Inheritance of a static object factory

[nomiz]

Dear all,

Could someone please explain what I am doing wrong below, and why this is not working?

Thanks!

In a dynamic library i have a class LibObject that has a static function createObject() which returns an instance of itself:

class LibObject
{
    private:
        int a;
 
    public:
        LibObject();
        static LibObject createObject(); // the LibObject factory
        int getA();
        void setA(int i);
}
 
LibOjcect::LibObject() {}
 
LibObject LibObject::createObject() { // the LibObject factory
    LibObject o;
    return o;   
}
 
int LibObject::getA() { return a }
 
void LibObject::setA(int i) { a = i }

class LibObject
{
    private:
        int a;

    public:
        LibObject();
        static LibObject createObject(); // the LibObject factory
        int getA();
        void setA(int i);
}

LibOjcect::LibObject() {}

LibObject LibObject::createObject() { // the LibObject factory
    LibObject o;
    return o;   
}

int LibObject::getA() { return a }

void LibObject::setA(int i) { a = i }

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Now in another class Object I am extending the LibObjectclass:

class Object : public LibObject
{
  public:
      Object();
}
 
Object::Object() {}

class Object : public LibObject
{
  public:
      Object();
}

Object::Object() {}

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Ok. So, now a third class would like to add Object objects to a QList:

class Test
{
    private:
        QList<Object> objects;
 
    public:
        Test();
}
 
Test::Test()
{
    objects.append(Object::createObject());
}

class Test
{
    private:
        QList<Object> objects;
    
    public:
        Test();
}

Test::Test()
{
    objects.append(Object::createObject());
}

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At compile time I get this error:
error: no matching function for call to 'QList<Object>::append(LibObject)'

So, is the static function createObject() not copied to the Object declaration?

[wayfaerer]

Shouldn't that be "QList<LibObject>" in class Test?

[nomiz]

Well, I would like it to be QList<Object>, because I want to add stuff in Object relative to LibObject..

[wayfaerer]

But LibObject::createObject() returns a LibObject, not an Object. You can make it return an Object if you want it to.

[nomiz]

Please explain. Do I have to re-declare the createObject() method in the Object class definition?

[wayfaerer]

I don't think so.

A base class factory method can (and often does) return an instance of a derived class. You would just change LibObject::createObject to:

Object LibObject::createObject() {
    Object o;
    return o;   
}

Object LibObject::createObject() {
    Object o;
    return o;   
}

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There's an excellent example of something similar here (in the "after" section): http://sourcemaking.com/design_patte...y_method/cpp/1

[nomiz]

No way! But then, shouldn't LibObject (the base class) be aware of its subclass (Object)? I think I don't exactly understand it yet, but I'll read your reference. Thanks!

[wayfaerer]

It blew my mind too.

It should be aware of the subclass, in the sense that it needs to #include the subclass's header file (or they could be in the same header file).