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From: Robert A Duff <bobduff@shell01.TheWorld.com>
Newsgroups: comp.lang.ada
Subject: Re: String filtering
Date: 03 Oct 2005 13:59:56 -0400
Organization: The World Public Access UNIX, Brookline, MA
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Date: 2005-10-03T13:59:56-04:00
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tmoran@acm.org writes:

> >   function Intern(Table: in out Symbol_Table; -- illegal!
> >                   X: String) return Symbol;
> >...
> >   Tok: Token := Get_Token(Stream);  [-- illegal]
> >...
> >   function Get_Line(S: in out Stream) return String; -- illegal!
> Isn't that why
>     function S'Input(Stream : access Ada.Streams.Root_Stream_Type'Class)
>     return T;
> takes an access parameter?

Yes.

> Is it very difficult to do the same with Intern or Get_Token or Get_Line?

It's a pain to have to declare things 'aliased' all over the place.
It's a "cry wolf" thing -- 'aliased' should mean I'm making
possibly-permanent pointers to that thing.  But here, we don't want
a pointer (except as a temporary by-reference parameter).

"aliased Blah'Class" means "Warning Will Robinson: this procedure might
save the pointer in a global data structure."  But that's not what
S'Input is doing.

And there's some run-time overhead for access parameters -- they carry
run-time accessibility-level info with them.

Furthermore, there's (annoyingly) no way to declare a formal parameter
aliased.  So you use 'access' where 'in out' should suffice, or you
declared tagged types that have no need for a tag.

Yes, there are workarounds for the lack of [in] out params on functions
-- but they have global consequences on your code.

- Bob