COM object and QtVariant casting issue

mlago

Hello every one,
I'm trying to use a COM object and I'm having trouble casting a returned function value. I am using Qt 5.7.0 and I got .h and .cpp file for COM object with dumpcpp tool.

class SCAPILIB_EXPORT IAttrList : public QAxObject
{
public:
    IAttrList(IDispatch *subobject = 0, QAxObject *parent = 0)
    : QAxObject((IUnknown*)subobject, parent)
    {
        internalRelease();
    }

    /*
    Method AddItem

    method AddItem

    */
    inline void AddItem(Attr Attribute, const QVariant& value);

    /*
    Method FindItemIndex

    method FindItemIndex

    */
    inline int FindItemIndex(int startIndex, SCAPILib::Attr Attribute);

    /*
    Method GetCount

    method GetCount

    */
    inline int GetCount();

    /*
    Method GetIndex

    method GetIndex

    */
    inline QVariant GetIndex(int index, Attr& pAttribute);

    /*
    Method GetItem

    method GetItem

    */
    inline QVariant GetItem(Attr Attribute);

    /*
    Method OutputToString

    method OutputToString

    */
    inline QString OutputToString();

    /*
    Method RestoreFromBlob

    method RestoreFromBlob

    */
    inline void RestoreFromBlob(QVariant blob);

    /*
    Method SaveToBlob

    method SaveToBlob

    */
    inline void SaveToBlob(QVariant& pBlob);

    /*
    Method SetIndex

    method SetIndex

    */
    inline void SetIndex(int index, SCAPILib::Attr Attribute, const QVariant& value);

// meta object functions
    static const QMetaObject staticMetaObject;
    virtual const QMetaObject *metaObject() const { return &staticMetaObject; }
    virtual void *qt_metacast(const char *);
};

The function GetItem always returned a QVariant, usually the QVariant contain a QString, QInt or QBool, but with a determinate attribute the returned value will be a pointer to the base class(IAttrList *).

inline QVariant IAttrList::GetItem(Attr Attribute)
{
    QVariant qax_result;
    void *_a[] = {(void*)&qax_result, (void*)&Attribute};
    qt_metacall(QMetaObject::InvokeMetaMethod, 12, _a);
    return qax_result;
}

I can use the function of the COM object just fine, except when it will be return a indicated class pointer. In this case de QVariant returnet type is (IUnknown *) and I not able to cast this to (IAttrLits *).

I'm tried with:

IAttrList * Attr = AttrList.GetItem(Some_Attribute).Value<IAttrList *>();

But compiler returned error: static assertion failed: qobject_cast requires the type to have a Q_OBJECT macro

Which I understand is normal because, according to QT documentation, classes derived from QAxObject can not contain the Q_OBJECT macro.

I have also tried with:

IAttrList * attr = static_cast<IAttrList *>(AttrList.GetItem(Some_attribute).data());

and

IAttrList * attr = static_cast<IAttrList *>(AttrList.GetItem(Some_attribute).value<void *>();

It's compile but the AttrList pointer does not seem to be valid because I get a segmentation fault to try use it.

I do not know what the problem might be, any subgestion?. Thanks in advanced.

sonulohani

Hi @mlago,

Two things to note here.

You have to add Q_OBJECT macro in private section of the class.
Before doing qvariant_cast, non-standard data types need to be registered, using the qRegisterMetaType() function

mlago

Hi @sonulohani ,
Thanks for the answer. The class was generated by dumpcpp from a COM object dll and I can't add Q_OBJECT macro because the class derive of QAxObject and not from QObject.

kshegunov

@mlago said in COM object and QtVariant casting issue:

the class derive of QAxObject and not from QObject.

QAxObject derives from QObject however. If I remember correctly, you can dynamic cast the unknown interface too. Something along the lines of:

IUnknown * interface; //< Comes from the COM API
IAttrList * attrList = dynamic_cast<IAttrList *>(interface);
if (attrList)  {
    // ... Do things
}
else  {
    // Not dealing with that type of object at all ...
}

mlago

Hi,
I tried with you say but not work:

error: cannot dynamic_cast 'interface' (of type 'struct IUnknown*') to type 'class IAttrList*' (source is a pointer to incomplete type)

I thinking that de right way is using QueryInterface but I have not been able to make it work yet. First all i dont have the UUID class because the dumpcpp dont generate any for this class. I tried asigned one with SetControl method, but in this case, the call:

HRESULT Hr = AttrList.QueryIntrface (IID_AttrList, (void **) & var);

Always returns, 0x80004001 E_NOTIMPL

I do not understand how something seemingly simple may be getting so complicated.

Thanks once again.

hskoglund

Hi, to get an instance of IAttrList, I think you need to create it first. (COM doesn't do that for you). Perhaps instead of calling QueryInterface, try a querySubObject(), something like:

QAxObject *iAttrList = AttrList.GetItem(Some_Attribute).querySubObject("IAttrList");

mlago

Thanks @hskoglund ,
The QVariant object returned by the function GetItem not have a QuerySubObject method and I can`t do the call in this way.

A casting from the QVariant to QAxObject is posible:

 QAxObject * axObj =AttrList.GetItem(some_attribute).value<QAxObject *>();

but return with a null pointer in axObj.

thank you anyway.

hskoglund

Hmm, if you have some .NET code that successfully retrieves an IAttrList class instance, could you post it here..

mlago

In case it helps, I bring another part of the code, of the class header file showing in my first post, where it refers to MetaTypeFuntionHelper, I have not found documentation about this struct:

template<>
struct QMetaTypeFunctionHelper<SCAPILib::IAttrList, /* Accepted */ true> {
    static void Destruct(void *t)
    {
        Q_UNUSED(t)
        static_cast<SCAPILib::IAttrList*>(t)->SCAPILib::IAttrList::~IAttrList();
    }
    static void *Construct(void *where, const void *t)
    {
        Q_ASSERT(!t);
        Q_UNUSED(t)
        return new (where) SCAPILib::IAttrList;
    }
#ifndef QT_NO_DATASTREAM
    static void Save(QDataStream &stream, const void *t) { stream << *static_cast<const SCAPILib::IAttrList*>(t); }
    static void Load(QDataStream &stream, void *t) { stream >> *static_cast<SCAPILib::IAttrList*>(t); }
#endif // QT_NO_DATASTREAM
};

Today I have also tried adding Q_DECLARE_METATYPE() macro to the end of the class declaration, outside the namespace:

namespace SCAPILib {
(...)
#endif
}
Q_DECLARE_METATYPE(SCAPILib::IAttrList)

But with these compiler errors:

qmetatype.h:1864: error: specialization of 'QMetaTypeId<SCAPILib::IAttrList>' after instantiation
     struct QMetaTypeId< TYPE >	 
qmetatype.h:1864: error: redefinition of 'struct QMetaTypeId<SCAPILib::IAttrList>'
     struct QMetaTypeId< TYPE >
qmetatype.h:1596: error: previous definition of 'struct QMetaTypeId<SCAPILib::IAttrList>'
 struct QMetaTypeId : public QMetaTypeIdQObject<T>