New style casting with QT

Question

I want to update my code from old style casting to new style one. I have some problems with understanding the different type of casts.

1 case

if (((QDataItem*)(*it))->GetType()==QDataItem::AudioTrack){

Here I have a class "QDataItem". This contain an enum of track types, like AudioTrack. Basing on a QTreeWidget I iterate through the QTreeWidget items. Each Item represents a QDataItem. Now with new casting I want to do:

if ((static_cast<QDataItem*>(*it))->GetType()==QDataItem::AudioTrack){

Is this there right way to do?

2 case In old style I have a twice casting

QAudioTrackItem *audio_track = (QAudioTrackItem*)(QDataItem*)(*it);

QAudioTrackItem is like QDataItem a class. I want to do here:

QAudioTrackItem *audio_track = reinterpret_cast<QAudioTrackItem*>(*it)

But I am not sure that this is correct in case of the missing QDataItem cast.

Is my result ok or do I have a bug?

Answer 1

Is this there right way to do?

Yes, (QDataItem*)(*it) and static_cast<QDataItem*>(*it) should be identical with your code.

But if your classes have such inheritance structure:

class QDataItem : QObject {};
class QAudioTrackItem : QDataItem {};

you should really consider using qobject_cast<>() instead:

if (auto item = qobject_cast<QAudioTrackItem *>(*it)) {
    ....
}

Is my result ok or do I have a bug?

Maybe, it depends on how your inheritance structure look like.

If they inherit just like my example above, it's totally OK to use reinterpret_cast<>() to convert any pointers between QObject, QDataItem and QAudioTrackItem.

But if your classes have multiple inheritance:

class QDataItem {};
class QAudioTrackItem : QObject, QDataItem {};

reinterpret_cast could kick your ass badly:

auto item = new QAudioTrackItem;

auto p1 = reinterpret_cast<QObject *>(item); // OK
auto p2 = reinterpret_cast<QAudioTrackItem *>(p1); // OK

auto p3 = reinterpret_cast<QDataItem *>(item); // Undefined Behavior
auto p4 = reinterpret_cast<QDataItem *>(p1); // Undefined Behavior