How can I use common variable in multiple cpp file in Qt..? - c++

Question

#include "main.h"
#include "ui_main.h"
Main::Main(QWidget *parent):
QMainWindow(parent),
ui(new Ui::Main)
{
ui->setupUi(this);
}
Main::~Main()
{
delete ui;
}
I have this header file main for Qt in which part should I create global variable

Answer 1

You can make use of extern variables. declare and define those variables
in any .h or .cpp file and you can use them in other files using extern
keyword and that variable name.
e.g. in my main.cpp I have Qstring A = "xyz";
suppose I have mainwindow.h and mainwindow.cpp
in mainwindow.h
extern Qstring A;
you will get the value of A in mainwindow.

Answer 2

If you really want to do that then you need to implement a Singleton class.
And after that you can use it like this:
class Common final
{
public:
Common& instance() // In the cpp.
{
static Common common;
return common;
}
static int value() // In the cpp.
{
return 11;
}
static std::string text() // In the cpp.
{
return "desired text";
}
private:
Common() // In the cpp.
{
}
};
Or just add some variables in a separate header file, but with OO I think it's better inside a class (or at least inside a namespace).

Answer 3

To use the variable of file in another file...
1.in the header file make a variable publically...
2.include that header file in another .cpp file...
3.to use the variable of that file make an object of the file and by using member operator...
//Main header file...
#ifndef MAINWINDOW_H
#define MAINWINDOW_H
#include <QMainWindow>
#include "a.h"
namespace Ui {
class MainWindow;
}
class MainWindow : public QMainWindow
{
Q_OBJECT
public:
explicit MainWindow(QWidget *parent = 0);
int i=10;
~MainWindow();
private slots:
void on_pushButton_clicked();
private:
Ui::MainWindow *ui;
};
#endif // MAINWINDOW_H
//in cpp file og
#include "ui_mainwindow.h"
MainWindow::MainWindow(QWidget *parent) :
QMainWindow(parent),
ui(new Ui::MainWindow)
{
ui->setupUi(this);
}
MainWindow::~MainWindow()
{
delete ui;
}
void MainWindow::on_pushButton_clicked()
{
a obj;
obj.setModal(true);
obj.exec();
}
//the cpp file in which you want to use the variable...
//here make an object of the header file an use member operator...
#include "a.h"
#include "ui_a.h"
#include "mainwindow.h"
a::a(QWidget *parent) :
QDialog(parent),
ui(new Ui::a)
{
ui->setupUi(this);
}
a::~a()
{
delete ui;
}
void a::on_pushButton_clicked()
{
MainWindow obj;
qDebug()<<obj.i;
}