Problems compiling when using an extern

[aarelovich]

Hi:

I have a rellly long process that produces about 700 Mb of a txt log output file. This is very hard to manage. So I want to divide the output in multiple smaller log files. This is what my main.cpp looks like

#include <QtGui/QApplication>
#include "mineedit.h"
#include "logoutput.h"
#include <iostream>
 
void messageHandling(QtMsgType type, const char *msg){
 
    if (ERRORLOGGER.isEmpty()){
        ERRORLOGGER = DEFERRORLOGGER;
    }
 
    std::cout << "In Message Handling" << std::endl;
    std::cout << "Writing to file" << ERRORLOGGER.toStdString() << std::endl;
 
    QFile file(ERRORLOGGER);
    file.open(QFile::Append);
    QTextStream stream(&file);
    switch (type) {
    case QtDebugMsg:
        stream << msg << "\n";
        file.close();
        break;
    case QtWarningMsg:
        stream << "WARNING: " << msg << "\n";
        file.close();
        break;
    case QtCriticalMsg:
        stream << "CRITICAL: " << msg << "\n";
        file.close();
        break;
    case QtFatalMsg:
        stream << "FATAL: " << msg << "\n";
        file.close();
        abort();
    }    
}
 
int main(int argc, char *argv[])
{
    ERRORLOGGER = DEFERRORLOGGER;
    qInstallMsgHandler(messageHandling);
    QApplication a(argc, argv);
    MineEdit w;
    w.show();
    return a.exec();
}

#include <QtGui/QApplication>
#include "mineedit.h"
#include "logoutput.h"
#include <iostream>

void messageHandling(QtMsgType type, const char *msg){

    if (ERRORLOGGER.isEmpty()){
        ERRORLOGGER = DEFERRORLOGGER;
    }

    std::cout << "In Message Handling" << std::endl;
    std::cout << "Writing to file" << ERRORLOGGER.toStdString() << std::endl;

    QFile file(ERRORLOGGER);
    file.open(QFile::Append);
    QTextStream stream(&file);
    switch (type) {
    case QtDebugMsg:
        stream << msg << "\n";
        file.close();
        break;
    case QtWarningMsg:
        stream << "WARNING: " << msg << "\n";
        file.close();
        break;
    case QtCriticalMsg:
        stream << "CRITICAL: " << msg << "\n";
        file.close();
        break;
    case QtFatalMsg:
        stream << "FATAL: " << msg << "\n";
        file.close();
        abort();
    }    
}

int main(int argc, char *argv[])
{
    ERRORLOGGER = DEFERRORLOGGER;
    qInstallMsgHandler(messageHandling);
    QApplication a(argc, argv);
    MineEdit w;
    w.show();
    return a.exec();
}

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And my logoutput.h is like

#ifndef LOGOUTPUT_H
#define LOGOUTPUT_H
 
#include <QString>
 
//----------------------------For outputting an error file--------------------------------
#define         DEFERRORLOGGER             "/home/aarelovich/Documents/log.err"
#define         FOLDER_OUTPUT_LOG          "./home/aarelovich/Documents"
extern QString  ERRORLOGGER;
 
#endif // LOGOUTPUT_H

#ifndef LOGOUTPUT_H
#define LOGOUTPUT_H

#include <QString>

//----------------------------For outputting an error file--------------------------------
#define         DEFERRORLOGGER             "/home/aarelovich/Documents/log.err"
#define         FOLDER_OUTPUT_LOG          "./home/aarelovich/Documents"
extern QString  ERRORLOGGER;

#endif // LOGOUTPUT_H

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Now in a part of my code I do:
ERRORLOGGER = name_of_current_log_file.

However I get the following compilation errors:
obj/main.o: In function `messageHandling(QtMsgType, char const*)':
/home/aarelovich/Dropbox/MineSim/main.cpp:8: undefined reference to `ERRORLOGGER'
/home/aarelovich/Dropbox/MineSim/main.cpp:9: undefined reference to `ERRORLOGGER'
/home/aarelovich/Dropbox/MineSim/main.cpp:13: undefined reference to `ERRORLOGGER'
/home/aarelovich/Dropbox/MineSim/main.cpp:15: undefined reference to `ERRORLOGGER'
obj/main.o: In function `main':
/home/aarelovich/Dropbox/MineSim/main.cpp:40: undefined reference to `ERRORLOGGER'
obj/mineedit.o:/home/aarelovich/Dropbox/MineSim/mineedit.cpp:101: more undefined references to `ERRORLOGGER' follow
collect2: ld returned 1 exit status

Can anyone please tell me what am I doing wrong? Or how I can dynamically change the output file in which I create my application log?

Thanks for any help

[mvuori]

Extern means that the variable is defined in a cpp file, which it is not yet. You need to add the line QString ERRORLOGGER; in your cpp file, preferably at the top of it, after the includes.

[aarelovich]

I did do that (in my main.cpp file) and I also removed logout.h. After that I only need to declare it as extern every time I need to use it in any part of the program that it is not in main.cpp. Thank you very much