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From: Ludovic Brenta <ludovic@ludovic-brenta.org>
Newsgroups: comp.lang.ada
Subject: Re: Forcing use of "constructors"
Date: Mon, 23 Nov 2009 05:38:03 -0800 (PST)
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Date: 2009-11-23T05:38:03-08:00
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xorque wrote:
> Hello, all.
>
> What's the preferred way to mandate that objects be created by
> the use of given "constructor" subprograms?
>
> By this, I mean that I want to create a type and the only way to
> get new values of this type is to use a subprogram that I provide.
>
> * It's a record type.
> * A few of the components must be set to "sensible" values before
> =A0 use.
> * What constitutes a "sensible" value isn't known until runtime.
> * I don't particularly want to add "is initialized" checks on all
> =A0 subprograms in the package.
>
> Regards,
> xw

Declare your type to have unknown discriminants:

package P is
   type T (<>) -- unknown discriminants: objects requires
initialization
      is [limited] private;
   function Construct return T;
private
   type T is [limited] record ... end record;
end P;

If the type is limited, the only way for a client to create an object
is to call Construct. If the type is nonlimited, the client has two
ways: call Construct or copy an existing object to a new one.

Inside Construct, you would normally use the extended return statement
(ARM 6.5(2.1/2)), e.g.

function Construct return T is
begin
   return Object : T do
      -- initialize some components
   end return;
end Construct;

You can also make your type controlled or limited controlled. You can
expose the controlledness in the public part or hide it in the private
part.

--
Ludovic Brenta;