Re: What is the point of Private?

[Peter Amey <peter.amey@praxis-cs.co.uk>]

Jeffrey Carter wrote:
> Simon Smith wrote:
> 
>> Why is it there? I am sure I have heard talk that its to stop the 
>> client from seeing exactly how the data is stored/manipulated however 
>> if people really did want to know this wouldn't they just scroll down 
>> the ada spec file and find out for themselves or am I missing 
>> something here?
> 
> 
> No, it was there originally to put stuff that the compiler needs to know 
> to compile client code, but that the client code doesn't need to know to 
> use the package. (Actually, the compiler probably doesn't need to know 
> that stuff, and it could be put in the body, but in the late 1970's 
> Ada's designers weren't willing to accept the extra work and overhead 
> that would entail.)

I think there are real compiler design issues here.  Modula 2 had 
"opaque types" which were conceptually similar to private types. 
However, Modula 2 had no equivalent of a package's private part which 
meant that the size of the opaque type wasn't known and that all 
implementations therefore required an opaque type to be implemented as a 
pointer.  Ada's scheme means the size is known to the compiler from the 
package spec alone.


> 
> Now, it also serves to put stuff that will be visible to the private 
> parts and bodies of children, but not to anyone else.
> 

Which is rather useful!

Peter