On Friday 02 December 2011 10:57:07 Stephen Kelly wrote:
> Hi,
>
> There is a lot of work going into the Qt metatype system at the moment, and
> I would like to put my oar in too.
>
> I would like to store the fact of whether a type inherits QObject with the
> qmetatype information.
>
> This would allow things like this:
>
> QVariant v = QVariant::fromValue(new QAction); // Any QObject subclass
>
> // Somewhere else:
>
> if (v.canConvert<QObject*>() {
> QObject *obj = v.value<QObject*>();
> // ...
> }
>
> // Now use the properties on obj, or connect to its signals and slots.
>
>
> Implementing this just needs a SFINAE template, and isn't unduly complex.
>
> The problem is that it introduces a slight source incompatibility. The
> SFINAE template requires that the full type be declared instead of just
> forward declared. That is, things like this, which are possible with Qt4
> would not work anymore:
>
> class MyClass;
> Q_DECLARE_METATYPE(MyClass*)
>
> void foo(MyClass *aFoo)
> {
> QVariant v = QVariant::fromValue(aFoo);
> }
>
> void choo(const QVariant &aChoo)
> {
> MyClass *c = aChoo.value<MyClass*>();
> }
>
> // MyClass still just forward declared!
>
> Similarly, qRegisterMetaType<MyClass*>() would require MyClass to be fully
> defined, whereas currently it does not. Being able to use these APIs when
> the tpr is not fully defined makes sense inside QtCore where types in QtGui
> need to be forward declared for example, but I don't think there's a great
> need to allow forward declared types outside of that case.devl
>
> To add a migration path I would add QVariant::fromForwardDeclared<T>() and
> qRegisterForwardDeclaredMetaType<T*>() (along with helper template methods)
> which don't use the SFINAE and therefore behave in Qt4 compatibility and
> Q_FORWARD_DECLARE_METATYPE() macro which would not use template methods
> which rely on SFINAE.
>
> It is very uncommon to use forward declared type like is currently allowed,
> so this would be optimizing for the more common case. I think this would be
> beneficial to QML related APIs.
>
> What do you think of the proposed change?
How much does it break to require type to be defined?
It is probably ok to force the type to be defined.
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