Re: Cannot alias null
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?
Re: Cannot alias null
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.
Re: Cannot alias null
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
Re: Cannot alias null
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;
Re: Cannot alias null
On Friday, 13 June 2014 at 15:05:49 UTC, Tom Browder via
Digitalmars-d-learn wrote:
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;
This seems to work fine for me. What's the problem?
Note this isn't *strictly* the same as what you have in C, as
null is not strongly bindded to a single type. If you want a
null pointer which is pre-emptivelly strongly, then you can use
an eponymous template:
enum nullptr(T) = (T*).init;
auto p = nullptr!t;
Throw in an alias, and you got it working exactly like an C:
enum nullptr(T) = (T*).init;
alias t_nullptr = nullptr!t;
struct t* t_ptr = t_nullptr;
That said, I'd advise against this. Just use null and move on.
It's idiomatic.
Re: Cannot alias null
On Fri, Jun 13, 2014 at 10:15 AM, monarch_dodra via
Digitalmars-d-learn digitalmars-d-learn@puremagic.com wrote:
On Friday, 13 June 2014 at 15:05:49 UTC, Tom Browder via Digitalmars-d-learn
wrote:
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;
This seems to work fine for me. What's the problem?
I use dmd -c .d (note file name change) and get:
t.d(2): Error: { } expected following aggregate declaration
t.d(2): Error: Declaration expected, not '*'
Taking Philpax's suggestion I try
$ cat t.d
struct t;
t* t_ptr = null;
$ dmd -c t.d
and get a good build.
Note this isn't *strictly* the same as what you have in C, as null is not
...
strongly bindded to a single type. If you want a null pointer which is
pre-emptivelly strongly, then you can use an eponymous template:
...
That said, I'd advise against this. Just use null and move on. It's
idiomatic.
So this is the correct (i.e., good enough) solution then:
$ cat t.d
struct t;
t* t_ptr = null;
Sounds good to me.
Thanks all.
Best,
-Tom
Re: Cannot alias null
On 06/12/2014 01:26 PM, Tom Browder via Digitalmars-d-learn wrote: This will not compile: alias blah = null; The dmd message are: di/test_hdr.d(10): Error: basic type expected, not null di/test_hdr.d(10): Error: semicolon expected to close alias declaration di/test_hdr.d(10): Error: Declaration expected, not 'null' Are there any other objects that cannot be aliased? alias works only with types. Being an expression (not an object), null cannot not work with alias. Ali
Re: Cannot alias null
On 6/12/14, 4:29 PM, Ali Çehreli wrote:
On 06/12/2014 01:26 PM, Tom Browder via Digitalmars-d-learn wrote:
This will not compile:
alias blah = null;
The dmd message are:
di/test_hdr.d(10): Error: basic type expected, not null
di/test_hdr.d(10): Error: semicolon expected to close alias declaration
di/test_hdr.d(10): Error: Declaration expected, not 'null'
Are there any other objects that cannot be aliased?
alias works only with types. Being an expression (not an object), null
cannot not work with alias.
Ali
void foo() {}
alias bar = foo();
Am I just misunderstanding what is meant by types?
Re: Cannot alias null
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.
Re: Cannot alias null
On 06/12/2014 01:36 PM, Andrew Edwards wrote:
void foo() {}
alias bar = foo();
Am I just misunderstanding what is meant by types?
Seems to be an old behavior. That does not compile with 2.066:
Error: function declaration without return type. (Note that constructors
are always named 'this')
The following compiles though:
alias bar = foo;
I stand corrected: alias works not only with types but with symbols as
well. I was right about the original code though: Aliases cannot be
used for expressions.
Ali
Re: Cannot alias null
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
Re: Cannot alias null
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
Re: Cannot alias null
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
Re: Cannot alias null
since null is a value maybe you want enum blah = null; you may also give it a type after the enum word
Re: Cannot alias null
On Thu, Jun 12, 2014 at 4:58 PM, Adam D. Ruppe via Digitalmars-d-learn digitalmars-d-learn@puremagic.com wrote: since null is a value maybe you want enum blah = null; That works. you may also give it a type after the enum word But I can't get any other variant to work so far. -Tom
Re: Cannot alias null
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.
Re: Cannot alias null
On Thursday, 12 June 2014 at 21:58:32 UTC, Adam D. Ruppe wrote:
since null is a value maybe you want
enum blah = null;
you may also give it a type after the enum word
I *think* the issue might be that null is an rvalue? Because
you can alias variable names all you want. I do it all the time
for templates where I *may* need a temporary.
eg:
void foo(T)(T val)
{
static if (isUnsigned!T)
alias uval = val;
else
auto uval = unsigned(val);
...
}
It's also quite useful with varargs:
alias a0 = args[0];
Also, you can't alias things like int.init either. I'm not sure
the rvalue thing is the source, because these work:
//struct S
{
static int i;
static int j() @property;
}
alias a = S.i;
alias b = S.j;
//
I'd consider filling a bug report.
Re: Cannot alias 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
