Thread overview
§ 28.3 Pointers and the Garbage Collector
Apr 07
kdevel
Apr 07
kdevel
April 07
In § 28.3 Pointers and the Garbage Collector [1] we read

   Do not add or subtract an offset to a pointer such that the result points
   outside of the bounds of the garbage collected object originally allocated.

      char* p = new char[10];
      char* q = p + 6; // ok
      q = p + 11;      // error: undefined behavior
      q = p - 1;       // error: undefined behavior

C and C++ allow a pointer to point to the (non-existing) element after the end
of the array:

   char *e = p + 10;

Does this point "outside of the bounds of the garbage collected object"?

In § 28.3 we also read

   Do not depend on the ordering of pointers:

      if (p1 < p2)  // error: undefined behavior
         ...

   since, again, the garbage collector can move objects around in memory.

In C and C++ we are used to read code like this:

   void foo ()
   {
      char *p = new char [10];
      char *e = p + 10;
      char *q;
      for (q = p; q < e; ++q)
         ...
   }

Does this for-loop "depend on the ordering of pointers"?

[1] https://dlang.org/spec/garbage.html#pointers_and_gc
April 07
On Sunday, 7 April 2019 at 10:05:26 UTC, kdevel wrote:
> In § 28.3 Pointers and the Garbage Collector [1] we read
>
>    Do not add or subtract an offset to a pointer such that the result points
>    outside of the bounds of the garbage collected object originally allocated.
>
> [...]

No the foo() code would work in D too but in D since we have a true array type with ptr+length you should not write this kind of code, which is not @safe BTW.
April 07
On Sunday, 7 April 2019 at 10:17:53 UTC, AltFunction1 wrote:
> On Sunday, 7 April 2019 at 10:05:26 UTC, kdevel wrote:
>> In § 28.3 Pointers and the Garbage Collector [1] we read
>>
>>    Do not add or subtract an offset to a pointer such that the result points
>>    outside of the bounds of the garbage collected object originally allocated.
>>
>> [...]
>
> No the foo() code would work in D too but in D since we have a true array type with ptr+length you should not write this kind of code, which is not @safe BTW.

I appreciate your constructive reply. What about the formal validity wrt. to
the documentation? Is char *e = p + 10; outside of the bounds of the garbage collected object? Does for (q = p; q < e; ++q) depend on the ordering of pointers?