Re: Type invariants and private extensions?

[Egil H H <ehh.public@gmail.com>]

On Tuesday, September 12, 2017 at 12:00:41 AM UTC+2, Victor Porton wrote:
> 
> It seems that you don't understand me: I need different boolean functions 
> for different derived tagged types. If we specify Type_Invariant or 
> Type_Invariant'Class this is the same function because it can be specified 
> only for the base class not my derived classes.
> 
> Or maybe I do not understand you? If so, please explain.
> 
> Hm, well, Type_Invariant'Class may point to dispatching function which can 
> be overridden in the derived classes. Isn't it what you mean?
> 
> 

Any Invariants declared for a parent type must also hold for all derived types, but derived types can add more. Think about what someone holding a variable of Parent'Class expects. The definition of Parent promises an invariant. But what if the actual type is a child? Mayhem will ensue if the child does not promise the same invariant, hence it is inherited.