Re: API design problem - buffer scatter I/O

[christoph.grein@eurocopter.com]

On 22 Nov., 23:16, Robert A Duff <bobd...@shell01.TheWorld.com> wrote:
> I didn't read the whole wikibook, but the particular part you point to
> above has quite a bit of misinformation.  Also, the early focus on
> "fat pointers" vs. "thin pointers" is confusing.  There's no reason
> to talk about such implementation details in an introductory tutorial.
> Especially if the details are wrong.
>
> There is no such concept as "fat pointer" or "thin pointer" in Ada!
>
> It does point to a real problem, though.  Given your above declarations,
> you might want to say:
>
>     This_Buf : aliased Stream_Element_Array (1..10);
>     That_Buf : aliased Stream_Element_Array (1..10_000);
>
>     Scatter_Input_Data ((This_Buf'Access, That_Buf'Access)); -- wrong!
>
> but that's illegal.  You can do this instead:
>
>     This_Buf : aliased Stream_Element_Array := (1..10 => <>);
>     That_Buf : aliased Stream_Element_Array := (1..10_000 => <>);
>
>     Scatter_Input_Data ((This_Buf'Access, That_Buf'Access)); -- OK

May be the nomenclature in wiki is wrong or confusing (and you always
have to be skeptical about the contents), but the incompatibilities
described there exist. And I remember a discussion with Robert Dewar
that Access_To_Address_Conversion does not work for access to
unconstrained just because of the bounds (in GNAT, this is just an
Unchecked_Conversion of the Address in either way).

So, Bob, could you please elaborate about what is wrong in this
description.

And I disagree that this should not be in wiki, because programmers
inevitable will fall into this trap (as I did) and the corresponding
RM pages are very difficult to grok. This is not an implementation
detail.

I do not know what a thin and a fat and a far pointer actually are,
I'm no Ada implementor. From my point of view as language user, a thin
pointer is simply an address, a fat pointer an address and the bounds.
So just the use of "thin" and "fat" is wrong, because it's an
implementation detail?