Class Promotion ?

[Walux]

Hi Qt Community ,

The answer i'm afraid can be very obvious , but code-safety is priority .

I have a class A , inherited by a class B.

In a certain state , i want my class A to "promote" to class B .
In other terms , i want the class A to "turn" into a class B that holds all the values of class A , and when i say "turn into" , i mean i want for the events that were occuring to the first instance of class A to still happen to the promoted class B.

Thanks you very much.

[A Former User]

I didn't get it. Also I don't get how this is related to Qt.

[mrjj]

Hi
You have
class B;
class A : public B

so A is always a B. ??

so I dont get it either. :)

besides it does remind me of
http://stackoverflow.com/questions/274626/what-is-object-slicing

[Walux]

@mrjj

It's actually :

class A;

class B : public A

Class A has key and mouse events , and multiple protected variables.
I want from my instance of class A to turn into a class B that contains copies of the values of the variables of class A while keeping the events working .

Thanks for your help , imma take a look at your link

[Walux]

@Wieland

To be more precise , both my classes A and B are subclassed QGraphicsItem ,
they only differ in the implementation of the virtual paint() method .

[mrjj]

@Walux
so if u had both paint functions in same class and switched
at some point, it would be A or B then?
So if only diff is painting, then if u can switch painting, all be ok ?

[Walux]

@mrjj

It would be in B's shape.

@mrjj said:

So if only diff is painting, then if u can switch painting, all be ok ?

I thought about it , but i'm planning on making more derived classes that'll hold new variables.

That 'switch' u mentionned is what i'm struggling to do.

[mrjj]

@Walux
the simple version is
class Master
all variables..

class A : Master
paintEvent

class B : Master
paintEvent

the more lame is
class A :B
PaintAsA(QPainter *)
PaintAsB(QPainter *)

paintevent(...)
if(UseB)
PaintAsB
else
PaintAsA

which is bad with more sublasses

[A Former User]

Why do your shapes of class A and B have to be related at all? When ever you have an object of A on your scene and you want to "turn it into" an object of B, just create a new object of B, copy the relevant data from A to B and destroy the original A object.

[Walux]

@Wieland

By doing that , i lose all the key and mouse events that happen to class A.

[A Former User]

You mean events that have been sent to A but haven't been "delivered" to it yet?

[Walux]

Yes , how can i assign all of them to the new class B ?

[A Former User]

Call QCoreApplication::processEvents before destroying A.

[Walux]

@mrjj

Maybe i should do this instead :

I start by creating a class B that uses the paintEvent of class A , then when i reach the certain state , it calls the paintEvent of class B .

I have no problem doing it now , since the difference is only in the painting , but later when the new classes have new variables , that'll be a problem .

[Walux]

@Wieland

Maybe a small piece of code will help me understand better , and thank you :)

[Walux]

I'm sure the answer i'm looking for lays somewhere between :

• dynamic_cast
• virtual
• operator=

[mrjj]

I dont understand why it ever need to change "type".
I think you need to rethink the design.
Normally u use base class pointers and childs for polymorph operations but
i cannot understand your use case.

Its like circle that must sudden draw as a rect?

[Walux]

@mrjj
@Wieland

I'll try to minimize the code , to help clarify my situation :

Class myScene : public QGraphicsScene
{
//...
void MousePressEvent(QGraphicsSceneMouseEvent *event)
//...
private:
A *a;
//...
}

"myScene.cpp"

myScene::myScene 
{
a = new A;
}

void  myScene::MousePressEvent(QGraphicsSceneMouseEvent *event)
{
a->doesSmth();
if(a->reachesSomePoint())
a = new B;   // Compiles good but i lose the events and the whole paint of a;
}

Class A : public QObject , public QGraphicsItem
{
//...
    A();
    QRectF boundingRect() const Q_DECL_OVERRIDE;
    QPainterPath shape() const Q_DECL_OVERRIDE;
    void paint(QPainter *painter, const QStyleOptionGraphicsItem *option,
               QWidget *widget) Q_DECL_OVERRIDE; // This is the first painting

    
//...
      protected:
qreal myValue1;
int myValue2;
//...
}

Class B : public Class A
{
        void paint(QPainter *painter, const QStyleOptionGraphicsItem *option,
               QWidget *widget) Q_DECL_OVERRIDE; // This is the second painting
//Future Variables
       private:
qreal futureValue1;
//...
}

[mrjj]

hi
You only need to go from base to child?
You can use a cast for that.
http://www.bogotobogo.com/cplusplus/upcasting_downcasting.php

and you dont lose A paint as you are free to call the A::paintEvent from
B::paintEvent.

[Walux]

@mrjj

Thanks for your help , glad you stayed with me till the bottom ;)
Imma read the content of your link :)