From: "Randy Brukardt" <randy@rrsoftware.com>
Subject: Re: 'for cursor in container.first loop' bug?
Date: Thu, 28 Aug 2014 14:56:18 -0500
Date: 2014-08-28T14:56:18-05:00 [thread overview]
Message-ID: <lto1h3$cja$1@loke.gir.dk> (raw)
In-Reply-To: 85y4u8xztj.fsf@stephe-leake.org
"Stephen Leake" <stephen_leake@stephe-leake.org> wrote in message
news:85y4u8xztj.fsf@stephe-leake.org...
> I'm trying to use the 'for cursor in container.first loop' syntax in Ada
> 2012 with my own container, and having lots of problems.
>
> So first I tried a simple example using Ada.Containers.Vector, with GNAT
> 2014:
>
> with Ada.Text_Io; use Ada.Text_Io;
> with Ada.Containers.Vectors;
> procedure Try_Containers
> is
> package Integer_Vectors is new Ada.Containers.Vectors (Natural,
> Integer);
> use Integer_Vectors;
>
> A : Vector := To_Vector (1, 10);
> begin
> Loop_1 :
> for Element of A loop
> Put_Line ("A (i) => " & Integer'Image (Element));
> -- can't do Element := 2;
> end loop Loop_1;
>
> Loop_2 :
> for Cursor in First (A) loop
> Put_Line ("A (I) => " & Integer'Image (Element (Cursor)));
> Replace_Element (A, Cursor, 2);
> Reference (A, Cursor) := 2;
> end loop Loop_2;
>
> end Try_Containers;
>
> This gives compilation errors:
>
> gnatmake -k ../try_containers.adb
> gcc -c -I../ -I- ../try_containers.adb
> try_containers.adb:17:28: invalid prefix in call
> try_containers.adb:17:28: invalid prefix in selected component "I572b"
> gnatmake: "../try_containers.adb" compilation error
>
> Same error (with a different generated name) with GnatPro 7.2.
>
> Commenting out loop_2 gives the expected results.
>
> Am I missing something simple, or is this a compiler bug? At the very
> least, it's not a good error message.
You're missing something simple: "First" is not an iterator (it returns a
cursor). You need to use one of the functions named "Iterate" that return a
value that has an iterator type:
Loop_2 :
for Cursor in Iterate (A) loop
I'd still call it a bug, though, because the error message is inpenatrable
for something simple (a violation of 5.5.2(3/3)). Indeed, this shouldn't
even resolve.
I checked the two more recent GNAT compilers that I have (Gnatpro 7.2.2 and
a wavefront), but they both have this message. I'd report it as a bug,
because you probably wasted at lot of time (and even some of mine) on this
rather simple mistake.
I was a bit surprised by this, because I thought the new ACATS tests covered
this. (After some checking...) Interestingly, there is a test that checks
for this sort of problem, and it produces sensible error messages for cases
like:
95. for Item in Sparse_Cursor loop --
ERROR:
|
>>> cannot iterate over "Cursor"
>>> to iterate directly over the elements of a container, write "of
Sparse_Cursor"
116. for Item of Sparse_Cursor loop --
ERROR:
|
>>> cannot iterate over "Cursor"
But your test (which looks the same to me) produces gibberish. This is
obviously more subtle than I understand. Time to get some use out of your
AdaCore support contract. :-)
Randy.
next prev parent reply other threads:[~2014-08-28 19:56 UTC|newest]
Thread overview: 6+ messages / expand[flat|nested] mbox.gz Atom feed top
2014-08-28 17:56 'for cursor in container.first loop' bug? Stephen Leake
2014-08-28 19:56 ` Randy Brukardt [this message]
2014-08-29 13:52 ` Stephen Leake
2014-08-28 20:46 ` Simon Wright
2014-08-29 3:16 ` Randy Brukardt
2014-08-29 14:03 ` Stephen Leake
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