qRegisterMetaType results in "call to implicitly-deleted copy constructor"

Marc_Van_Daele

I want to dynamically create some components using QMetaType::type and QMetaType::create
This works fine with the following (dummy) component.

class Test1
{
public:
    Test1(){};
};

but fails when I use a subclass of QObject

class Test2 : public QObject
{
    Q_OBJECT
public:
    explicit Test2(QObject *parent = 0) : QObject(parent)  {}

signals:

public slots:
};

I get a compile error on the following code snippet in main.cpp

    qRegisterMetaType<Test2>("Test2");
    int id2 = QMetaType::type("Test2" );
    void * ptr2 = QMetaType::create( id2 );

The error that I get is

../../TestCreate/main.cpp:23:12: warning: unused variable 'ptr1' [-Wunused-variable]
    void * ptr1 = QMetaType::create( id1 );
           ^
../../TestCreate/main.cpp:24:12: warning: unused variable 'ptr2' [-Wunused-variable]
    void * ptr2 = QMetaType::create( id2 );
           ^
In file included from ../../TestCreate/main.cpp:1:
In file included from /Users/marc/Qt5.6/5.6/clang_64/lib/QtGui.framework/Headers/QGuiApplication:1:
In file included from /Users/marc/Qt5.6/5.6/clang_64/lib/QtGui.framework/Headers/qguiapplication.h:37:
In file included from /Users/marc/Qt5.6/5.6/clang_64/lib/QtCore.framework/Headers/qcoreapplication.h:40:
In file included from /Users/marc/Qt5.6/5.6/clang_64/lib/QtCore.framework/Headers/qobject.h:48:
/Users/marc/Qt5.6/5.6/clang_64/lib/QtCore.framework/Headers/qmetatype.h:760:32: error: call to implicitly-deleted copy constructor of 'Test2'
            return new (where) T(*static_cast<const T*>(t));
                               ^ ~~~~~~~~~~~~~~~~~~~~~~~~~
/Users/marc/Qt5.6/5.6/clang_64/lib/QtCore.framework/Headers/qmetatype.h:1689:83: note: in instantiation of member function 'QtMetaTypePrivate::QMetaTypeFunctionHelper<Test2, true>::Construct' requested here
                                   QtMetaTypePrivate::QMetaTypeFunctionHelper<T>::Construct,
                                                                                  ^
/Users/marc/Qt5.6/5.6/clang_64/lib/QtCore.framework/Headers/qmetatype.h:1717:12: note: in instantiation of function template specialization 'qRegisterNormalizedMetaType<Test2>' requested here
    return qRegisterNormalizedMetaType<T>(normalizedTypeName, dummy, defined);
           ^
../../TestCreate/main.cpp:17:5: note: in instantiation of function template specialization 'qRegisterMetaType<Test2>' requested here
    qRegisterMetaType<Test2>("Test2");
    ^
../../TestCreate/test2.h:6:15: note: copy constructor of 'Test2' is implicitly deleted because base class 'QObject' has a deleted copy constructor
class Test2 : public QObject
              ^
/Users/marc/Qt5.6/5.6/clang_64/lib/QtCore.framework/Headers/qobject.h:461:20: note: 'QObject' has been explicitly marked deleted here
    Q_DISABLE_COPY(QObject)
                   ^
/Users/marc/Qt5.6/5.6/clang_64/lib/QtCore.framework/Headers/qglobal.h:318:5: note: expanded from macro 'Q_DISABLE_COPY'
    Class(const Class &) Q_DECL_EQ_DELETE;\
    ^
2 warnings and 1 error generated.

If I understand this error correctly, qRegisterMetaTypeuses internally a copy constructor (which is deleted explicitly on QObject).
How can I work around this?

Thanks for your help,

Marc

SGaist

Hi,

Since Test2 is a QObject based class, you should register Test2 *.

[incomplete see @kshegunov's answer]

kshegunov

@Marc_Van_Daele
This thread from the interest mailing list should be useful.

@SGaist
This will not help (if you wish, you can see the thread for more details as to why).

Marc_Van_Daele

@kshegunov Thanks for the interesting thread!

kshegunov

@Marc_Van_Daele
No problem. My advice is to create a factory for your objects as Thiago suggested, because it's the cleanest solution and doesn't presume anything on the part of the QObject instances themselves (like an invokable constructor).

SGaist

Argle, I didn't saw I hit reply… I was looking for that email exchange.

Thanks for the pointers.