I think there are two separate notions being discussed here - First is the notion of a partial function - assignment and copy are both partial functions and have a precondition that the rvalue be in a valid state. The second is the notion of a runtime type where once the type is determined it is fixed and assignment becomes defined only on items of the same type. I tend to shy away from implementing such types and usually prefer to document that assignment won't throw (perhaps unless some subtype throws) if the two objects are of the same runtime type (or have the same topology - or whatever a correct term would be for the circumstance). There are cases where I think the notion of a runtime type is valid even when it makes assignment more partial than it would be otherwise - being partial doesn't effect the concept at all - but such a design choice should be weighed carefully. Sean