Empty associative array crashes program

[Steven Schveighoffer]

On Mon, 01 Apr 2013 19:12:52 -0400, Jonathan M Davis <jmdavisProg at gmx.com>  
wrote:

> On Monday, April 01, 2013 22:32:01 =?UTF-8?B?Ikx1w61z?=.Marques
> <luismarques at gmail.com>@puremagic.com wrote:
>> I added a bug to the database, because the following code results
>> in a segfault (DMD v2.062, OS X 10.8.3, 64-bit):
>>
>> #!/usr/local/bin/rdmd
>>
>> @safe:
>>
>> void main()
>> {
>> int[string] a;
>> a["foo"] = 0;
>> a.remove("foo");
>> assert(a != null); // segfault (not exception)
>> }
>>
>> Now I come here for some further clarifications. While it seems
>> fairly clear to me that the assert should not crash the program
>> with a segmentation fault (at least with @safe), is the assert
>> actually true? (Is it even specified by the language if the AA
>> can become null again, after removing the last item?)
>>
>> Second, I didn't test this in other OSes, but assuming the bug is
>> reproducible, am I actually the first one to trip on this issue?
>> It seems too basic, but I didn't find a duplicate in the bug
>> database.
>
> The problem is that you're using != with null. Don't do that. If you  
> want to
> check whether something is null, use the is operator:
>
> assert(a !is null);
>
> If you use == or !=, it's going to do compare the objects, not their
> pointers/references, and in the case of an AA, that means that it's  
> going to
> dereference them both and compare their contents. And since null is well,
> null, dereferencing it results in a segfault.
>
> If you try and compare objects with null with ==, you get an error. The
> compiler should probably do the same with AAs but apparently doesn't.
>
> So, if there's a bug, it's the fact that the compiler allows the  
> comparison in
> the first place. Otherwise, there's no bug here.
>

No, AA's are not classes (which BTW have had that problem fixed), they are  
pImpl structs.  The equals operator should check for null before comparing  
the contents.  It is a valid bug.

-Steve