Q_D macro not working

[Ash V]

@Axel-Spoerl
Hey Axel !
I sincerely hope you're having a great day.

I've come across this issue today in my Qt Creator, that the Q_D macro is not able to fetch data from my private class.

Specifically,

• I'm calling the d->privatePrintMethod() in the constructor of my SecondClass
• It calls the q->printMethod() which tends to access the int d->etaD but is not able to get it

dqpointer.hpp

#ifndef DQPOINTER_H
#define DQPOINTER_H

#include <QDebug>

class FirstClassPrivate;
class FirstClass
{
public:
    FirstClass();

protected:
    FirstClass(FirstClassPrivate* d);
    FirstClassPrivate* d_ptr;
private:
    Q_DECLARE_PRIVATE(FirstClass)
};


class SecondClassPrivate;
class SecondClass : public FirstClass
{
public:
    SecondClass();
    void printMethod();

protected:
    SecondClass(SecondClassPrivate* d);
private:
    Q_DECLARE_PRIVATE(SecondClass)
};

#endif // DQPOINTER_H

dqpointer_p.hpp

#ifndef DQPOINTER_P_H
#define DQPOINTER_P_H

class FirstClass;
class FirstClassPrivate
{
public:
    FirstClassPrivate()
    {
        qDebug() << "FirstClassPrivate Constructor 1\n";
    }

    FirstClassPrivate(FirstClass* q)
        : q_ptr(q)
    {
        qDebug() << "FirstClassPrivate Constructor 2\n";
    }

    bool etaA;
    char etaB;

protected:
    FirstClass* q_ptr;
private:
    Q_DECLARE_PUBLIC(FirstClass)
};


class SecondClass;
class SecondClassPrivate : public FirstClassPrivate
{
public:
    SecondClassPrivate()
    {
        qDebug() << "SecondClassPrivate Constructor 1\n";
    }

    SecondClassPrivate(SecondClass* q)
        : FirstClassPrivate(q)
    {
        qDebug() << "SecondClassPrivate Constructor 2\n";
    }

    void privatePrintMethod()
    {
        Q_Q(SecondClass);
        q->printMethod();
    }

    float etaC;
    int etaD = 10;

private:
    Q_DECLARE_PUBLIC(SecondClass)
};

#endif // DQPOINTER_P_H

dqpointer.cpp

#include "dqpointer.h"
#include "dqpointer_p.h"

FirstClass::FirstClass()
    : d_ptr(new FirstClassPrivate(this))
{
    qDebug() << "FirstClass Constructor 1\n";
}

FirstClass::FirstClass(FirstClassPrivate* d)
    : d_ptr(d)
{
    qDebug() << "FirstClass Constructor 2\n";
}


SecondClass::SecondClass()
    : FirstClass(new SecondClassPrivate)
{
    qDebug() << "SecondClass Constructor 1\n";

    Q_D(SecondClass);
    d->privatePrintMethod();
}

SecondClass::SecondClass(SecondClassPrivate* d)
    : FirstClass(d)
{
    qDebug() << "SecondClass Constructor 2\n";
}

void SecondClass::printMethod()
{
    int n;

    // Does not work
    // Q_D(SecondClass);
    // n = d->etaD;

    // Works
    n = 10;

    for(int i = 1; i <= n; i++)
        qDebug() << i;
}

main.cpp

#include <QCoreApplication>
#include <QDebug>

#include "dqpointer.h"

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

    SecondClass* s = new SecondClass;

    return a.exec();
}

It'd be a great help from your side if you could help me out in this issue.

[Ash V]

@Christian-Ehrlicher
Hey Christian, here's the issue I'm getting.

// Does not work
Q_D(SecondClass);
n = d->etaD;

// Works
n = 10;

[Christian Ehrlicher]

@Ash-V said in Q_D macro not working:

but is not able to get it

What does it mean? Do you get anything compiler error or do you don't get the expected value?

[Ash V]

@Christian-Ehrlicher

Also Christian,
I'd be very helpful of you if you could explain to me when I can use the const keyword in the Q_D and the Q_Q macros.

I used const in Q_D() like this,

SecondClass::SecondClass()
    : FirstClass(new SecondClassPrivate)
{
    qDebug() << "SecondClass Constructor 1\n";

    Q_D(const SecondClass);
    d->privatePrintMethod();
}

but it throws this error.

/dqpointer.cpp: error: passing ‘const SecondClassPrivate’ as ‘this’ argument discards qualifiers [-fpermissive]
/dqpointer.cpp: In constructor ‘SecondClass::SecondClass()’:
/dqpointer.cpp: error: passing ‘const SecondClassPrivate’ as ‘this’ argument discards qualifiers [-fpermissive]
      |     d->privatePrintMethod();
      |     ~~~~~~~~~~~~~~~~~~~~~^~

[Christian Ehrlicher]

@Ash-V said in Q_D macro not working:

const keyword in the Q_D and the Q_Q macros

What has this to do with Qt? You can not call a non-const function from a const object. Make your function const.

[Ash V]

@Christian-Ehrlicher
And this doubt Christian ?
The first one.

See, I've uploaded the output screenshots.

[JonB]

@Ash-V
May I ask: I thought you were a "beginner" at Qt, maybe C++ too (but perhaps not). If you know what you are doing, and have a good usage case, using all the Q_D and d_ptr stuff may be fine. But I, and most people, have never used them, so I just wonder whether you might be over-complicating things.