[Qt-interest] Constructing a QEnum with only a metatypeid.

[Thiago Macieira]

On Tuesday, 9 de August de 2011 11:44:48 MARTIN Pierre wrote:
> THank you for your time Thiago!
> 
> >> Can someone provide me with an example of code showing how to construct a
> >> QVariant "v", based on a given QVariant::UserType "t" where "t" refers to
> >> a
> >> type registered for an enum type?
> > 
> > QVariant v(t, 0);
> 
> Ha, that's what i was doing already… But the docs seem to not recommend
> using that constructor, hence my question :)

I don't know why you want that...

> > The value will be default constructed. I hope you like your enums of value
> > 0.
>
> Isn't there a way to tell QVariant that the given metatype "x" gets
> outputted as "int"? It would be OK to not have the Cnames in the output,
> just their integal casts / values.

No.

> > The 0 above is not the value, it's a null pointer.
> 
> However: The data pointed by the second argument seems to be copied into an
> internal location. When i do: int const			*pVal	= &val;

> QVariant			var		(t, pVal);

> Where val is an int value of my enum, everything is working ok, and the
> variant doesn't seem to store the value rather than the memory address
> (Which is good for me, and makes things partially work after all). But to
> recover the value of the variant, i have to do a ugly: int const	*prVal	
=
> (int const*)v.constData();

That constructor is meant to be used as:

	Type x = initialiseIt();
	QVariant v(qMetaTypeId<Type>(), &x);

That will create a QVariant whose value contains a copy of the type pointed. 
It's functionally equivalent to:

	QVariant v = QVariant::fromValue(x);

Hence why you shouldn't use the constructor.

> And it works, but defeats part of QVariant's functionality… Also, i checked
> at this point, the memory address returned by constData is really different
> than the one given in the constructor… Which makes me say that the data is
> actually copied rather than the memory address itself.

QVariant is value-based. Its data belongs to it and no one else.

> > It will print the type name but not the value. Printing the value is not
> > possible.
> 
> Can i have a hint where to search in the sources "why"? Wouldn't it be
> possible to get the int value outputted at all?

Yes: src/corelib/kernel/qvariant.cpp.

The "why" you get by reading the entire file and realising that there's no 
mechanism to tell QVariant and QMetaType that this type is isomorphic to int.

Note: enums are not necessarily isomorphic to int! They could be the size of 
short (so there are 2 garbage bytes in a 4-byte read) or the size of a long, 
which can be 8 bytes long.

> Any other idea for working QVariants and enums, other than a more ugly "if"s
> branches?

-- 
Thiago Macieira - thiago (AT) macieira.info - thiago (AT) kde.org
   Software Architect - Intel Open Source Technology Center
      PGP/GPG: 0x6EF45358; fingerprint:
      E067 918B B660 DBD1 105C  966C 33F5 F005 6EF4 5358
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