Qt5: How to get rid of the compiler warnings for this Singleton?

Question

I am using Qt5 on a Windows 7 platform.
I've implemented a Singleton for a database I work with.
So far it's ok, it works fine, but when I compile the code I always get 2 warnings related to copy constructor and to assignment operator.

Here is the code:

class DataBase : public QObject
{
    Q_OBJECT

public:
    static DataBase * instance(QObject * parent = 0);
    static void destroy();
    //
    QString openDataBaseConnection();
    void closeDataBaseConnection(QString & connectionName);

private:
    DataBase(QObject * parent);
    ~DataBase();
    DataBase(DataBase const &){} // <- copy constructor
    DataBase & operator = (DataBase const &){} // <- assignment operator
    static DataBase * pInstance;
};

And here are the compiler warnings:

1) Base class QObject should be explicitly initialized in the copy constructor
2) No return statement in function returning non-void (that's for the assignment operator code line).

Well, what can I do in order to finally get rid of these 2 warnings?

score 1 · Accepted Answer · edited May 23 '17 at 12:15

1

Try to initialize QObject base with the same parent as other has:

DataBase(DataBase const& other) :
QObject(other.parent())
// copy-construct members
{
}

The operator= should look like:

DataBase &operator=(DataBase const& other)
{
    QObject::operator=(other);
    // copy-assign members
    return *this;
}

The warning is about that you forgot to return *this;.

Note that what you're doing are not default implementations. They do nothing!

You'll probably want to use default keyword (if you're compiling under C++11 or later) to leave the implementation of these function up to the compiler:

DataBase(DataBase const &) = default;
DataBase &operator=(DataBase const&) = default;

edited May 23 '17 at 12:15

Community

1
1

answered Dec 23 '15 at 21:41

LogicStuff

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What do you mean by "not the default implementation. They do nothing"? Please elaborate a little bit, I am beginner in C++ ... – סטנלי גרונן Dec 23 '15 at 21:49
When you're defining the special member functions yourself, you have to implement what you want them to do manually (copying, `return`ing `*this;`). – LogicStuff Dec 23 '15 at 21:52
1, Yes, no more warning on copy constructor. And 2: stupid me, I forgot to return something there... Thanks! :) – סטנלי גרונן Dec 23 '15 at 21:53
Oh, yes, I forgot to mention that. – LogicStuff Dec 23 '15 at 22:01
1

No problem... I added `CONFIG += c++11` in the .pro file and now it compiles like a charm! :) – סטנלי גרונן Dec 23 '15 at 22:04
I suppose this will work for me too: `DataBase(DataBase const &) = delete;` and `DataBase &operator=(DataBase const&) = delete;` – סטנלי גרונן Dec 24 '15 at 22:04
Of course, if you don't need to copy the class, or you want to prevent yourself from doing so (the copy might be expensive or just a wrong thing to do). – LogicStuff Dec 24 '15 at 22:08

Qt5: How to get rid of the compiler warnings for this Singleton?

1 Answers1