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From: "Jeffrey R. Carter" <spam.jrcarter.not@spam.not.acm.org>
Newsgroups: comp.lang.ada
Subject: Re: deferred constants
Date: Wed, 7 Jul 2021 22:40:25 +0200
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On 7/7/21 9:31 PM, Matt Borchers wrote:
> Is it possible to define a constant in a package body that has it's initialization deferred to elaboration?
> 
> with Gnat.RegExp;
> package body
>      pat : constant Gnat.RegExp.RegExp;
> begin
>      pat := Gnat.RegExp.compile( "..." );
> end;

Deferred constants are defined in ARM 7.4 
(http://www.ada-auth.org/standards/rm12_w_tc1/html/RM-7-4.html), which says they 
may only appear in the visible part of a pkg spec, and the full declaration must 
appear in the private part of the same pkg. So what you're trying is illegal.

In cases like this, you declare the object as a variable, with comments 
indicating that it is set later and is then constant.

of course, in the example, one can simply do

Pat : constant Gnat.RegExp.RegExp := Gnat.RegExp.Compile ( "..." );

but there are cases where this is not possible.

-- 
Jeff Carter
"He had no conception of the instrument. He
was blowing into it."
Take the Money and Run
135