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From: "Randy Brukardt" <randy@rrsoftware.com>
Newsgroups: comp.lang.ada
Subject: Re: Ada 2012 : aliased parameters ?
Date: Tue, 29 Mar 2011 19:12:43 -0500
Organization: Jacob Sparre Andersen Research & Innovation
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Date: 2011-03-29T19:12:43-05:00
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"Florian Weimer" <fw@deneb.enyo.de> wrote in message 
news:87lizxiy6c.fsf@mid.deneb.enyo.de...
>* AdaMagica:
>
>> Aliased parameters are meant to cure a deficiency in e.g. containers.
>> With current Ada, it's awkward to replace an element of a container.
>>
>> Now if you make the curser an aliased parameter, the function result
>> may dereference safely the cursor, since the cursor and the result are
>> tightly coupled,
>
> But this is already the case in Ada 2005.  The lifetime of passed-in
> objects extends beyond the immediate need for evaluating the
> expression:
>
> | Leaving an execution happens immediately after its completion,
> | except in the case of a _master_: the execution of a body other than
> | a package_body; the execution of a statement; or the evaluation of
> | an expression, function_call, or range that is not part of an
> | enclosing expression, function_call, range, or simple_statement
> | other than a simple_return_statement.

This works in your example because you avoid the type system completely and 
thus the accessibility checks. It also would work if you don't care that it 
can raise Program_Error if someone passes in a local variable. That runtime 
source of failure is not acceptable, and the use of alised parameters makes 
it a compile-time check.

                                     Randy.