www.digitalmars.com         C & C++   DMDScript  

digitalmars.D.learn - array !is null fails

reply BCS <ao pathlink.com> writes:
this works:

assert((cast(char*)null)[0..0] !is null);

this dosn't

assert((cast(char*)null)[0..10] !is null);

DMD v1.005 linux

Am I missing somthing?
Feb 12 2007
parent reply John Demme <me teqdruid.com> writes:
BCS wrote:


 this works:
 
 assert((cast(char*)null)[0..0] !is null);
 
 this dosn't
 
 assert((cast(char*)null)[0..10] !is null);
 
 DMD v1.005 linux
 
 Am I missing somthing?


Seems like a safety thing to me.  If an array has a non-zero length, the
still points at null, you shouldn't be looking at it.  A zero length array
is always valid, however.  A zero-length array is still an array, not
null... depending on your definition.

Dunno what the implications of such a discontinuity are, but you could just
use .ptr or (!is null && .length > 0) if you want.

-- 
~John Demme
me teqdruid.com
http://www.teqdruid.com/
Feb 12 2007
parent reply BCS <ao pathlink.com> writes:
Reply to John,


 BCS wrote:
 

 this works:
 
 assert((cast(char*)null)[0..0] !is null);
 
 this dosn't
 
 assert((cast(char*)null)[0..10] !is null);
 
 DMD v1.005 linux
 
 Am I missing somthing?
 


 Seems like a safety thing to me.  If an array has a non-zero length,
 the still points at null, you shouldn't be looking at it.


That's why it's in the assert


  A zero
 length array is always valid, however.  A zero-length array is still
 an array, not null... depending on your definition.
 


Ah... I though that [i..i].length == 1? 


 Dunno what the implications of such a discontinuity are, but you could
 just use .ptr or (!is null && .length > 0) if you want.
 


What good is the (array !is null) if it doesn't tell you when the pointer 
is null?
Feb 12 2007
parent reply "Jarrett Billingsley" <kb3ctd2 yahoo.com> writes:
"BCS" <ao pathlink.com> wrote in message 
news:ce0a334373d48c91d12a1b9c0ec news.digitalmars.com...

 Ah... I though that [i..i].length == 1?


Nope, since the second index is noninclusive, [i..i + 1] will be one element 
(just i), and [i..i] is 0.


 What good is the (array !is null) if it doesn't tell you when the pointer 
 is null?


Well if you want to see if the pointer is null then use "if(array.ptr is 
null)".  Though I'll agree with you that the discontinuity between 
zero-length null pointer arrays and more-than-zero-length null pointer 
arrays is a bit odd.
Feb 13 2007
parent reply BCS <ao pathlink.com> writes:
Reply to Jarrett,


 "BCS" <ao pathlink.com> wrote in message
 news:ce0a334373d48c91d12a1b9c0ec news.digitalmars.com...
 

 Ah... I though that [i..i].length == 1?
 


 Nope, since the second index is noninclusive, [i..i + 1] will be one
 element (just i), and [i..i] is 0.


Oh, Duh. And I've had to correct others on that one  :b


 

 What good is the (array !is null) if it doesn't tell you when the
 pointer is null?
 


 Well if you want to see if the pointer is null then use "if(array.ptr
 is null)".  Though I'll agree with you that the discontinuity between
 zero-length null pointer arrays and more-than-zero-length null pointer
 arrays is a bit odd.
 


What else could be null? I don't see anything else that it could mean other 
than "is the pointer null".
Feb 13 2007
parent reply Frits van Bommel <fvbommel REMwOVExCAPSs.nl> writes:
BCS wrote:

 Reply to Jarrett,
 

 "BCS" <ao pathlink.com> wrote in message
 news:ce0a334373d48c91d12a1b9c0ec news.digitalmars.com...


 What good is the (array !is null) if it doesn't tell you when the
 pointer is null?


 Well if you want to see if the pointer is null then use "if(array.ptr
 is null)".  Though I'll agree with you that the discontinuity between
 zero-length null pointer arrays and more-than-zero-length null pointer
 arrays is a bit odd.


 What else could be null? I don't see anything else that it could mean 
 other than "is the pointer null".


'null', when interpreted as an array, is an _empty_ array with the 
pointer being null.

This allows it to be a reasonable default initializer for dynamic 
arrays, and means (arr == null) checks whether the array is empty.
Feb 13 2007
parent reply BCS <BCS pathlink.com> writes:
Frits van Bommel wrote:

 BCS wrote:
 

 Reply to Jarrett,


 "BCS" <ao pathlink.com> wrote in message
 news:ce0a334373d48c91d12a1b9c0ec news.digitalmars.com...


 What good is the (array !is null) if it doesn't tell you when the
 pointer is null?


 Well if you want to see if the pointer is null then use "if(array.ptr
 is null)".  Though I'll agree with you that the discontinuity between
 zero-length null pointer arrays and more-than-zero-length null pointer
 arrays is a bit odd.


 What else could be null? I don't see anything else that it could mean 
 other than "is the pointer null".


 
 
 'null', when interpreted as an array, is an _empty_ array with the 
 pointer being null.
 
 This allows it to be a reasonable default initializer for dynamic 
 arrays, and means (arr == null) checks whether the array is empty.


Ok, I see that. But aren't the vast majority of comparisons ageist null 
asking "Is this readable?" That is the meaning I would expect. Then 
again it does bring in a corner case (length checked iff ptr!=null).

It's quite a bother though.

void fn(char[] c)
{

	//works untill c.ptr == null && c.length != 0

	assert(c !is null);
	writef("%s\n", c);
}
Feb 13 2007
parent reply Frits van Bommel <fvbommel REMwOVExCAPSs.nl> writes:
BCS wrote:

 Frits van Bommel wrote:

 'null', when interpreted as an array, is an _empty_ array with the 
 pointer being null.

 This allows it to be a reasonable default initializer for dynamic 
 arrays, and means (arr == null) checks whether the array is empty.


 
 Ok, I see that. But aren't the vast majority of comparisons ageist null 
 asking "Is this readable?" That is the meaning I would expect. Then 
 again it does bring in a corner case (length checked iff ptr!=null).
 
 It's quite a bother though.
 
 void fn(char[] c)
 {
 
     //works untill c.ptr == null && c.length != 0
 
     assert(c !is null);
     writef("%s\n", c);
 }


I'd argue that if you have an array with a non-zero length but null 
pointer, you've already messed up somewhere. It should have been checked 
at whatever place sliced the null pointer. (The only way to obtain such 
an array would be either slicing a raw pointer or using pointer 
casts/unions to access memory as a different type than it actually is, 
right?)

And it'd get even worse if you take a non-empty slice from such an array 
that doesn't start at index 0. Then you have'd an array with non-zero 
length and non-null pointer that is nevertheless very much invalid, and 
without any way to detect it[1].



[1]: That is, short of indexing it and detecting the segfault/access 
violation caused, or resorting to system-dependent checks like "is it in 
the first page of memory".
Feb 13 2007
parent BCS <BCS pathlink.com> writes:
Frits van Bommel wrote:

 
 
 I'd argue that if you have an array with a non-zero length but null 
 pointer, you've already messed up somewhere. It should have been checked 
 at whatever place sliced the null pointer.


Yup. But then again asserts should be used to check stuff you otherwise 
can't control (incoming arguments and returns from 3rd party code etc.)

I go by the assumption that if an assert *ever* trips then, by 
definition, you have a bug (unless you are trying to trip it in a 
unittest or something).

Conditions that might happen without bugs should be handled with exceptions.


 (The only way to obtain such 
 an array would be either slicing a raw pointer or using pointer 
 casts/unions to access memory as a different type than it actually is, 
 right?)
 
 And it'd get even worse if you take a non-empty slice from such an array 
 that doesn't start at index 0. Then you have'd an array with non-zero 
 length and non-null pointer that is nevertheless very much invalid, and 
 without any way to detect it[1].
 


Maybe there should be a gc.IsValid(void[]) function that returns true if 
  the full length of the array is readable (or is GC allocated if the 
first is to hard).

I wonder what it would take to make asserts do that check when given an 
array.

char[] c
assert(c); // is c "good"?



 
 
 [1]: That is, short of indexing it and detecting the segfault/access 
 violation caused, or resorting to system-dependent checks like "is it in 
 the first page of memory".
Feb 13 2007