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Date: 28 Jan 93 15:58:29 GMT
From: progers@ajpo.sei.cmu.edu (Pat Rogers)
Subject: Re: private types and recompilation
Message-ID: <1993Jan28.105829.5919@sei.cmu.edu>
List-Id: <comp.lang.ada>

In article <9301271722.aa25516@Paris.ics.uci.edu> kanderso@mabillon.ICS.UCI.EDU
 (Kenneth Anderson) writes:
>In comp.lang.ada you write:
>
>>When Ada 83 was designed, why did the designers choose to put
>>the details of private types in package specifications, rather than
>>in package bodies (which is more in the spirit of information hiding, and
>>better supports independent compilation).
>
>I'm not sure, but I think because the compiler needs to know the size
>of the types so that it can allocate space for parameters in the
>subprograms that are defined in the spec.
>
>

Well, not really that, so much as that objects of the private type can be 
allocated by users of the package before the package body is ever compiled.  
So the compiler must have sufficient info from the package spec.  Incomplete
type definitions might help you (sometimes called the "Deferred Implementation
Scheme"):

package P is

  type Exported is private;
  ...  operations on the type Exported ...
private
  type Implementation; -- incomplete type def
  type Exported is access Implementation;
end P;

package body P is
  
  type Implementation is ...

end P;

Note however that the language doesn't always force a recompilation, say,
when a comment is added, or a private part is altered. It depends upon
what was changed, but some implementations are "smart" about it.

Pat