there is no such thing as heritage of attributes in OO -languages, so I guess the ez-crew will not implement that.
Can't you structure your content in such a way that you could decide on a higher level (subtree, section) whether to show the left menu or not? I've never encountered a situation where I needed this on object level.
Inheritance is of course a key concept in OO languages. There was a discussion on this topic (ezp class inheritance) during the summer camp, but nothing decided yet. A virtual datatype that encapsulates any existing class would be a possibility too.
The idea of having custom defined attributes (like dublin core) for all classes has been brought up before in the forums too.
Antoine, I would reconsider what you want to achieve like Tom mentioned, maybe provide a "switch" with custom defined template variables to wether or not display a left-menu. Otherwise, you'll have to edit every class ...
But we're still talking about content classes here, and not about an average OO-class. I wasn't aware of the fact that the crew was considering it, it might be interesting to solve other problems.
Thanks for your answers.
In fact, I would like to define for some objects which items should be displayed in left menu (menu will be hidden if there's no items).
That's why I need to have an "Object relations" attribute for these objects. Putting this into templates wouldn't be enough flexible. I think I'll create this attribute for most used classes (Folder, Article and Product) and I'll test in templates if this attribute exists for current node (in data_map). If not, menu will be hidden. So I wont have to edit every classes.