'QObject::QObject' cannot access private member declared in class 'QObject'

Question

class CHIProjectData : public QObject
{
public:
    CHIProjectData();
    CHIProjectData(QMap<QString,QString> aProjectData,
                   CHIAkmMetaData* apAkmMetaData = 0,
                   QObject* parent = 0);
private:
    QMap <QString,QString> m_strProjectData;
    CHIAkmMetaData* m_pAkmMetaData;
};

CHIProjectData::CHIProjectData(QMap<QString,QString> aProjectData,
                               CHIAkmMetaData* apAkmMetaData,
                               QObject* aParent)
    :
    QObject(aParent)
{
        m_strProjectData = aProjectData;
        m_pAkmMetaData = apAkmMetaData;
}

Why does it give the "'QObject::QObject' cannot access private member declared in class 'QObject'" error?

Answer 1

I'm guessing that your CHIProjectData class is being copied somewhere (using the compiler-generated copy constructor or assignment operator). QObject cannot be copied or assigned to, so that would cause an error. However, the compiler has no line to point to for the error, so it chooses some line in the file (the final brace is common, since that is when the compiler knows if it should generate those functions or not, after parsing the class declaration to see if they already exist).

Answer 2

The default constructor for QObject must be private and the error you are getting is quite likely to do with CHIProjectData::CHIProjectData (default constructor) implicitly trying to invoke base class's default constructor. If you look at QObject you would most likely find that it's defined something like this:

class QObject {
    QObject(); //private contructor, derived classes cannot call this constructor
public:
    QObject(QObject* aParent);
};

The solution is to make default QObject constructor protected or public or call other constructor overload from the default CHIProjectData constructor:

CHIProjectData::CHIProjectData() : QObject(NULL){
}

Answer 3

Adding a copy constructor to CHIProjectData class did the trick.

Answer 4

When using QObject subclass objects try to manipulate with pointers.

take the problematic scenario

myObject = MyObjectClass() 

in this case its more clean to have

MyObjectClass *myObject;
//code
myObject = new MyObjectClass;

This would remove the need for object copying and assignments by using reference copying and assignments.

Answer 5

In my case the problem was that the Q_OBJECT macro silently introduces a private: specifier, even within a struct:

struct myClass : public QObject {
   Q_OBJECT
   // everything here is private now...
}