Downloading txt file with qtnetwork

I am trying to download a txt file from a url in QT but i can't seem to make it work.
I am following this guide https://wiki.qt.io/Download_Data_from_URL. I implemented the filedownloader class exactly like it's made in the guide, but when i try to use it like specified in the guide I cannot make it work. I created a slot to be called when the download is finished, but if i try to call the downloader inside like the guide it says it is an undeclared identifier.
Does anyone know how to correctly implement this code?

this is the .cpp of my code

#include "mainwindow.h"
#include "ui_mainwindow.h"
#include "mainwindow.h"
#include <QStringList>
#include <QCoreApplication>
#include <QFile>
#include <QFileInfo>
#include <QList>
#include <QtNetwork/QNetworkReply>
#include <QStringList>
#include <QTimer>
#include <QUrl>
#include <QtNetwork/QNetworkAccessManager>
#include <QtNetwork/QNetworkRequest>
#include <filedownloader.h>
#include <iostream>
#include <QObject>
MainWindow::MainWindow(QWidget *parent)
    : QMainWindow(parent)
    , ui(new Ui::MainWindow)
{
    ui->setupUi(this);
    QUrl emiurl( "url of my file");
    // call to the downloader class.In the guide it's made differently, but it doesn't work
    FileDownloader emiload(emiurl,this); 
   //this connect links the end of the download with the textwriter slot
 QObject::connect(&emiload, SIGNAL (downloaded()), this, SLOT (textwriter()));  

}
>MainWindow::~MainWindow()
{
    delete ui;
}
//slot needed to create the txt file from the downloaded one
void MainWindow::textwriter() 
{
 QByteArray emibyte;
 emibyte=emiload->downloadedData(); //this line gives me error 
 QFile emifile("emi.txt");
 emifile.open(QIODevice::WriteOnly);
 std::cout << emibyte.size() << std::endl;
 QDataStream out(&emifile);
 out << emibyte;
 std::cout << emifile.size() << std::endl;
}

Now here's the .h

#ifndef MAINWINDOW_H
#define MAINWINDOW_H
#include "filedownloader.h"
#include <QMainWindow>
#include <QtNetwork/QNetworkAccessManager>
QT_BEGIN_NAMESPACE
namespace Ui { class MainWindow; }
QT_END_NAMESPACE
class MainWindow : public QMainWindow
{
    Q_OBJECT
public:
    MainWindow(QWidget *parent = nullptr);
    ~MainWindow();
private:
    Ui::MainWindow *ui;
private slots:
 void textwriter();
};
#endif // MAINWINDOW_H

To make the undeclared identifier go away and successfully compile, you need to add FileDownloader to the class' declaration. This way, it will be known throughout the class.

I chose to go with the approach that's usual in Qt, to declare a pointer to FileDownloader.

#pragma once // <--- this is supported by virtually any compiler today

#include <QMainWindow>
#include <QtNetwork/QNetworkAccessManager>
QT_BEGIN_NAMESPACE
namespace Ui { class MainWindow; }
QT_END_NAMESPACE

class FileDownloader; // <-- forward declaration is enough, but you can also #include "filedownloader.h"

class MainWindow : public QMainWindow
{
    Q_OBJECT
public:
    MainWindow(QWidget* parent = nullptr);
    ~MainWindow();
private:
    Ui::MainWindow* ui = nullptr;
    FileDownloader* emiload = nullptr; // <--- the important line!
private slots:
    void textwriter();
};

And then instantiate and call emiload in the constructor:

MainWindow::MainWindow(QWidget *parent)
    : QMainWindow(parent)
    , ui(new Ui::MainWindow)
{
    ui->setupUi(this);
    // create an instance of FileDownloader with "new".
    emiload = new FileDownloader(QUrl("url of my file"), this); 
    // using member pointer connection available since Qt5
    connect(emiload, &FileDownloader::downloaded, this, &MainWindow::textwriter);  
}