Policies/Library Code Policy/Shared D-Pointer Example
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.
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);
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); }
};
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);
int sumOfIntegers() const;
protected:
KFooDerived(KFooDerivedPrivate &dd, QObject *parent);
private:
Q_DECLARE_PRIVATE(KFooDerived)
};
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
{
// because of 'const' use d_func here instead of Q_D
return d_func()->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! */
}
int KFooDerived::sumOfIntegers() const
{
return d_func()->someInteger + d_func()->anotherInteger;
}