My header file:

#ifndef DIALOGTEST_H
#define DIALOGTEST_H

#include <QDialog>

class QPushButton;
class QComboBox;
class QVBoxLayout;
class QHBoxLayout;
class QLabel;

class DialogTest : public QDialog
{
    Q_OBJECT

public:
    DialogTest(QWidget *parent = nullptr);

private slots:
    void saveBtnClicked();
    void itemChanged(int);
    void init();
    void load();

private:
    QComboBox* cboBox = nullptr;

    QHBoxLayout* topHLayout = nullptr;
    QHBoxLayout* middleHLayout = nullptr;
    QHBoxLayout* bottomHLayout = nullptr;
    QVBoxLayout* mainLayout = nullptr;

    QLabel* select = nullptr;
    QLabel* ItemSelected = nullptr;
    QPushButton* save = nullptr;
};
#endif // DIALOGTEST_H

The implementation:

#include "DialogTest.h"
#include <QPushButton>
#include <QComboBox>
#include <QHBoxLayout>
#include <QVBoxLayout>
#include <QMessageBox>
#include <QSettings>
#include <QLabel>
#include <QDebug>

DialogTest::DialogTest(QWidget *parent)
    : QDialog(parent)
{
    select = new QLabel(tr("Select an Item"));
    cboBox = new QComboBox;
    ItemSelected = new QLabel(tr("Selected Item"));
    save = new QPushButton(tr("&Save"));

    topHLayout = new QHBoxLayout;
    topHLayout->addWidget(select);
    topHLayout->addWidget(cboBox);

    middleHLayout = new QHBoxLayout;
    middleHLayout->addWidget(ItemSelected);
    middleHLayout->addStretch();

    bottomHLayout = new QHBoxLayout;
    bottomHLayout->addStretch();
    bottomHLayout->addWidget(save);

    mainLayout = new QVBoxLayout;
    mainLayout->addLayout(topHLayout);
    mainLayout->addLayout(middleHLayout);
    mainLayout->addLayout(bottomHLayout);

    setLayout(mainLayout);
    
    init();
    load();

    connect(save, &QPushButton::clicked, this, &DialogTest::saveBtnClicked);
    connect(cboBox, SIGNAL(currentIndexChanged(int)), this, SLOT(itemChanged(int)));
}

void DialogTest::saveBtnClicked()
{
   QSettings settings;
   settings.setValue("settings", cboBox->currentIndex());
   QMessageBox::information(this, "Save", "Your item is saved, Please re-open the application");
}

void DialogTest::itemChanged(int index)
{
  ItemSelected->setText(QString::number(index) + " = " + cboBox->currentText());
}

void DialogTest::init()
{
    cboBox->clear();

    for (int i=0; i<10; i++)
        cboBox->addItem("Item number " + QString::number(i));
}

void DialogTest::load()
{
   QSettings settings;
   QVariant value = settings.value("settings",0);

   bool ok;
   int index = value.toInt(&ok);

   if(!ok)
   {
       QMessageBox::critical(this, "Error", "Error in loading the file");
       return;
   }

   if(index < cboBox->count()) 
    cboBox->setCurrentIndex(index);
  
   else 
    cboBox->setCurrentIndex(0);
}

The problem is that the program can't return the value and it's always "Item number 0"!