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From: "Jeffrey R. Carter" <spam.jrcarter.not@spam.not.acm.org>
Newsgroups: comp.lang.ada
Subject: Re: how to use "in out" for the "self" parameter in a function in a
 protected object?
Date: Sun, 17 Sep 2017 10:36:51 +0200
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Date: 2017-09-17T10:36:51+02:00
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On 09/17/2017 12:40 AM, Frank Buss wrote:
> 
> What's the rationale for this behavior and where can I find the definition, that 
> it is not allowed? Doesn't look useful to me, because I can do this with a 
> procedure. In fact now I have written a procedure with an "out" parameter and it 
> works. But it is annoying to declare an extra temporary variable for it when I 
> call it. Would be nice, if I could do this with a function.

Modifying the state of a protected object requires exclusive access to it (a 
lock of some sort). Simply reading the state does not require a lock, and 
multiple reads may proceed in parallel. The definition of POs in 1995 used 
protected functions for non-exclusive, read-only access, and protected 
procedures and entries for exclusive write access. One can debate whether this 
was the best decision, but that's the way POs are defined.

One way to get a function that modifies the state is to have a function that 
calls a procedure of the PO and returns the value it obtains. Taken to its 
logical conclusion, the PO can be hidden in a pkg body and accessed through 
public subprograms.

-- 
Jeff Carter
"My little plum, I am like Robin Hood. I take from
the rich, and I give to the poor. ... Us poor."
Poppy
96