From: Ludovic Brenta <ludovic@ludovic-brenta.org>
Subject: Re: Forcing use of "constructors"
Date: Mon, 23 Nov 2009 05:38:03 -0800 (PST)
Date: 2009-11-23T05:38:03-08:00 [thread overview]
Message-ID: <27db03bc-9655-4a65-a730-bcad83bfe616@h2g2000vbd.googlegroups.com> (raw)
In-Reply-To: 15055d2d-0dd5-4c84-bd4a-1cc60e9dea2d@p32g2000vbi.googlegroups.com
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
> use.
> * What constitutes a "sensible" value isn't known until runtime.
> * I don't particularly want to add "is initialized" checks on all
> 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;
next prev parent reply other threads:[~2009-11-23 13:38 UTC|newest]
Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top
2009-11-23 11:41 Forcing use of "constructors" xorque
2009-11-23 13:38 ` Ludovic Brenta [this message]
2009-11-23 14:15 ` xorque
2009-11-23 14:20 ` Maciej Sobczak
2009-11-23 16:18 ` Robert A Duff
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