Unresolved external symbol

[A Former User]

Hello. Given the following code:

// portal.h
#ifndef PORTAL_H
#define PORTAL_H

#include <QObject>
#include <QPointer>

class Portal : public QObject
{
    Q_OBJECT
public:
    explicit Portal(QObject *parent = nullptr);
    ~Portal();

    static Portal* GetPortal();

    bool connected;

signals:
    void StateChanged();

private:
    static QPointer<Portal> self;

};

#endif // PORTAL_H

// portal.cpp
#include "Portal.h"

Portal::Portal(QObject *parent)
    : QObject{parent}
{
    self = this;
}

Portal::~Portal()
{
    delete self;
    self = NULL;
}

Portal* Portal::GetPortal()
{
    return self;
}

I get the follwing error: Portal.obj:-1: error: LNK2001: unresolved external symbol "private: static class QPointer<class Portal> Portal::self" (?self@Portal@@0V?$QPointer@VPortal@@@@A) for the file portal.obj. I've tried everything I could think of and I can't find a way to fix it.

[JonB]

@Mandar1jn
You have to define storage for class static variable declarations:

private:
    static QPointer<Portal> self;

In the .cpp you need somewhere outside of anything else, e.g. before the constructor:

static QPointer<Portal> Portal::self;

Are you trying to turn C++ into Python? ;-)

[SGaist]

Hi and welcome to devnet,

You just declared your static variable but you never initialized it.

That said, why such a singleton ?

[A Former User]

@JonB thank you. That solved my issue