,Qt::KeepAspectRatio);
imageObject = new QImage();
imageObject->load(filename);
image = QPixmap::fromImage(*imageObject);
im = image.scaled(400,400,Qt::KeepAspectRatio);
scene = new QGraphicsScene(this);
scene->addPixmap(image);
scene->setSceneRect(image.rect());
ui->mainimage->setScene(scene);
}
I have spent 2 whole days on debugging this, but still have no idea. I am looking forward to any advice and support!
BTW The Refreshfilename Function works fine, so I did not paste it here.

Answer 1

Cause
Since I do not know what RefreshFilename(); does, I cannot tell exactly what the cause is.
However, I see a major flaw in your code, i.e. you create a new scene each time MainWindow::loadimage is called and this causes a memory leak.
When you provide more details I will be more specific here.
Solution
Set the scene once and add a QGraphicsPixmapItem to it, then in the loadImage update the pixmap of the item.
Keep the current number in a class attribute.
Again, I will be more specific once details are added.
Example
In any case (waiting for you to provide a MVCE), I have prepared a working example according to your task description:
#define IMAGE_COUNT 99
MainWindow::MainWindow(QWidget *parent) :
QMainWindow(parent),
m_imgNum(0),
m_item(new QGraphicsPixmapItem())
{
auto *widget = new QWidget(this);
auto *layoutMain = new QVBoxLayout(widget);
auto *layoutButtons = new QHBoxLayout();
auto *btnPrev = new QPushButton(tr("Previous"), this);
auto *btnNext = new QPushButton(tr("Next"), this);
auto *view = new QGraphicsView(this);
view->setScene(new QGraphicsScene(this));
view->scene()->addItem(m_item);
layoutButtons->addStretch();
layoutButtons->addWidget(btnPrev);
layoutButtons->addWidget(btnNext);
layoutButtons->addStretch();
layoutMain->addWidget(view);
layoutMain->addLayout(layoutButtons);
setCentralWidget(widget);
resize(640, 480);
loadImage();
connect(btnPrev, &QPushButton::clicked, [this](){
if (m_imgNum > 0)
m_imgNum--;
else
m_imgNum = IMAGE_COUNT;
loadImage();
});
connect(btnNext, &QPushButton::clicked, [this](){
if (m_imgNum < IMAGE_COUNT)
m_imgNum++;
else
m_imgNum = 0;
loadImage();
});
}
void MainWindow::loadImage()
{
m_item->setPixmap(QString("images/image_%1.jpg").arg(m_imgNum, 2, 10, QChar('0')));
}
where m_imgNum, m_item and loadImage are declared in the header as:
private:
inline void loadImage();
int m_imgNum;
QGraphicsPixmapItem *m_item;