• Qian Xu (30/30) Mar 31 2009 Hi All,
• Unknown W. Brackets (8/47) Mar 31 2009 Please remember, strings are not objects.
• Frits van Bommel (13/15) Mar 31 2009 They're not the same operation. (Though there was quite a large debate o...
• Sergey Gromov (4/14) Mar 31 2009 When you compare null to an array, null is first converted into an empty
• BCS (3/10) Mar 31 2009 FWIW while that may be logically correct, I don't think that's how it ac...

Qian Xu <quian.xu stud.tu-ilmenau.de> writes:

Hi All,

When I was trying to learn how char-array works, I found something
unexpected.

-------------------------- code ------------------------------
module string_test;

void main()
{
  // test 1
  assert(null == "", "null is empty"); // No segfault

  // test 2
  char[] test; // test = null;
  assert(test is null, "undefined_string is null");
  assert(test == "", "undefined_string (null) is empty");
  assert(test.length == 0, "undefined_string.length == 0");

  // test 3
  test = "";
  assert(test !is null, "empty_string is NOT null");
  assert(test == "", "empty_string is empty");
  assert(test.length == 0, "empty_string.length == 0");

  // test 4
  test = "hello";
  assert(test !is null, "non_empty_string is NOT empty");
  assert(test != "", "non_empty_string is NOT empty");
  assert(test.length > 0, "non_empty_string.length > 0");
}
-------------------------- code ------------------------------

I just wondered, why the first test does not lead to a segfault. Is this an
undocumented compiler feature? I have tested it with gdc and gdmd. Both no
segfault.


--Qian

"Unknown W. Brackets" <unknown simplemachines.org> writes:

Please remember, strings are not objects.

Therefore, a comparison against null does not cause a segfault, as it 
might with an object.  In the case of arrays, "test == null" and "test 
is null" should be the same operation.

In contrast, if you had null == new Object(), you would've seen:

name.d(6): Error: use 'is' instead of '==' when comparing with null

-[Unknown]


Qian Xu wrote:
 Hi All,
 
 When I was trying to learn how char-array works, I found something
 unexpected.
 
 -------------------------- code ------------------------------
 module string_test;
 
 void main()
 {
   // test 1
   assert(null == "", "null is empty"); // No segfault
 
   // test 2
   char[] test; // test = null;
   assert(test is null, "undefined_string is null");
   assert(test == "", "undefined_string (null) is empty");
   assert(test.length == 0, "undefined_string.length == 0");
 
   // test 3
   test = "";
   assert(test !is null, "empty_string is NOT null");
   assert(test == "", "empty_string is empty");
   assert(test.length == 0, "empty_string.length == 0");
 
   // test 4
   test = "hello";
   assert(test !is null, "non_empty_string is NOT empty");
   assert(test != "", "non_empty_string is NOT empty");
   assert(test.length > 0, "non_empty_string.length > 0");
 }
 -------------------------- code ------------------------------
 
 I just wondered, why the first test does not lead to a segfault. Is this an
 undocumented compiler feature? I have tested it with gdc and gdmd. Both no
 segfault.
 
 
 --Qian

Frits van Bommel <fvbommel REMwOVExCAPSs.nl> writes:

Unknown W. Brackets wrote:
 In the case of arrays, "test == null" and "test 
 is null" should be the same operation.

They're not the same operation. (Though there was quite a large debate on these 
newsgroups a while back because a lot of people thought they *should* be, as
you 
said)

The difference:
'==', when used on arrays, compares first the length and then all elements
(only 
if the lengths are equal, otherwise the result is already known).
Since 'null' contains no elements, only the length gets compared for (null == 
test). Either the other array is also empty and they're therefore equal, or the 
other array isn't and they aren't.
'is', when used on arrays, compares both the length and the pointer. It only 
returns true if both are equal. It never looks at the elements.

So (null == ""), but (null !is "").

Sergey Gromov <snake.scaly gmail.com> writes:

Tue, 31 Mar 2009 16:29:30 +0200, Qian Xu wrote:

 When I was trying to learn how char-array works, I found something
 unexpected.
 
 -------------------------- code ------------------------------
 module string_test;
 
 void main()
 {
   // test 1
   assert(null == "", "null is empty"); // No segfault

When you compare null to an array, null is first converted into an empty
array, then a comparison takes place.  This happens because arguments to
comparison operator are first converted to a common type.

BCS <ao pathlink.com> writes:

Reply to Sergey,

 Tue, 31 Mar 2009 16:29:30 +0200, Qian Xu wrote:
 
 assert(null == "", "null is empty"); // No segfault

 When you compare null to an array, null is first converted into an
 empty array, then a comparison takes place.  This happens because
 arguments to comparison operator are first converted to a common type.
 

FWIW while that may be logically correct, I don't think that's how it actually 
gets implemented.