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From: "Jeffrey R. Carter" <spam.jrcarter.not@spam.not.acm.org>
Newsgroups: comp.lang.ada
Subject: Re: Running a preprocessor from GPS?
Date: Sat, 1 Aug 2015 11:59:58 -0700
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Date: 2015-08-01T11:59:58-07:00
List-Id: <comp.lang.ada>

On 08/01/2015 11:15 AM, Paul Rubin wrote:
> 
> Is there a good place to read about this?  I had the impression that
> package initialization runs unconditionally.  EGarrulo was asking how to
> get it to run only in response to a runtime request that might never
> happen.

It depends on what you mean by "initialize". Elaboration of a library pkg occurs
at start up unconditionally. There's no requirement that a pkg be a library pkg,
so that could be a way to get pkg elaboration to happen later, though it's not
common.

Initialization of an object happens when the object is created. For an object
created by an object declaration, that happens when the declaration is elaborated:

package body P is
   State : Integer := Integer'First;

If the initialization is complex, it can be done in the executable part of the
pkg body when that is elaborated:

package body P is
   State : Integer;
   ...
begin
   -- complex code that assigns a value to State
end P;

If initializing State requires information that the pkg can't get by itself,
then initialization can be performed by an operation called by a client:

package P is
   State       : Integer;
   Initialized : Boolean := False;

   procedure Initialize (Value : in Integer) is
   begin
      if Initialized then
         raise Already_Initialized;
      end if;

      State := Value;
      Initialized := True;
   end Initialize;

   procedure Op is
   begin
      if not Initialized then
         raise Not_Initialized;
      end if;
      ...
   end Op;

This latter case delays initialization to response to a run-time request that
might never happen. Allocation of storage for State still happens during
elaboration of P. Delaying allocation of storage as well is straightforward and
left as an exercise for the reader.

-- 
Jeff Carter
"I don't know why I ever come in here. The
flies get the best of everything."
Never Give a Sucker an Even Break
102