[Development] QDateTime is missing shared null optimization

[Milian Wolff]

Hello all,

I just realized that QDateTime in Qt 5 is still not implementing the "shared null optimization" - I'm not sure 
it's the right term.

What I mean is that you'll incur a heap allocation when doing

QDateTime invalidDate;

See:

QDateTime::QDateTime()
    : d(new QDateTimePrivate)
{
}

Most other classes in Qt are cheap to generate in an empty/invalid state, not so for QDateTime. Is there a 
reason for that, or is it just an oversight? If so, I'd like to amend that. Would it be preferred to introduce a 
shared null like QTypedArrayData::sharedNull, or should I rather use a nullptr to indicate the invalid state, 
like in QImage? Or am I missing something and these tricks are not applicable to QDateTime?

Grepping for QSharedDataPointer, I find many more classes that do not leverage a shared null state. Should 
we try to implement some helper functions/macros there which can be leveraged by every user of this 
class to implement a shared null state?

Bye 

-- 
Milian Wolff | milian.wolff at kdab.com | Software Engineer
KDAB (Deutschland) GmbH&Co KG, a KDAB Group company
Tel: +49-30-521325470
KDAB - The Qt Experts
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