How to declare global variable in QT?

[TomNow99]

@Christian-Ehrlicher Thank you for answer. In last post I paste all my deb class code.

Here is my mainWindow code:

#ifndef MAINWINDOW_H
#define MAINWINDOW_H

#include <QMainWindow>
#include "deb.h"
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;
};
#endif // MAINWINDOW_H

#include "mainwindow.h"
#include "ui_mainwindow.h"
#include <QDebug>
MainWindow::MainWindow(QWidget *parent)
    : QMainWindow(parent)
    , ui(new Ui::MainWindow)
{
    ui->setupUi(this);
    qDebug()<<x;
}

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

I don't have other x function or variable.

[Christian Ehrlicher]

@TomNow99 said in How to declare global variable in QT?:

I don't have other x function or variable.

You have: https://doc.qt.io/qt-5/qwidget.html#x-prop

Use ::x - but it's c++ basics I would guess and has nothing to do with Qt :)

[TomNow99]

@Christian-Ehrlicher It was.... Wow :D Thank you very much! Yes, x and y is position :D Thank you!

[JonB]

@TomNow99
If in your deb.h file you show you have int x=4; in addition to extern int x;, then at least when you include that into multiple .cpp files you will end up with "multiple definitions of x variable" when you link. Definitions/initializations of "global" variables belong in one .cpp file, not in a .h file. EDIT Ah perhaps you are already doing this, I can't figure where what you show is situated.

OK, I see what is @Christian-Ehrlicher is spotting. One obvious conclusion: if you are going to have "global" variables, don't name them anything like x!!

[TomNow99]

@JonB When I delete this line "extern int x" from deb.h I have error in mainWindow "use of undeclared identifier xx".

[JonB]

@TomNow99
Of course, why would you delete the extern?? As I said, I was talking about the int x = 4 line, which I (at least originally) thought was in your deb.h. If that is in a .cpp and not a .h then you are fine....

[TomNow99]

@JonB @Christian-Ehrlicher I have one more question.

I need only one thing to have everything, what I would like.

When I look at std::cout I can use it everywhere ( of course when I include iostream library ), but I can't create next std::ostream object ( because of protected constructor ). I would like the same in my project ( can't create other than that global object ).

But when I change public to private constructor of my class, I will not create object in deb.cpp ( here I would like to create my object. Only here! ). I know that I can create some function singleton, but I would like that trick, which is in iostream and cout.

So I would like have:

#ifndef DEB_H
#define DEB_H

class deb
{
private:
    deb();
};

extern deb xx;
#endif // DEB_H

and

#include "deb.h"

deb xx; // error - constructor is private

deb::deb()
{

}

[Christian Ehrlicher]

You mix something up - std::cout is no std::ostream - it's a global object which takes a std::ostream via the << operator.
std::ostream is a base class of the different output possibilities like iostream or ofstream.

[TomNow99]

@Christian-Ehrlicher Here:

https://en.cppreference.com/w/cpp/io/cout

I see ( on the top ):

extern std::ostream cout;

EDIT:
So I tried create std::ostream cout2; and I can't

EDIT2:
So cout is object of class... ? Which one?

[jsulm]

@TomNow99

extern std::ostream cout;

declares a variable, so an instance/object.

[TomNow99]

@jsulm Yes, but here I see that cout is object of std::sotream class.