Declaring an object as extern

Question

I am trying to declare an object as extern because I want a thread to be able to access and update it from a different file. But I get the following error message when I try to compile my code:

In file included from main.cpp:1:
dialog.h:43:6: error: storage class specified for ‘temperatureValue’
   43 |      extern QLabel *temperatureValue;
      |      ^~~~~~
dialog.h:44:6: error: storage class specified for ‘humidityValue’
   44 |      extern QLabel *humidityValue;
      |      ^~~~~~
dialog.h:45:6: error: storage class specified for ‘CO2Value’
   45 |      extern QLabel *CO2Value;
      |      ^~~~~~
dialog.h:46:6: error: storage class specified for ‘lightingValue’
   46 |      extern QLabel *lightingValue;
      |      ^~~~~~
dialog.h:47:6: error: storage class specified for ‘letterGrade’
   47 |      extern QLabel *letterGrade;

Here is the header file where I am declaring my extern objects. Why am I getting this error and how should I be declaring the objects so I can use them globally?

#ifndef DIALOG_H
#define DIALOG_H

#include <iostream>
#include <string>
#include <QLabel>
#include <QDialog>
#include <QtWidgets>
#include <QVBoxLayout>
#include <QGridLayout>
#include <QGroupBox>
#include <QtCore>
#include <QtGui>
#include <QtCharts>
#include <QChart>
#include <QChartView>
#include <QDate>
#include <QTime>
#include <QDateTime>
#include <QFile>
#include <QString>
#include <QStringList>
#include <QTextStream>
#include <QLineSeries>
#include <QIODevice>
#include <QDir>
#include <Qt>
#include <QDateTimeAxis>
#include <QValueAxis>
#include <QComboBox>
#include <QBarSeries>
#include <QFileSystemWatcher>

namespace MainWindow
{
    class Dialog : public QDialog
    {
        Q_OBJECT

        public:
            Dialog(QWidget *parent = nullptr);
            ~Dialog();

        // create update graph functions
        private slots:
            void delayUpdateChart();
            void updateChart();
            void changePet();
            void expandGraph();
            
        private:

            //Create the layouts that will make up the GUI
            QGridLayout *mainLayout;
            QGridLayout *leftLayout;
            QVBoxLayout *centerLayout;
            QVBoxLayout *rightLayout;
            QWidget *leftWidget;
            QWidget *centerWidget;
            QWidget *rightWidget;

            //Create all the labels that will be used to display environmental information
            QLabel *temperatureLabel;
            QLabel *humidityLabel;
            QLabel *CO2Label;
            QLabel *lightingLabel;
            extern QLabel *temperatureValue;
            extern QLabel *humidityValue;
            extern QLabel *CO2Value;
            extern QLabel *lightingValue;
            extern QLabel *letterGrade;
            QLabel *petName;

        //Create widgets that contain the labels
            QWidget *temperatureWidget;
            QWidget *humidityWidget;
            QWidget *CO2Widget;
            QWidget *lightingWidget;

        // Create file watcher for data
            QFileSystemWatcher *fileWatcher;

        // Create chart 
            QChartView *chartview;

        // Create chart type selection
            QComboBox *typeComboBox;
            QComboBox *rangeComboBox;

        // Create pet selection
        QComboBox *petSelector;

        // Create expand graph check box
        QCheckBox *graphCheckBox;

            void leftVerticalLayout();
            void centerVerticalLayout();
            void rightVerticalLayout();
        void readLastLine();

        // Create helper functions for the graph
            QLineSeries* getLineSeries(std::string filename, int typeInd, int rangeInd);
            QDateTimeAxis* getDTAxis();
            QValueAxis* getValueAxis(std::string label);
    
    };
}

#endif // DIALOG_H

Answer 1

extern a class member is not allowed, If we can do that, for instances of a class type, compiler don't know which address to resolve while linking. you can do like this:

//A.h
#ifndef _A_H_
#define _A_H_
struct A{
    int value;
    
};
extern A a;
#endif

//A.cpp
#include "A.h"
A a;

//main.cpp
#include <iostream>

#include "A.h"
int main() {

std::cout<<a.value<<std::endl;   
}

In your codes, extern Dialog's intance instead of its members.