http://d.puremagic.com/issues/show_bug.cgi?id=3990

           Summary: Deferencing a dynamic array as pointer
           Product: D
           Version: 2.041
          Platform: x86
        OS/Version: Windows
            Status: NEW
          Keywords: accepts-invalid
          Severity: normal
          Priority: P2
         Component: DMD
        AssignedTo: nobody@puremagic.com
        ReportedBy: bearophile_hugs@eml.cc


--- Comment #0 from bearophile_hugs@eml.cc 2010-03-20 05:57:22 PDT ---
This D2 code is currrently allowed, but I think it has to become a syntax error, dynamic arrays are not pointers:

void main() {
   int[] a1 = [5, 4, 3];
   assert(*a1 == 5);
   alias typeof(a1) T1;
   assert(is(typeof(*T1)));
   int* p1 = cast(int*)a1;
   assert(p1 == a1.ptr);
}

----------------

Similar code can be written for a fixed-size array like: int[3] a2 = [5, 4, 3];

For fixed-size arrays such conversions to pointers can be more acceptable.

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--- Comment #1 from bearophile_hugs@eml.cc 2010-05-06 04:15:13 PDT ---
Another case that I think is related:


import std.stdio: writeln;
struct Arr(int N) {
    int[N] data;
    alias data this;
}
void main() {
    auto p = new Arr!(10);
    *p = 10;
    writeln(p.data); // Output: 10 10 10 10 10 10 10 10 10 10
}

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--- Comment #2 from bearophile_hugs@eml.cc 2011-04-26 10:39:52 PDT ---
More very good rationale from Steven Schveighoffer, that I think settles the situation:

http://www.digitalmars.com/webnews/newsgroups.php?art_group=digitalmars.D&article_id=135391

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--- Comment #3 from Kenji Hara <k.hara.pg@gmail.com> 2011-10-31 04:30:21 PDT ---
(In reply to comment #0)

1.
>    assert(*a1 == 5);
>    alias typeof(a1) T1;
>    assert(is(typeof(*T1)));

Dereferencing dynamic or static array without -d option is deprecated. I think this is 'accepts-invalid'.

2.
>    int* p1 = cast(int*)a1;
>    assert(p1 == a1.ptr);

Explicit casting is still valid.

  cast(int*)a1

means

  cast(int*)a1.ptr

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--- Comment #4 from Kenji Hara <k.hara.pg@gmail.com> 2011-10-31 04:33:36 PDT ---
(In reply to comment #1)

This is not related.

>     auto p = new Arr!(10);
>     *p = 10;

means

  Arr!(10)* p = new Arr!(10);
  (*p).data = 10;  // see alias this after deref-ing of p

And, the assignment an element type into static array is valid.

  int[10] sa;
  sa = 10;  // translated to sa[] = 10

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--- Comment #5 from bearophile_hugs@eml.cc 2011-10-31 05:36:43 PDT ---
(In reply to comment #3)

> >    assert(*a1 == 5);
> >    alias typeof(a1) T1;
> >    assert(is(typeof(*T1)));
> 
> Dereferencing dynamic or static array without -d option is deprecated. I think this is 'accepts-invalid'.

Don agrees:

http://www.digitalmars.com/webnews/newsgroups.php?art_group=digitalmars.D&article_id=148054


> 2.
> >    int* p1 = cast(int*)a1;
> >    assert(p1 == a1.ptr);
> 
> Explicit casting is still valid.
> 
>   cast(int*)a1
> 
> means
> 
>   cast(int*)a1.ptr

I don't see the need to accept this cast. There is the ".ptr" so this cast is not useful in practice. And generally this cast is refused by DMD 2.056 as you see in the following program, so you are introducing/leaving an useless special case:


struct Foo {
    int* p;
    size_t n;
}
void main() {
    Foo f;
    auto x = cast(int*)f;
}


So I think cast(int*)a1 should be forbidden.

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--- Comment #6 from bearophile_hugs@eml.cc 2011-10-31 05:39:53 PDT ---
(In reply to comment #4)

> This is not related.

You are right, it's not related, it's a different problem.


> And, the assignment an element type into static array is valid.
> 
>   int[10] sa;
>   sa = 10;  // translated to sa[] = 10

It's currently valid, but I think it should be forbidden. I think D should require the [] here. See bug 3971

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--- Comment #7 from Kenji Hara <k.hara.pg@gmail.com> 2011-10-31 07:43:01 PDT ---
(In reply to comment #5)
> I don't see the need to accept this cast. There is the ".ptr" so this cast is not useful in practice. And generally this cast is refused by DMD 2.056 as you see in the following program, so you are introducing/leaving an useless special case:
> 
[snip]
> So I think cast(int*)a1 should be forbidden.

The title of this issue is "Deferencing a dynamic array as pointer".
Therefore even if you think it is not useful, we should not discuss it in here.

Instead, you can file it as a new issue.

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Kenji Hara <k.hara.pg@gmail.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |patch


--- Comment #8 from Kenji Hara <k.hara.pg@gmail.com> 2011-10-31 07:49:27 PDT ---
https://github.com/D-Programming-Language/dmd/pull/483

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--- Comment #9 from bearophile_hugs@eml.cc 2011-10-31 12:24:36 PDT ---
(In reply to comment #7)

Thank you for the patch.


> The title of this issue is "Deferencing a dynamic array as pointer".
> Therefore even if you think it is not useful, we should not discuss it in here.
>
> Instead, you can file it as a new issue.

I have lumped them together because to me they look like quite similar issues: in both cases an array is seen as a pointer. But OK, I have opened the new report bug 6869

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