Re: A question about private types

[Adam Beneschan <adambeneschan@gmail.com>]

On Thursday, June 12, 2014 8:03:05 AM UTC-7, Mike H wrote:
> ... Or, at lest, I expect the answer to be about private types.

> My instinct is that the package below is vulnerable to erroneous use
> because the "Address" component of "Cell_type" can be changed. I suspect
> that this vulnerability could be removed if "Grid_index_type" is made
> read only. I have attempted to make it a private type but it then
> becomes non-discrete and can no longer be used as a parameter in
> functions such as "Line_of".
> 
> Help please! And, preferably, in a form where the new source code is
> sufficiently transparent to be understood when doing a "walk-through"
> with a C++ speaker.

If you want to prevent clients from modifying the "Address" of a Cell_Type, the answer is to make Cell_Type (not Grid_Index_Type) private, and add procedures/functions for retrieving the fields from Cell_Type that you want to give them permission to retrieve, and for setting the fields that you want to give them permission to set.

By the way, I don't understand this:
  
> I have attempted to make [Grid_Index_Type] a private type but it then 
> becomes non-discrete and can no longer be used as a parameter in 
> functions such as "Line_of".

Making Grid_Index_Type private would cause problems for your Grid_Type definition (although you might be able to use Ada 2012's user-defined indexing features to get around it).  But why would Grid_Index_Type no longer be usable as a parameter to Line_Of?  I'm asking this just in case you have some misunderstanding about the language that needs to be corrected.

                              -- Adam