qt5 (cpp) : how to center the QApp.

app.cpp

#include "app.h"
#include "ui_app.h"

app::app(QWidget *parent)
    : QMainWindow(parent)
    , ui(new Ui::app)
{
    ui->setupUi(this);
    connect(this->ui->hashButton, SIGNAL (released()), this, SLOT (handleHashButton()));
    this->ui->input->setFocus();
    hash_algo_list += {"SHA256","SHA1","SHA224","SHA384", "SHA512", "MD4","MD5"};
    this->ui->hash_cb->addItems(hash_algo_list);
    this->center();
}



void app::center(){
    // geometry of the main window
    QRect qr = this->frameGeometry();

    // center point of screen
    QScreen *qc = QGuiApplication::primaryScreen();
    QPoint cp = qc->geometry().center();

    // move rectangle's center point to screen's center point
    qr.moveCenter(cp);

    // top left of rectangle becomes top left of window centering it
    this->move(qr.topLeft());
}


void app::handleHashButton(){
    QByteArray origin = this->ui->input->text().toUtf8();
    QByteArray hashed;
    switch (this->ui->hash_cb->currentIndex()) {
        case 0:
            hashed = QCryptographicHash::hash(origin, QCryptographicHash::Sha256);
            break;
        case 1:
            hashed = QCryptographicHash::hash(origin, QCryptographicHash::Sha1);
            break;
        case 2:
            hashed = QCryptographicHash::hash(origin, QCryptographicHash::Sha224);
            break;
        case 3:
            hashed = QCryptographicHash::hash(origin, QCryptographicHash::Sha384);
            break;
        case 4:
            hashed = QCryptographicHash::hash(origin, QCryptographicHash::Sha512);
            break;
        case 5:
            hashed = QCryptographicHash::hash(origin, QCryptographicHash::Md4);
            break;
        case 6:
            hashed = QCryptographicHash::hash(origin, QCryptographicHash::Md5);
            break;
    }
    this->ui->hash_result->setText(hashed.toHex());
}

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


void app::on_input_returnPressed()
{
    this->handleHashButton();
}

void app::on_hash_cb_currentTextChanged()
{
    if (!(this->ui->input->text() == ""))
        this->handleHashButton();
}

app.h

#ifndef APP_H
#define APP_H

#include <QMainWindow>
#include <QDesktopWidget>
#include <QScreen>
#include <QCryptographicHash>
#include <QDebug>

QT_BEGIN_NAMESPACE
namespace Ui { class app; }
QT_END_NAMESPACE

class app : public QMainWindow
{
    Q_OBJECT

public:
    app(QWidget *parent = nullptr);
    ~app();


private slots:
    void handleHashButton();
    void on_input_returnPressed();
    void on_hash_cb_currentTextChanged();
    void center();

private:
    Ui::app *ui;
    QStringList hash_algo_list;
};
#endif // APP_H