Qt GUI print global variables

[jonagf]

Hi!

I'm very new at Qt and c++ programming, and I created a Qt GUI and have some global variables that I change when my callbacks are called.
I would like to print those global variables in my terminal, so I wrote in my main.cpp the following:

#include "ros/ros.h"
 
#include "mainwindow.h"
#include <QApplication>
#include "globals.h"
#include "qtgui/GUIDados.h"
 
using namespace std;
 
namespace global2
{
int numberOfMotors;
int intensidade[16];
int local[16];
}
 
int main(int argc, char *argv[])
{
 
    QApplication a(argc, argv); //creates a QApplication object
    MainWindow w; //creates the main window object
    w.show();
 
    cout << "test: " << global2::numberOfMotors << endl;
    return a.exec(); //enter its event loop
 
}

#include "ros/ros.h"

#include "mainwindow.h"
#include <QApplication>
#include "globals.h"
#include "qtgui/GUIDados.h"

using namespace std;

namespace global2
{
int numberOfMotors;
int intensidade[16];
int local[16];
}

int main(int argc, char *argv[])
{

    QApplication a(argc, argv); //creates a QApplication object
    MainWindow w; //creates the main window object
    w.show();

    cout << "test: " << global2::numberOfMotors << endl;
    return a.exec(); //enter its event loop

}

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However, I always get test 0 and nothing else. In my mainwindow.cpp I am able to write those global variables in a label in my gui, like for example:
(this is just an excerpt of my mainwindow.cpp code)

#include "ros/ros.h"
 
#include "mainwindow.h"
#include "ui_mainwindow.h"
#include <QtGui/QPushButton>
#include "globals.h"
#include <QString>
#include <string>
#include <sstream>
#include "QMessageBox"
 
 
namespace global
{
int numberOfMotors;
int intensidade[16];
int local[16];
}
 
using namespace std;
 
void MainWindow::on_sendButton_clicked()
{
    QMessageBox::information(this,tr("Titulo"),tr("escolheu: %1").arg( global::intensidade[0]));
    ui->label1->setText(QString::number(global::numberOfMotors)); 
}

#include "ros/ros.h"

#include "mainwindow.h"
#include "ui_mainwindow.h"
#include <QtGui/QPushButton>
#include "globals.h"
#include <QString>
#include <string>
#include <sstream>
#include "QMessageBox"


namespace global
{
int numberOfMotors;
int intensidade[16];
int local[16];
}

using namespace std;

void MainWindow::on_sendButton_clicked()
{
    QMessageBox::information(this,tr("Titulo"),tr("escolheu: %1").arg( global::intensidade[0]));
    ui->label1->setText(QString::number(global::numberOfMotors)); 
}

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Can you explain me what I'm doing wrong?

[jefftee]

Your main window's event loop isn't started until line 25 is executed in your main routine, so events are not processed until after you've already executed line 24 and printed out the value. BTW, global variables are evil and should be avoided at all costs...

[d_stranz]

If this is your actual code and you really are declaring the variables in the global2 namespace twice, then the ones declared in main.cpp are not the same as the ones in MainWindow.cpp and so are never initialized. That the call to cout prints "0" and not gibberish is probably the result of a friendly compiler that initializes variables to zero in release mode for you.

Time to read up on your basic C++ on variables and scoping. Here's a start.

@jefftee: This has nothing to do with the event loop or event processing. cout is plain old C++. And unfortunately, Arduino and RasPi coding books have brought back the evil global variable with a vengeance. From the names of the variables, this seems to be what this is about.

[jefftee]

@jefftee: This has nothing to do with the event loop or event processing. cout is plain old C++. And unfortunately, Arduino and RasPi coding books have brought back the evil global variable with a vengeance. From the names of the variables, this seems to be what this is about.

Thanks, I jumped to the wrong conclusion... I still feel that global variables are an indication that you have poorly designed your classes/interfaces and a crutch used by too many people just to make it work...

Edit: after reviewing the code, he's creating the same variable names in two different namespaces (global and global2) so they are indeed different variables and I don't see where they are ever initialized either.

[d_stranz]

he's creating the same variable names in two different namespaces (global and global2)

Ah, and I missed that detail. Even if he does initialize the variables in the "global" namespace, the variables in "global2" remain uninitialized.

And I agree about global variables also. The closest I ever come are static const variables in cpp module scope or singleton classes that can only have one instance. But as I said, pick up any Arduino or Raspberry Pi coding book and they are in every example. It might be the most straightforward way to teach coding for single board computers, but it teaches bad habits that don't scale well when used in real-world applications.