GZE wrote:
-To constrain a wall on the first floor in such a way that it stays on top of a wall on the ground floor when this last one is moved.
-To constrain a toilet to always stays on the midpoint of a wall, even after it is stretched.
These are not possible in ArchiCAD as you describe them. As Lew points out you can use a multi story wall and it will remain aligned from floor to floor since it is a single entity. This is not really what you are asking for.
This sounds like a set up for a comparison to Revit which is fundamentally based on such relationships. This can be a mixed blessing. The relationships can cause problems as well as prevent them, and as the project becomes more complex so do the relationships.
ArchiCAD is only beginning to incorporate such explicit element relationships. Otherwise it is up to the user to manage them. In your wall example the simple way to maintain the relationship between the stacked walls (and floors, roofs, beams, etc.) is to simply use the marquee tool to stretch/move them as needed. What I like about this is that ArchiCAD is only doing specifically what you tell it to and not making changes based on complex relationships that you may or may not remember.
In the toilet example you would have to drag it back to align with the wall's center point, but it is hard for me to imagine why stretching a wall would commonly require a toilet to move. Toilets are most often set to a distance off a side wall or centered between toilet partitions.
