Q_DECLARE_METATYPE for template class

silicomancer

I just wrote a little templated container class. Something like this:

@
template <typename T> class Container: public ContainerBase
{
......
T data;
};
@

I need to register all instances of that template automatically to meta type system and binary data streams. I was able to implement automatic creation and registration of data streams. It was also no big issue to call qRegisterMetaType() automatically.

However Q_DECLARE_METATYPE() is a bit more complicated.

Using the macro directly turned out to be impossible. So I played around, copied the original macro and adapted it a bit:

@
template <typename T> struct QMetaTypeId<Container<T>>
{
enum { Defined = 1 };
static int qt_metatype_id()
{
static QBasicAtomicInt metatype_id = Q_BASIC_ATOMIC_INITIALIZER(0);
if (const int id = metatype_id.loadAcquire()) return id;
const int newId = qRegisterMetaType<Container<T>>(typeid(Container<T>).name(), reinterpret_cast<Container<T>*>(quintptr(-1)));
metatype_id.storeRelease(newId);
return newId;
}
};
@

This works fine. But I would like to avoid copying Qt internals.

Is there any solution for that problem that involves official Qt API only?

NicuPopescu

Q_DECLARE_METATYPE_TEMPLATE_1ARG(Container)

silicomancer

Thanks for your answer! That macro looks very promising. I replaced my snippet by

@ Q_DECLARE_METATYPE_TEMPLATE_1ARG(Container) @

but I get the usual static assert

@ "Type is not registered, please use the Q_DECLARE_METATYPE macro to make it known to Qt's meta-object system" @

However I do not understand why. I would have expected that it works.

NicuPopescu

Here I tested and it works, even I tested the metatype ids for some instances and looks ok: are you sure that error comes from where you call the macro definition for templates?

and one more issue: you can still improve your workaround to make it completely generic ... for this have a look at qt code how they use strays "" ... I'm not good at this but I tested like there and it works

silicomancer

Yes, I checked the "Type is not registered" error. It definitely corresponds to the missing container declaration.

BTW that Container and ContainerBase class is not a QList or QSet or something. I should have mentioned that these are custom classes holding a single data instance. They are not similar to the Qt container.

I did some tests. This one works too (however this isn't what I want to do):

@Q_DECLARE_METATYPE(Container<ConcreteType>) @

I also tried to declare the contained type first before declaring the container:

@Q_DECLARE_METATYPE(ConcreteType)
Q_DECLARE_METATYPE_TEMPLATE_1ARG(Container); @

This gives me a different error "value_type is not an element of Container<T>" in this line of qmetatype.h:

@template<typename T, bool = QMetaTypeId2<typename T::value_type>::Defined> @

Then I added

@typedef T value_type; @

to my Container class. Now I get an error about a missing iterator.

My impressions is that there are lot of concrete features that are expected being implemented in the provided container class. I can't fulfil all of them (especially iterators). Is that macro specialized for Qt containers and alike?

Any ideas on this?

NicuPopescu

how I tested here:

in simple main window app, in mainwindow.h:

@
...
class ContainerBase
{

};

template <class T> class Container: public ContainerBase
{

T data;
};

Q_DECLARE_METATYPE_TEMPLATE_1ARG(Container)
@

what is ConcreteType, a built in one?

NicuPopescu

bq. This gives me a different error “value_type is not an element of Container<T>” in this line of qmetatype.h:
template<typename T, bool = QMetaTypeId2<typename T::value_type>::Defined>

this line does not exist here! :) I use qt 5.1.1 mingw

silicomancer

I used an enum class as concrete type and I am using Qt5.2.0 beta here.
How did you involve QVariant?

This is a minimum example that breaks compilation when used in one of projects headers:

@
template <class T> class Container
{
};

Q_DECLARE_METATYPE_TEMPLATE_1ARG(Container)

inline void Test()
{
QVariant x = QVariant::fromValue(Container<int>());
}
@

NicuPopescu

it gives no error here ...

@Container<int> y = x.value< Container<int> >();@

returns the correctly too

shall I understand that Q_DECLARE_METATYPE_TEMPLATE_1ARG is working now?

silicomancer

No, it does NOT work. I also tried the code in one if the Qt examples to make sure there are no collisions in my custom project.

Adding the following code:

@
#include <QVariant>

template <class T> class Container
{
};

Q_DECLARE_METATYPE_TEMPLATE_1ARG(Container)

inline void Test()
{
QVariant x = QVariant::fromValue(Container<int>());
}

@

to the bottom of the tabdialog.h header of TabWidget example returns errors on building:

@
C:\Qt\Qt5.2.0\5.2.0-beta1\msvc2012\include\QtCore\qmetatype.h:1372: Fehler:C2039: 'value_type': Ist kein Element von 'Container<T>'
with
[
T=int
]
C:\Qt\Qt5.2.0\5.2.0-beta1\msvc2012\include\QtCore/qmetatype.h(1578): Siehe Verweis auf die Instanziierung der gerade kompilierten Klassen-template "QtPrivate::SequentialContainerConverterHelper<T>".
with
[
T=Container<int>
]

..... and a lot more

@

If it works for you I suppose this is a Qt 5.2.0 beta problem. I am using 5.2.0 beta, VS2012, ANGLE, 32 Bit.

Rigel

I did this test with Qt5.1.1 (VS2012, ANGLE, 32 Bit) and it worked.

I also did it with Qt5.2.0-beta1 and it failed.

So I can definitely confirm that there is some strange issue with Qt5.2.0-beta1 (VS2012, ANGLE, 32 Bit).

SGaist

Hi,

It sounds like a regression (I may be wrong though), there has been a lot of work on QMetatype lately. Can you open a bug report so it might get fixed before 5.2 official release ?

NicuPopescu

bq. I did this test with Qt5.1.1 (VS2012, ANGLE, 32 Bit) and it worked.
I also did it with Qt5.2.0-beta1 and it failed.
So I can definitely confirm that there is some strange issue with Qt5.2.0-beta1 (VS2012, ANGLE, 32 Bit).

I can confirm these tests too, but I want to do one more with Qt 5.2.0-beta1 with mingw

NicuPopescu

I found a solution for Qt 5.2.0-beta1:

@class ContainerBase: public QString /!!!inherits a core class which is compatible with the new meta type system!!!/
{

};

template <class T> class Container: public ContainerBase
{
T data;

};

Q_DECLARE_METATYPE_TEMPLATE_1ARG(Container)@

silicomancer

EDIT: I created an issue, see https://bugreports.qt-project.org/browse/QTBUG-34957

silicomancer

Update:

According to the report (see above), the Q_DECLARE_METATYPE_TEMPLATE_1ARG in Qt5 seem to work as expected.

Unfortunately they do not intend to support registering template types (using a new public macro). So I will need to stick to my nasty workaround :-(

Anyway thanks for your help!