[Development] Meaning of Q_PRIMITIVE_TYPE?

[Marc Mutz]

Hi,

Over at http://codereview.qt-project.org/21518, we're discussing whether QUuid 
is Q_PRIMITIVE_TYPE or only Q_MOVABLE_TYPE.

The documentation of Q_DECLARE_TYPEINFO says Q_PRIMITIVE_TYPE means the type 
is a POD, without constructors or destructors. According to that definition, 
QUuid is not primitive. But neither would be QFixed and QFlags, which have 
been marked primitive in the past.

I think the intention of Q_PRIMITIVE_TYPE, and the way it is used in Qt, is to 
mark types that act as ints do, ie. there's no class invariant, so 
char[sizeof(type)] = { 0 } is equal to the default-constructed value, 
uninitialised memory represents a valid value[1], and the object owns no 
resources. It is in this sense that QFixed, QFlags and QUuid are primitive 
types.

[1] This doesn't hold for cups_option_t, btw, which is marked primitive, but 
is a struct{char*a,*b;}, so uninitialised memory does not represent a valid 
value, even though, technically, it's a POD.

So I'd like to propose to extend the scope of Q_PRIMITIVE_TYPE to cover such 
non-POD classes that are nevertheless close enough to PODs.

That means:
1a. memset(0, &t, sizeof(T)) constructs a valid object, and that object is
   equal to T(), if T has a default constructor.
-alternatively-
1b Any bit pattern represents a valid object (one that the dtor can be called 
on).
2. memcpy() creates a valid independent copy of the object
3. The dtor doesn't need to be run.

With 1a+2+3, primitiveness is a strict superset of PODness.

If Jędrzej is right, though, and the intent was for primitive types to not 
require initialisation, ie. the 1b alternative is used, PODness is no longer 
sufficient for primitiveness. And cups_option_t must be downgraded to 
movable.

Looking at existing usage, I think 1a+2+3 comes closest to what everyone 
thinks primitive types mean, so I'd like to update the docs accordingly.

Opinions?

Thanks,
Marc

-- 
Marc Mutz <marc.mutz at kdab.com> | Senior Software Engineer
KDAB (Deutschland) GmbH & Co.KG, a KDAB Group Company
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