Weird problem, with MainWindow class variables.

[Rondog]

In your code you have this:

@
QApplication a(argc, argv);
MainWindow w;
Own own;
...
@

These exist only in main() which will run until a.exec() returns. For the classes 'w' and 'own' is there a pointer, or some other reference to these classes that you can use outside of main()?

For this code:

@
MainWindow mw;
int width, height;

    height = mw.GetImageHeight();
    width = mw.GetImageWidth();

@

GetImageHeight() and GetImageWidth() should return zero or possibly some other random value as it is another, unrelated class.

[AndreasA9]

Sorry, i don't understand, what you are trying to say to me.

[andreyc]

In your example (below), are you trying to get width and height from MainWindow before app.exec() ?
@
MainWindow mw;
int width, height;

    mw.GetImageHeight(height);
    mw.GetImageWidth(width);

@

If so then MainWindow::imageHeight and imageWidth are not initialized and you are getting whatever random numbers are there.

[AndreasA9]

No, im not trying to get them before app.exec()
getImageHeight and getImageWidth are exectued after i press a button on my form and long proccess is done with network communication is done

[AndreasA9]

I post my full source code here

mainwindow.h
@
#ifndef MAINWINDOW_H
#define MAINWINDOW_H

#include <QMainWindow>
#include "networking.h"
namespace Ui {
class MainWindow;
}

class MainWindow : public QMainWindow
{
Q_OBJECT

public:

explicit MainWindow(QWidget *parent = 0);
void GetImageHeight(int &height);
void GetImageWidth(int &width);

~MainWindow();

private slots:

void on_pushButton_clicked();
void on_WidthSlider_valueChanged(int value);
void on_HeightSlider_valueChanged(int value);

private:
int imageWidth;
int imageHeight;

QUrl q;
Ui::MainWindow *ui;

};

#endif // MAINWINDOW_H

@

networking.h

@
#ifndef NETWORKING_H
#define NETWORKING_H

#include <QtNetwork/QNetworkAccessManager>
#include <QtNetwork/QNetworkRequest>
#include <QMessageBox>
#include "mainwindow.h"

class networking : public QObject
{
Q_OBJECT

public:
    networking();

    void on_download_triggered(QUrl url);
    bool IsNetworkAvailable();


private:
    QNetworkAccessManager *manager = new QNetworkAccessManager(this);
    void ProcessPicture(QImage *image);

private slots:
void onNetworkReply(QNetworkReply* reply);

};

#endif // NETWORKING_H

@

mainwindow.cpp

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

MainWindow::MainWindow(QWidget *parent) :
QMainWindow(parent),
ui(new Ui::MainWindow)
{

ui->setupUi(this);
this->setFixedSize(378, 304);

}

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

void MainWindow::on_pushButton_clicked()
{

if(ui->URLbox->text().isEmpty()) QMessageBox::information(0, "Warning", "No URL inserted!");

else
{
    networking *n = new networking();
    q = ui->URLbox->text();
    if(n->IsNetworkAvailable()) n->on_download_triggered(q);
}

}

void MainWindow::GetImageHeight(int &height) {height = imageHeight;}

void MainWindow::GetImageWidth(int &width) {width = imageWidth; }

void MainWindow::on_WidthSlider_valueChanged(int value)
{
imageWidth = value;
QString valueString = QString::number(value);
ui->WidthLabel->setText(valueString);
}

void MainWindow::on_HeightSlider_valueChanged(int value)
{
imageHeight = value;
QString valueString = QString::number(value);
ui->HeightLabel->setText(valueString);
}

@

networking.cpp

@#include "networking.h"
#include <QtNetwork/QNetworkReply>
#include <QFileDialog>

networking::networking()
{
connect(manager, SIGNAL(finished(QNetworkReply*)), this, SLOT(onNetworkReply(QNetworkReply*)));
}
void networking::on_download_triggered(QUrl url)
{
manager->get(QNetworkRequest(QUrl(url)));
}

void networking::ProcessPicture(QImage *image)
{
MainWindow *mw = new MainWindow();

int width, height;

mw->GetImageHeight(height);
mw->GetImageWidth(width);


qDebug() << height;
qDebug() << width;

QImage newimage = image->scaled(width, height, Qt::KeepAspectRatio);
QString path =  QFileDialog::getSaveFileName(0, tr("Save Image"), "C://", tr("Image Files (*.png *.jpg *.bmp)"));

newimage.save(path);

}

void networking::onNetworkReply(QNetworkReply* reply)
{
if(!reply->error())
{
QImage *img = new QImage();
img->loadFromData(reply->readAll());

    if(img->isNull())
            QMessageBox::critical(0, "Error", "Inserted URL is not an image!");

    ProcessPicture(img);

}
else
{
    QVariant statusCode = reply->attribute(QNetworkRequest::HttpStatusCodeAttribute);
    int status = statusCode.toInt();

    qDebug() << status;

    switch(status)
    {
        case 0:
        {
            QMessageBox::critical(0, "Error", "URL doesn't exist!");
            break;
        }
        case 404:
        {
            QMessageBox::critical(0, "Error", "404 not found!");
            break;
        }
    }
}

}

bool networking::IsNetworkAvailable()
{
QNetworkAccessManager m;
QNetworkReply *check = m.get(QNetworkRequest(QUrl("google.com")));
return (!check->error()) ? true : false;
}
@

[Rondog]

I think this is your problem:

@
void networking::ProcessPicture(QImage *image)
{
MainWindow *mw = new MainWindow();

int width, height;

mw->GetImageHeight(height);
mw->GetImageWidth(width);

qDebug() << height;
qDebug() << width;

...
}
@

The class '*mw' is unrelated to the mw class created in main(). The member imageWidth and imageHeight are not updated in this version.

To verify this you could create a global pointer of the MainWindow class and see if this solves the problem:

@
MainWindow *g_mw;

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    MainWindow w;
    Own own;

    g_mw = &mw;

 
    QObject::connect(&w, SIGNAL(heightValueChanged(int)), &own, SLOT(imageHeight(int)));
    QObject::connect(&w, SIGNAL(widthValueChanged(int)), &own, SLOT(imageWidth(int)));
 
    w.show();
 
    return a.exec();
}

@

and then in your network module

@
#include "networking.h"
#include <QtNetwork/QNetworkReply>
#include <QFileDialog>

extern MainWindow *g_mw;

networking::networking()
{
...
}
...
void networking::ProcessPicture(QImage *image)
{
// MainWindow *mw = new MainWindow();

int width, height;

g_mw->GetImageHeight(height);
g_mw->GetImageWidth(width);


qDebug() << height;
qDebug() << width;

...
}
@

Note: I wouldn't use a global variable like this. You are better to pass a pointer (maybe as part of the constructor of your Networking class?). It is just a simple way to verify this is the problem without changing too much.

[andreyc]

Rondog is right. You are creating new MainWindow object and it is not related to your main MainWindow.

Another option to solve it would be to pass a MainWindow pointer to networking ctor and keep it there. Then in networking::ProcessPicture() use that pointer.
networking.h

@
#ifndef NETWORKING_H
#define NETWORKING_H

class MainWindow;
 
class networking : public QObject
{

...
public:
networking(MainWindow* mw);

...
private:
MainWindow* m_mw;

};
#endif // NETWORKING_H

@

networking.cpp
@
#include "networking.h"
#include "MainWindow.h"
#include <QtNetwork/QNetworkReply>
#include <QFileDialog>

networking::networking(MainWindow* mw) : m_mw(mw)
{

...
}

void networking::ProcessPicture(QImage *image)
{
// MainWindow *mw = new MainWindow();

int width, height;

m_mw->GetImageHeight(height);
m_mw->GetImageWidth(width);


qDebug() << height;
qDebug() << width;

...
}
@

[AndreasA9]

Thanks, it worked! I didn't know, why i didn't realise fault myself. Quite logical. Thanks again!

[AndreasA9]

One last question, why i had to write class MainWindow; to my networking.h file.

[andreyc]

It is a forward declaration. Compiler needs to know that a type exists but does not need to know the details. It works with the pointers and the references. Helps to avoid including unneeded headers.