On Thu, Jun 12, 2014 at 03:26:13PM -0500, Tom Browder via Digitalmars-d-learn wrote:
> This will not compile:
> 
>   alias blah = null;
[...]

'null' is a value, not a type. Try:

	alias blah = typeof(null);


T

-- 
If it's green, it's biology, If it stinks, it's chemistry, If it has numbers it's math, If it doesn't work, it's technology.

On Thu, Jun 12, 2014 at 3:42 PM, H. S. Teoh via Digitalmars-d-learn <digitalmars-d-learn@puremagic.com> wrote:
> On Thu, Jun 12, 2014 at 03:26:13PM -0500, Tom Browder via Digitalmars-d-learn wrote:
>> This will not compile:
>>
>>   alias blah = null;
> [...]
>
> 'null' is a value, not a type. Try:
>
>         alias blah = typeof(null);

Great, that works!

What I was really trying to do was D'ify C expressions like this:

  typedef ((struct t*)0) blah;

So, taking your advice, I found this to work (at least it compiles as a translation:

  alias blah = typeof(null);

Thanks,T and Ali.

Best,

-Tom

On 06/12/2014 02:06 PM, Tom Browder via Digitalmars-d-learn wrote:

> What I was really trying to do was D'ify C expressions like this:
>
>    typedef ((struct t*)0) blah;

Is that actually a function pointer typedef? I can't parse that line. :)

> So, taking your advice, I found this to work (at least it compiles as
> a translation:
>
>    alias blah = typeof(null);

I suspect you need something else. :)

Ali

On Thu, Jun 12, 2014 at 4:17 PM, Ali Çehreli <digitalmars-d-learn@puremagic.com> wrote:
> On 06/12/2014 02:06 PM, Tom Browder via Digitalmars-d-learn wrote:
...
>> What I was really trying to do was D'ify C expressions like this:
>>
>>    typedef ((struct t*)0) blah;
...
>> So, taking your advice, I found this to work (at least it compiles as a translation:
>>
>>    alias blah = typeof(null);
>
> I suspect you need something else. :)

Undoubtedly, indeed!  [Still a WIP (work in progress).]

Best,

-Tom

On Thursday, 12 June 2014 at 20:44:16 UTC, H. S. Teoh via Digitalmars-d-learn wrote:
> On Thu, Jun 12, 2014 at 03:26:13PM -0500, Tom Browder via Digitalmars-d-learn wrote:
>> This will not compile:
>> 
>>   alias blah = null;
> [...]
>
> 'null' is a value, not a type. Try:
>
> 	alias blah = typeof(null);
>
>
> T

Yet you can alias variables...

int i;
alias j = i;

So there's something special about "null".

On 06/12/2014 03:38 PM, monarch_dodra wrote:

> Yet you can alias variables...
>
> int i;
> alias j = i;

Initially I forgot about the fact that symbols can be alias'ed as well. So that's fine.

> So there's something special about "null".

The difference is that null is an expression. It is the same limitation as not being able to alias a literal.

    alias zero = 0;
    alias blah = null;

Those two declarations fail for the same reason:

  Error: basic type expected, not 0
  Error: semicolon expected to close alias declaration
  Error: basic type expected, not null
  Error: semicolon expected to close alias declaration

The pair of error messages are somewhat silly: The first one is misleading because as we know, it should say "basic type *or symbol* expected"; and the second one is bogus because there actually is a semicolon there: :p

Ali

On Thursday, 12 June 2014 at 21:07:47 UTC, Tom Browder via Digitalmars-d-learn wrote:
> What I was really trying to do was D'ify C expressions like this:
>
>   typedef ((struct t*)0) blah;

This doesn't compile for me with GCC, and I don't know what it's supposed to mean. ((struct t*) 0) is a value, not a type...

Where does it come from?

On Thursday, 12 June 2014 at 22:54:20 UTC, Ali Çehreli wrote:
> On 06/12/2014 03:38 PM, monarch_dodra wrote:
> > So there's something special about "null".
>
> The difference is that null is an expression. It is the same limitation as not being able to alias a literal.
>
>     alias zero = 0;
>     alias blah = null;

Oh! Right. That makes sense.

So you should use enum instead:
enum zero = 0;
enum blah = null;

Thanks.

On Fri, Jun 13, 2014 at 7:59 AM, via Digitalmars-d-learn <digitalmars-d-learn@puremagic.com> wrote:
> On Thursday, 12 June 2014 at 21:07:47 UTC, Tom Browder via Digitalmars-d-learn wrote:
>>
>> What I was really trying to do was D'ify C expressions like this:
>>
>>   typedef ((struct t*)0) blah;
>
>
> This doesn't compile for me with GCC, and I don't know what it's supposed to
> mean. ((struct t*) 0) is a value, not a type...

Sorry, you're correct.  It is from a C macro and would be used for an rvalue.  Something like this:

$ cat chdr.h
struct t;
#define t_nullptr ((struct t*)0)
struct t* t_ptr = t_nullptr;

After pre-processing with "gcc -E -P" that should read:

$ cat chdr.h.i
struct t;
struct t* t_ptr = ((struct t*)0);

which does compile.

So I'm not sure how to translate that into D.   I do know my first attempt here doesn't work, even with it being surrounded by extern (C) {}:

$ cat chdr.d
struct t;
struct t* t_ptr = null;

> Where does it come from?

The usage comes from many of the C API headers in the BRL-CAD package
(http://brlcad.org).

Best,

-Tom

On Friday, 13 June 2014 at 15:05:49 UTC, Tom Browder via Digitalmars-d-learn wrote:
> On Fri, Jun 13, 2014 at 7:59 AM, via Digitalmars-d-learn
> <digitalmars-d-learn@puremagic.com> wrote:
>> On Thursday, 12 June 2014 at 21:07:47 UTC, Tom Browder via
>> Digitalmars-d-learn wrote:
>>>
>>> What I was really trying to do was D'ify C expressions like this:
>>>
>>>   typedef ((struct t*)0) blah;
>>
>>
>> This doesn't compile for me with GCC, and I don't know what it's supposed to
>> mean. ((struct t*) 0) is a value, not a type...
>
> Sorry, you're correct.  It is from a C macro and would be used for an
> rvalue.  Something like this:
>
> $ cat chdr.h
> struct t;
> #define t_nullptr ((struct t*)0)
> struct t* t_ptr = t_nullptr;
>
> After pre-processing with "gcc -E -P" that should read:
>
> $ cat chdr.h.i
> struct t;
> struct t* t_ptr = ((struct t*)0);
>
> which does compile.
>
> So I'm not sure how to translate that into D.   I do know my first
> attempt here doesn't work, even with it being surrounded by extern (C)
> {}:
>
> $ cat chdr.d
> struct t;
> struct t* t_ptr = null;
>
>> Where does it come from?
>
> The usage comes from many of the C API headers in the BRL-CAD package
> (http://brlcad.org).
>
> Best,
>
> -Tom

Remove the struct from the pointer:

struct t;
t* t_ptr = null;