Just a friendly reminder about using arrays in boolean conditions

[kdevel]

On Sunday, 17 November 2024 at 16:47:39 UTC, Nick Treleaven wrote:
> [...]
> It was deprecated and then un-deprecated - see 
> https://issues.dlang.org/show_bug.cgi?id=4733#c38 for why it 
> was problematic, possible solutions and also a link to a 2015 
> discussion.

Problematic is valid code like

    if (auto arr = make_me_an_array ()) {
       ...
    }

There is another interesting aspect as Steven pointed out in that
2015 discussion [1]:

    || The "truthiness" of an array says it's true ONLY if both the
    || pointer and length are 0.
    | Ugh, *false* only if they are both 0. If either are not zero,
    | then it's true.

And later he stated [2]

    | arr.ptr == null -> arr contains a null pointer (length could
    | technically be non-zero).

My question is: Is it possible that a valid D program gets into a
state where an array has ptr == null and length > 0? If so, how?

Such a "null array" can at least not be printed:

    import std.stdio;

    void main ()
    {
       int [] arr;

       struct V {
          size_t length;
          void *ptr;
       }
       auto q = cast (V *) &(arr);
       (*q).length = 2;

       writeln (arr); // segfault in formatValueImpl
    }


[1] Re: string <-> null/bool implicit conversion
     https://forum.dlang.org/post/mr54qg$15qj$1@digitalmars.com

[2] https://forum.dlang.org/post/mr72sf$2sc7$1@digitalmars.com