When to use 'Class in a parameter list

["Marin David Condic" <marin.condic.auntie.spam@pacemicro.com>]

O.K. Here's something I thought I understood but given behavior of some code
I have, now I'm questioning what is happening.

If I have a tagged type "Base_Type" that has an operation on it, called "Op"
and I derive a new type called "Child_Type" that does not require changes to
"Op", do I make the parameter Base_Type or Base_Type'Class? (My
understanding was that I could make it Base_Type, but then calls to it with
a Child_Type would require explicit type conversion. Making it 'Class would
accept anything of that class without conversion. Apparently the compiler is
swollowing it without type conversion - which is now confusing me.)

In code:

procedure Op (Base : in out Base_Type) ;
...
X : Child_Type ;
...
Op (X) ; --  Why is this working without a Base_Type (X) conversion???

So unless I'm doing something strange that is causing some corner-case to
come up, I'm now wondering why I would need Base_Type'Class as a parameter
type? I was under the impression that I would use 'Class if I wanted to make
an operation that worked on anything derived from the class without explicit
conversion. (Possible to override it in a child class, AFAIK...)

My understanding of when to *NOT* use the 'Class was if I was building an
operation I expected to override (possibly calling the parent operation
within it - using a type conversion). So when it is not overriden, and
control goes to the parent op without an explicit conversion, then when do
you need the 'Class? I must be missing something here......(I need to do
this sort of thing more often - it all evaporates if you don't use it!!!)

MDC
--
Marin David Condic
Senior Software Engineer
Pace Micro Technology Americas    www.pacemicro.com
Enabling the digital revolution
e-Mail:    marin.condic@pacemicro.com
Web:      http://www.mcondic.com/