Get warnings by using a QObject class in DLL

billliu

I created a DLL and used it in another project. I found all DLL classes which inherit from QObject cause warings about dllimport, so I just made a very simple class called "MyLib" to create a DLL and to see if it will still hapen.

In mylib.h:

#ifndef MYLIB_H
#define MYLIB_H

#include "MyLib_global.h"
#include <QObject>

class MYLIB_EXPORT MyLib :  public QObject
{
     Q_OBJECT
public:
    MyLib();
    void Hello();
};

#endif // MYLIB_H

When the class inherits from QObject, it can run but the warnings show up:

warning: 'static void MyLib::qt_static_metacall(QObject*, QMetaObject::Call, int, void**)' redeclared without dllimport attribute: previous dllimport ignored [-Wattributes]
debug\moc_mylib.cpp:55:6: warning: 'static void MyLib::qt_static_metacall(QObject*, QMetaObject::Call, int, void**)' redeclared without dllimport attribute: previous dllimport ignored [-Wattributes]
void MyLib::qt_static_metacall(QObject _o, QMetaObject::Call _c, int _id, void _a)
^~~~~
warning: 'MyLib::staticMetaObject' redeclared without dllimport attribute after being referenced with dll linkage
debug\moc_mylib.cpp:63:26: warning: 'MyLib::staticMetaObject' redeclared without dllimport attribute after being referenced with dll linkage
const QMetaObject MyLib::staticMetaObject = { {
^~~~~~~~~~~~~~~~
warning: 'virtual const QMetaObject MyLib::metaObject() const' redeclared without dllimport attribute: previous dllimport ignored [-Wattributes]
debug\moc_mylib.cpp:74:20: warning: 'virtual const QMetaObject MyLib::metaObject() const' redeclared without dllimport attribute: previous dllimport ignored [-Wattributes]
const QMetaObject MyLib::metaObject() const
^~~~~
warning: 'virtual void MyLib::qt_metacast(const char)' redeclared without dllimport attribute: previous dllimport ignored [-Wattributes]
debug\moc_mylib.cpp:79:7: warning: 'virtual void* MyLib::qt_metacast(const char*)' redeclared without dllimport attribute: previous dllimport ignored [-Wattributes]
void MyLib::qt_metacast(const char _clname)
^~~~~
warning: 'virtual int MyLib::qt_metacall(QMetaObject::Call, int, void)' redeclared without dllimport attribute: previous dllimport ignored [-Wattributes]
debug\moc_mylib.cpp:87:5: warning: 'virtual int MyLib::qt_metacall(QMetaObject::Call, int, void**)' redeclared without dllimport attribute: previous dllimport ignored [-Wattributes]
int MyLib::qt_metacall(QMetaObject::Call _c, int _id, void **_a)
^~~~~

If I remove QObject and test again, the warnings will disappear. I am using Qt6.0.
Does anyone know how to solve this problem?

Christian Ehrlicher

@billliu said in Get warnings by using a QObject class in DLL:

MYLIB_EXPORT

Your definition of the macro is wrong when you use the library. Please read the docs.

billliu

@Christian-Ehrlicher said in Get warnings by using a QObject class in DLL:

Your definition of the macro is wrong when you use the library. Please read the docs.

Do you mean I should define Q_DECL_EXPORT and Q_DECL_IMPORT correctly?
I think they are correct because a header file created by Qt defines them.
When I move the cursor to "MYLIB_EXPORT", it also seems correct, as shown below.

My shared library:

My client:

Christian Ehrlicher

You must not add mylib.h to your pro-file of MyTest.pro - it does not belong to this project and triggers a moc run for it.

billliu

@Christian-Ehrlicher said in Get warnings by using a QObject class in DLL:

You must not add mylib.h to your pro-file of MyTest.pro - it does not belong to this project and triggers a moc run for it.

I think it's right.
Thanks for your help!