View source for Policies/Library Code Policy/Shared D-Pointer Example

The following example illustrates the [[Policies/Library_Code_Policy#Shared D-Pointers|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 ==
<code cppqt>
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); }
};
</code>

== The header file, with Q_DECLARE_PRIVATE ==
<code cppqt>
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 setSomeInteger(int i);
    int sumOfIntegers() const;
protected:
    KFooDerived(KFooDerivedPrivate &dd, QObject *parent);
private:
    Q_DECLARE_PRIVATE(KFooDerived)
};
</code>

== The .cpp file ==
<code cppqt>
/*** 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;
}
</code>