Problem with variable in main.cpp and mainwindow.cpp

[adis1001]

Hi.
I need a help.
I have a variable int bit2, which is a mainwindow.h.
In main.cpp I calculate her value, and I want to send this value to mainwindow.cpp. Is this possible?
I include part of the cod:

mainwindow.h

#ifndef MAINWINDOW_H
#define MAINWINDOW_H
 
 
#include <QMainWindow>
static int bit2;
namespace Ui {
class MainWindow;
 
}
 
class MainWindow : public QMainWindow
{
    Q_OBJECT
 
 
public:
    explicit MainWindow(QWidget *parent = 0);
    ~MainWindow();
    int flag;
 void zapisz(int wartosc);
 
 
 
private slots:
 
 
private:
    Ui::MainWindow *ui;
 
 
};
 
#endif // MAINWINDOW_H

#ifndef MAINWINDOW_H
#define MAINWINDOW_H


#include <QMainWindow>
static int bit2;
namespace Ui {
class MainWindow;

}

class MainWindow : public QMainWindow
{
    Q_OBJECT


public:
    explicit MainWindow(QWidget *parent = 0);
    ~MainWindow();
    int flag;
 void zapisz(int wartosc);



private slots:


private:
    Ui::MainWindow *ui;


};

#endif // MAINWINDOW_H

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main.cpp

int main(int argc, char **argv)
{
    QApplication a(argc, argv);
    MainWindow w;
    w.show();
...
...
w.zapisz(51);

int main(int argc, char **argv)
{
    QApplication a(argc, argv);
    MainWindow w;
    w.show();
...
...
w.zapisz(51);

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mainwindow.cpp

#include "mainwindow.h"
#include "ui_mainwindow.h"
 
 
MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::MainWindow)
{
    ui->setupUi(this);
    QPixmap pix("/home/bananapi/wizu-build-desktop-Qt_4_8_2__System__Release/new_grn.bmp");
    QPixmap pix2("/home/bananapi/wizu-build-desktop-Qt_4_8_2__System__Release/new_red.bmp");
 
    ui->label->setPixmap(pix);
    ui->label_2->setPixmap(pix);
    ui->label_3->setPixmap(pix);
    ui->label_4->setPixmap(pix);
    flag=0;
int bit;
bit=bit2;//TU JEST BLAD ;/
...
...
void MainWindow::zapisz(int wartosc)
{
    bit2=wartosc;
}

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


MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::MainWindow)
{
    ui->setupUi(this);
    QPixmap pix("/home/bananapi/wizu-build-desktop-Qt_4_8_2__System__Release/new_grn.bmp");
    QPixmap pix2("/home/bananapi/wizu-build-desktop-Qt_4_8_2__System__Release/new_red.bmp");

    ui->label->setPixmap(pix);
    ui->label_2->setPixmap(pix);
    ui->label_3->setPixmap(pix);
    ui->label_4->setPixmap(pix);
    flag=0;
int bit;
bit=bit2;//TU JEST BLAD ;/
...
...
void MainWindow::zapisz(int wartosc)
{
    bit2=wartosc;
}

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When I write in mainwindow.cpp ¨bit=51¨ (value which I want calculate in main.cpp) programm is working. In my opinion bit2 in main.cpp and bit2 in mainwindow.cpp is different variable. Can someone help? There is any way to send value from main.cpp to mainwindow.cpp?

[jefftee]

Don't use a global variable for this. Either pass the variable to the constructor of MainWindow or add a method to MainWindow where you can pass the variable as a function argument and store in class member variable, etc.

Edit: I see now that you did try to add a method to set the value, but instead of declaring bit2 as a static global variable, make it a private member variable of MainWindow, then assign the value passed in your MainWindow::zapisz method.

// MainWindow.h
class MainWindow
{
...
private:
    int bit2;
}
 
// MainWindow.cpp
void MainWindow::zapisz(int wartosc)
{
    bit2 = wartosc;
}

// MainWindow.h
class MainWindow
{
...
private:
    int bit2;
}

// MainWindow.cpp
void MainWindow::zapisz(int wartosc)
{
    bit2 = wartosc;
}

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Does that not do what you want?

[adis1001]

I made in private, like you, but still isn´t work. While I use a method m.zapisz(51), value 51 is in bit2, or m.bit2? Because if in m.bit2, mainwindow.cpp can´t see this value :/

[stampede]

As a general note, don't define static variables in header files, as each .cpp file which includes the header will get its own copy of the variable.
Typically you only declare the variable in header:

// header.h
extern int my_var;

// header.h
extern int my_var;

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and define it in exactly one .cpp file:

// impl.cpp
#include "header.h"
 
int my_var = 0;

// impl.cpp
#include "header.h"

int my_var = 0;

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In your case I suggest not using the global variable at all anyway.

[adis1001]

Hi, Iḿ not sure I understand you.
I should write extern int bit in private section in mainwindow.h?
And then, where I should write this:

// impl.cpp
#include "header.h"

int my_var = 0;


impl.cpp is my main.cpp or mainwindow.cpp? and int my_var is my bit?

[stampede]

I should write extern int bit in private section in mainwindow.h?

No no, just do that as @jefftee suggested - remove the static variable completely. Use only member variable in the MainWindow class.
I mentioned the "extern" keyword as a sidenote, so that you know it for future.

[adis1001]

Ok, i did it how @jefftee show, but still doesn´t work how I want :/ In main.cpp i have w.zapisz(51), so bit should have value 51, but in mainwindow.cpp bit don´t have a value :/

[anda_skoa]

Are you sure you are reading the variable after your wrote to it?

Cheers,
_

[jefftee]

Repost your current code for all of main.cpp, MainWindow.cpp, and MainWindow.h.