QGraphicsItem inheritance issues

wazzdaman

Hello,

I'm working on my custom little toy in Qt5.15/C++ which uses my own QGraphicsTextItem, QGraphicsLineItem and QGraphicsRectItem based classes. They share common QGraphicsItem related methods like a method for setting flags, overridden itemChange() implementation and my extra custom methods too.

It's not very elegant to have all this code duplicated in my customized classes but all my efforts were unsuccessful due to inheritance issues.

How can I elegantly solve this problem?

Thanks a lot!

JonB

@wazzdaman
Since C++ does not allow you to alter the base class to extend it, either write "helper" functions/class which take a parameter of a QGraphicsItem or possibly make your classes for all of these derive from an extra class with the desired functionality in addition to deriving from QGraphicsItem.

wazzdaman

@JonB Thanks for your answer! Could you write a short sample code for the second option (not with the helper functions)?

jsulm

@wazzdaman Why do you need sample code for that? You should know how to subclass a class:

class MyClass : public QGraphicsItem
...

JonB

@wazzdaman
For the "second option", multiple inheritance, I had in mind you would be using stuff like

class YourHelperGraphicsItem

class YourGraphicsTextItem : public QGraphicsTextItem, public YourHelperGraphicsItem

class YourGraphicsLineItem : public QGraphicsLineItem, public YourHelperGraphicsItem

for each of the QGraphics...Items you want to use. So that you can put "shared functionality" in YourHelperGraphicsItem. (There are issues, though, e.g. YourHelperGraphicsItem cannot derive from QGraphicsItem.) That is the only way I know to "extend" the shared functionality in C++ since you cannot alter the base class (QGraphicsItem). I don't know whether anyone else can achieve something with C++ templates.

wazzdaman

@jsulm Thanks!

@JonB I see, thanks for the clarification!