Qt 4.8, CPP : mutiple definiation

[Anas_Deshmukh]

hi everyone,

I am having multiple defination err in my qt desktop application (qt 4.8.6 + ubuntu 12.04).

below is my demo code.

// global.h
#ifndef GLOBAL_H
#define GLOBAL_H

#include <QtCore>
#include <QObject>
#include <QDebug>
QString temp;

#endif // GLOBAL_H

// class1.h

#ifndef CLASS1_H
#define CLASS1_H

#include <QtCore>
#include <QObject>
#include <QDebug>

class class1: public QObject
{
    Q_OBJECT
public :
    Q_INVOKABLE QString class1_foo();
};

#endif // CLASS1_H

// class2.h

#ifndef CLASS2_H
#define CLASS2_H

#include <QtCore>
#include <QObject>
#include <QDebug>


class class2: public QObject
{
    Q_OBJECT
public :
    Q_INVOKABLE QString class2_foo();
};

#endif // CLASS2_H

//class1.cpp

#include "class1.h"
#include "class2.h"

#include "global.h"

QString class1::class1_foo() {

    qDebug() << "this is class 1";


    return "this is class 1";
}; 

// class2.cpp

#include "class1.h"
#include "class2.h"

#include "global.h"

QString class2::class2_foo() {

    class1 class1_obj;
    class1_obj.class1_foo();

    qDebug() << "this is class 2";
    return "this is class 2";
} 

//errors :
/home/user/anas/qtProject/build-multiple_def_err-Desktop-Debug/class2.o:-1: In function `QFlagsQIODevice::OpenModeFlag::QFlags(QIODevice::OpenModeFlag)':

/home/user/anas/qtProject/multiple_def_err/class2.cpp:6: multiple definition of `temp'
/home/user/anas/qtProject/multiple_def_err/class1.cpp:7: error: first defined here

: error: collect2: error: ld returned 1 exit status

[Vinod Kuntoji]

@Anas_Deshmukh ,

declare like this,
const QString temp;

[Venkatesh V]

@Anas_Deshmukh
Hi,
This error is because of adding include statement of class A in class B and class B in class A.
in class A you dont requires class B statement. remove that one, it will works fine.

[VRonin]

your linker is right. You can't have the same temp defined in 2 different object files.

the solution is change QString temp; into extern QString temp; and then create a global.cpp file in which you put

#include "global.h"
QString temp;

[Anas_Deshmukh]

@VRonin Thanak you. your solution work fine