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digitalmars.D - Minor issue - zero-length fixed size arrays in variable-sized

reply Jarrett Billingsley <jarrett.billingsley gmail.com> writes:
I noticed in the spec on arrays that "A [fixed-size] array with a
dimension of 0 is allowed, but no space is allocated for it. It's
useful as the last member of a variable length struct.."  This sounds
like C99's "flexible array members," where a struct can have an array
as its last element that isn't given a size, specifically for allowing
variable-sized structs.

Well, the issue with a zero-length fixed-size array is that.. uh, you
can't access anything out of it.  The compiler disallows any indexing
of a zero-length array with constant indices, and at runtime, all
accesses caught by the array bounds checking.  Weirder still, the .ptr
of any zero-length array is always null, so you can't even do things
like "arr.ptr[5] = x;" (which would be perfectly acceptable in my
opinion).

Just a silly issue.
Jul 08 2009
next sibling parent bearophile <bearophileHUGS lycos.com> writes:
Jarrett Billingsley:

I have "solved" that problem compiling in release mode...


 I noticed in the spec on arrays that "A [fixed-size] array with a
 dimension of 0 is allowed, but no space is allocated for it.



The future most important implementation of D, LDC, allocates a "bit" for it (I
have no idea where this bit goes):
http://www.dsource.org/projects/ldc/wiki/Docs#Zero-lengthstaticarrays

Bye,
bearophile
Jul 08 2009
prev sibling next sibling parent reply Tim Matthews <tim.matthews7 gmail.com> writes:
Jarrett Billingsley wrote:

 I noticed in the spec on arrays that "A [fixed-size] array with a
 dimension of 0 is allowed, but no space is allocated for it. It's
 useful as the last member of a variable length struct.."  .......





It would be interesting if anyone has yet to find this useful. In c it 
could be used more directly. In D I think its not much use apart from 
the offset of property. It would be real bad to have to use it like this:

import std.stdio;

struct S
{
     int a;
     uint msgSize;
     char[0] message;
}

void setMessage(S* s, const char[] msg)
{
     (cast(char*)(s+S.message.offsetof))[0..s.msgSize] = msg[0..s.msgSize];
}

char[] getMessage(S* s)
{
     return (cast(char*)(s+S.message.offsetof))[0..s.msgSize];
}

S* createS(uint msgSize)
{
     S* s = cast(S*)((new ubyte[S.sizeof+msgSize]).ptr);
     s.msgSize = msgSize;
     return s;
}

void main()
{
     S* s = createS(5);
     setMessage(s, "hello");
     writeln(getMessage(s));
     setMessage(s, "world");
     writeln(getMessage(s));
}
Jul 08 2009
parent reply Daniel Keep <daniel.keep.lists gmail.com> writes:
You'd have to be mad to use it like that.

struct S
{
    int a;
    private { size_t size; char[0] data; }
    char[] message() { return data.ptr[0..length]; }
    size_t length() { return size; }

    S* opCall(size_t size)
    {
        return cast(S*)((new ubyte[S.sizeof+size]).ptr);
    }
}

void main()
{
    auto s = S(5);
    s.message[] = "hello";
    writeln(s.message);
    s.message[] = "world";
    writeln(s.message);
}
Jul 08 2009
next sibling parent Tim Matthews <tim.matthews7 gmail.com> writes:
Daniel Keep wrote:

 You'd have to be mad to use it like that.



I am mad.


 
 struct S
 {
     int a;
     private { size_t size; char[0] data; }
     char[] message() { return data.ptr[0..length]; }
     size_t length() { return size; }
 
     S* opCall(size_t size)
     {
         return cast(S*)((new ubyte[S.sizeof+size]).ptr);
     }
 }
 
 void main()
 {
     auto s = S(5);
     s.message[] = "hello";
     writeln(s.message);
     s.message[] = "world";
     writeln(s.message);
 }



I don't know what you'd have to be to use it like that though ^ :)
Jul 08 2009
prev sibling parent Tim Matthews <tim.matthews7 gmail.com> writes:
Steven Schveighoffer wrote:

  Having the compiler do the right
 thing is a much better option :)
 
 -Steve



Have you reported this issue as a bug/enhancement?
Jul 10 2009
prev sibling next sibling parent Jarrett Billingsley <jarrett.billingsley gmail.com> writes:
On Thu, Jul 9, 2009 at 2:38 AM, Daniel Keep<daniel.keep.lists gmail.com> wr=
ote:

 You'd have to be mad to use it like that.

 struct S
 {
 =A0 =A0int a;
 =A0 =A0private { size_t size; char[0] data; }
 =A0 =A0char[] message() { return data.ptr[0..length]; }
 =A0 =A0size_t length() { return size; }

 =A0 =A0S* opCall(size_t size)
 =A0 =A0{
 =A0 =A0 =A0 =A0return cast(S*)((new ubyte[S.sizeof+size]).ptr);
 =A0 =A0}
 }

 void main()
 {
 =A0 =A0auto s =3D S(5);
 =A0 =A0s.message[] =3D "hello";
 =A0 =A0writeln(s.message);
 =A0 =A0s.message[] =3D "world";
 =A0 =A0writeln(s.message);
 }



You can't.  The rest of my post explains that T[0].ptr is always null,
for some reason.

Currently I'm doing almost the same thing, though (cast(char*)(this +
1)[0 .. size]).
Jul 09 2009
prev sibling next sibling parent "Steven Schveighoffer" <schveiguy yahoo.com> writes:
On Thu, 09 Jul 2009 09:47:44 -0400, Jarrett Billingsley  
<jarrett.billingsley gmail.com> wrote:


 On Thu, Jul 9, 2009 at 2:38 AM, Daniel Keep<daniel.keep.lists gmail.com>  
 wrote:

 You'd have to be mad to use it like that.

 struct S
 {
    int a;
    private { size_t size; char[0] data; }
    char[] message() { return data.ptr[0..length]; }






...




 You can't.  The rest of my post explains that T[0].ptr is always null,
 for some reason.

 Currently I'm doing almost the same thing, though (cast(char*)(this +
 1)[0 .. size]).



Would something like this work?

(&data)[0..length]

-Steve



 Jul 09 2009



prev sibling next sibling parent  Jarrett Billingsley <jarrett.billingsley gmail.com>  writes:


On Thu, Jul 9, 2009 at 1:09 PM, Steven Schveighoffer<schveiguy yahoo.com> wrote:


 Would something like this work?

 (&data)[0..length]



Nope; &data is a char[0]*, and slicing it will get you a char[0][].
Which is pretty useless :D



 Jul 09 2009



prev sibling  parent  "Steven Schveighoffer" <schveiguy yahoo.com>  writes:


On Thu, 09 Jul 2009 20:24:16 -0400, Jarrett Billingsley  
<jarrett.billingsley gmail.com> wrote:


 On Thu, Jul 9, 2009 at 1:09 PM, Steven  
 Schveighoffer<schveiguy yahoo.com> wrote:


 Would something like this work?

 (&data)[0..length]



 Nope; &data is a char[0]*, and slicing it will get you a char[0][].
 Which is pretty useless :D



d'oh!

Yeah, the only improvement I can make on your code then is:

(cast(char*)&data)[0..length];

Or the more general:

(cast(typeof(data[0])*)&data)[0..length]

Which still involves a cast.  Crappy.  Having the compiler do the right  
thing is a much better option :)

-Steve



 Jul 10 2009