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From: "Randy Brukardt" <randy@rrsoftware.com>
Newsgroups: comp.lang.ada
Subject: Re: Access types as parameters
Date: Fri, 17 Jul 2009 20:03:12 -0500
Organization: Jacob Sparre Andersen
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Date: 2009-07-17T20:03:12-05:00
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"rickduley" <rickduley@gmail.com> wrote in message 
news:8410fc60-9b8a-4f82-92fc-622a6bbe5931@i18g2000pro.googlegroups.com...
> I don't quite see what you mean by:
>
>> But this does not means � access My_Type'Class � is better beceause it
>> is � safer � (quotes, beceause the other way is not always not safe),
>> as sometime, My_Access_Type is mandatory, depending on what the
>> function have to do with the reference.
>
>If the function is called, and the actual parameter is valid, what
>difference can it make what the function does with the data?

He might be referring to the fact that the uses of the anonymous access 
parameter might raise Program_Error (as there is a dynamic accessibility 
check) while the named example is either going to be legal (and work 
properly) or illegal.

My answer to the original question would have been:

>Given:
>
>   type My_Type is  ...;
>and
>   type My_Access_Type is access all My_Type'Class;
>
>what is the practical difference between:
>
>   function My_Function (Thing : access My_Type'Class) return Positive;
>and
>   function My_Function (Thing : My Access_Type) return Positive;

The first has a runtime parameter passing overhead that the latter does not, 
and depending on how the parameter is used inside of the function, a 
significant possibility of raising Program_Error later. (Something Bob Duff 
calls a "tripping hazard"). This latter problem is especially bad as it is 
likely to be missed in unit testing (in that sense, it is similar to 
assuming the lower bound of a string is 1 - tests often make the same 
assumption).

So it is best to avoid the first form unless you have a particular need for 
dispatching on an access value (which won't happen here, because the 
designated type is class-wide).

                                             Randy.