class Ui::MainWindow has no member named horizontalSlider

[alexcodercpp]

I was following a signal&slot tutorial I found out on youtube on how to create a progressBar and horizontalSlider just by coding, without drag&drop.

This is the code (mainwindow.cpp file):

#include "mainwindow.h"
#include "ui_mainwindow.h"
 
MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::MainWindow)
{
   ui->setupUi(this);
    connect(ui->horizontalSlider,
            SIGNAL(valueChanged(int)),
            ui->progressBar,
            SLOT(setValue(int)));
}
 
MainWindow::~MainWindow()
{
    delete ui;
}

#include "mainwindow.h"
#include "ui_mainwindow.h"

MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::MainWindow)
{
   ui->setupUi(this);
    connect(ui->horizontalSlider,
            SIGNAL(valueChanged(int)),
            ui->progressBar,
            SLOT(setValue(int)));
}

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

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As you see, all the job was to add the text in the constructor.
It fails to compile on my laptop: "class Ui::MainWindow has no member named horizontalSlider and class Ui::MainWindow has no member named progressBar".
However on the video I was following it compiles and the author didn't make any additional writings.
I would highly appreciate your suggestions, guys.
I'm using Qt 5.4.1, maybe it's because of version inconsistencies and that tutorial is outdated?