[Solved] Qt5 qMetaTypeConstructHelper ? (object factory)

[Sam]

Is qMetaTypeConstructHelper deprecated in Qt 5. I am trying to use

@template <>
void *qMetaTypeConstructHelper<MyModule>(const MyModule *)
{
return new MyModule();
}

int MyModule::s_moduleID = qRegisterMetaType<MyModule>("MyModule");
@

i get the following errors during compilation :

MyModule.cpp:17: error: C2143: syntax error : missing ';' before '<'
MyModule.cpp:17: error: C2988: unrecognizable template declaration/definition
MyModule.cpp:17: error: C2059: syntax error : '<'

How to get this working ?

[Sam]

From Qt 5 "qmetatype.h":http://code.woboq.org/qt5/qtbase/src/corelib/kernel/qmetatype.h.html i found that there is a definition like

@namespace QtMetaTypePrivate {
template <typename T, bool Accepted = true>
struct QMetaTypeFunctionHelper {
static void Delete(void t)
{
delete static_cast<T>(t);
}

static void *Create(const void *t)
{
    if (t)
        return new T(*static_cast<const T*>(t));
    return new T();
}

static void Destruct(void *t)
{
    Q_UNUSED(t) // Silence MSVC that warns for POD types.
    static_cast<T*>(t)->~T();
}

static void *Construct(void *where, const void *t)
{
    if (t)
        return new (where) T(*static_cast<const T*>(t));
    return new (where) T;
}

....
}
@

But its defined in a struct which is under QtMetaTypePrivate namespace, How should i use this ?

[lgeyer]

You shouldn't.

Why do you want to use it in the first place?

[Sam]

In the above example "MyModule" class is derived from QFrame, and for my application i need to use a copy constructor , but since the copy constructor is DISABLED in the private section , it always returned "error: C2248: 'QFrame::QFrame' : cannot access private member declared in class 'QFrame'

so in order to use that in Qt 4 i tried to use

@template <>
void *qMetaTypeConstructHelper<MyModule>(const MyModule *)
{
return new MyModule();
}

int MyModule::s_moduleID = qRegisterMetaType<MyModule>("MyModule");@

but somehow while porting to Qt 5 the above template is not available i need to find an alternate solution to this.

[lgeyer]

The copy constructor is disabled for a reason (and the template specialization for qMetaTypeConstructHelper does not work as copy constructor; it just creates a <code>new MyModule</code> using the default constructor).

So let's go back another step - why do you need a copy constructor in the first place?
What is the problem you are trying to solve?

If you are out for registering a non-copyable type with the meta type system don't, as the meta type system relies on a publicly available copy constructor (which is false for any QObject subclass). If you need to create a QObject by its name use a factory.

[Sam]

[quote author="Lukas Geyer" date="1358775900"]
If you are out for registering a non-copyable type with the meta type system don't, as the meta type system relies on a publicly available copy constructor (which is false for any QObject subclass). If you need to create a QObject by its name use a factory.[/quote]

You got it right, I actually want to create a QObject by its name, I too agree with using the facory implementation. Now I need to put the factory in place and test.

Thanks for the reply.

[Sam]

I did some search and got an implementation of factory pattern :

@#include <QMap>
#include <QString>

template<class Base> Base* new_object(const QString&);

template<class Base>
class Factory
{ // singleton class, one each base class
Factory() {}
Factory(const Factory&);
Factory& operator=(const Factory&);
~Factory() { qDeleteAll(creators); }
friend Base* new_object<>(const QString&);
public:
Base* create(const QString& nm) const
{
return (creators.contains(nm) ? creators[nm]->create() : NULL);
}

class Creator 
{ 
public:
    virtual ~Creator() {}
    virtual Base* create() const = 0;
};

static Factory& instance() 
{
    static Factory p_instance;
    return p_instance;
}

void unregisterCreator(const QString& nm) 
{
    if (creators.contains(nm)) 
    {
        delete creators[nm];
        creators.remove(nm);
    }
}
void registerCreator(const QString& nm, const Creator* c) 
{
    unregisterCreator(nm);
    creators.insert(nm, c);
}

private:
QMap<QString, const Creator*> creators;
};

template<class Base> Base* new_object(const QString& nm)
{
return Factory<Base>::instance().create(nm);
}

template<class Derived, class Base> class Register
{
typedef Derived* (*CreateFn) ();

class Creator : public Factory<Base>::Creator 
{ 
public:
    Creator(CreateFn fn) : createFn(fn) {}
    virtual Derived* create() const 
    {
        return (createFn ? createFn() : new Derived(NULL));
    }
    CreateFn createFn;
};

public:
Register(const QString& nm, CreateFn fn = NULL)
{
Factory<Base>::instance().registerCreator(nm, new Creator(fn));
}
};@

To create an instance we can use

@Factory<QObject>::instance().create("RegisterMe")@

or

@QObject *o = new_object<QObject>("RegisterMe");@

See "Simple-yet-powerful-object-factory":http://www.qtcentre.org/threads/8714-Simple-yet-powerful-object-factory

There is another implementation provided by you at "Elegant factory design":http://qt-project.org/forums/viewthread/21540 discussion.