The following example illustrates the shared d-pointer concept. The class presented here derives from QObject to illustrate parameter-passing in the constructor; it is not necessary to be like that in your code.

[edit] The header file, without Q_DECLARE_PRIVATE

class KFooBasePrivate;
class KFooBase : public QObject
{
public:
    KFooBase(QObject *parent);
    void setSomeInteger(int i);
    int someInteger() const;
protected:
    KFooBasePrivate * const d_ptr;
    KFooBase(KFooBasePrivate &dd, QObject *parent);
private:
    friend class KFooBasePrivate;
    inline KFooBasePrivate *d_func() { return d_ptr; }
    inline const KFooBasePrivate *d_func() const { return d_ptr; }
};
 
class KFooDerivedPrivate;
class KFooDerived : public KFooBase
{
public:
    KFooDerived(QObject *parent);
    void setAnotherInteger(int i);
    int anotherInteger() const;
    int sumOfIntegers() const;
protected:
    KFooDerived(KFooDerivedPrivate &dd, QObject *parent);
private:
    friend class KFooDerivedPrivate;
    inline KFooDerivedPrivate *d_func()
    { return reinterpret_cast<KFooDerivedPrivate *>(d_ptr); }
    inline const KFooDerivedPrivate *d_func() const
    { return reinterpret_cast<KFooDerivedPrivate *>(d_ptr); }
};

[edit] The header file, with Q_DECLARE_PRIVATE

class KFooBasePrivate;
class KFooBase : public QObject
{
public:
    KFooBase(QObject *parent);
    void setSomeInteger(int i);
    int someInteger() const;
protected:
    KFooBasePrivate * const d_ptr;
    KFooBase(KFooBasePrivate &dd, QObject *parent);
private:
    Q_DECLARE_PRIVATE(KFooBase)
};
 
class KFooDerivedPrivate;
class KFooDerived : public KFooBase
{
public:
    KFooDerived(QObject *parent);
    void setAnotherInteger(int i);
    int anotherInteger() const;
    int sumOfIntegers() const;
protected:
    KFooDerived(KFooDerivedPrivate &dd, QObject *parent);
private:
    Q_DECLARE_PRIVATE(KFooDerived)
};

[edit] The .cpp file

/*** KFooBase ***/
class KFooBasePrivate
{
public:
    virtual ~KFooBasePrivate() { }
    int someInteger;
};
 
KFooBase::KFooBase(QObject *parent) 
    : QObject(parent), d_ptr(new KFooBasePrivate)
{
}
 
KFooBase::KFooBase(KFooBasePrivate &dd, QObject *parent)
    : QObject(parent), d_ptr(&dd)
{
}
 
KFooBase::~KFooBase()
{
    delete d_ptr;
}
 
void KFooBase::setSomeInteger(int i)
{
    Q_D(KFooBase);
 
    d->someInteger = i;
}
 
int KFooBase::someInteger() const
{
    // in const functions add 'const' to the class name
    Q_D(const KFooBase);
 
    return d->someInteger;
}
 
/*** KFooDerived ***/
 
class KFooDerivedPrivate: public KFooBasePrivate
{
public:
    int anotherInteger;
};
 
KFooDerived::KFooDerived(QObject *parent)
    : KFooBase(*new KFooDerivedPrivate, parent)
{
}
 
KFooDerived::KFooDerived(KFooDerivedPrivate &dd, QObject *parent)
    : KFooBase(dd, parent)
{
}
 
KFooDerived::~KFooDerived()
{
    /* no need to delete the d-pointer! */
}
 
void KFooDerived::setAnotherInteger(int i)
{
    Q_D(KFooDerived);
 
    d->anotherInteger = i;
}
 
int KFooDerived::anotherInteger() const
{
    // in const functions add 'const' to the class name
    Q_D(const KFooDerived);
 
    return d->anotherInteger;
}
 
int KFooDerived::sumOfIntegers() const
{
    // in const functions add 'const' to the class name
    Q_D(const KFooDerived);
 
    return d->someInteger + d->anotherInteger;
}