From the D Blog -- Interfacing D with C: Strings Part One

[Виталий Фадеев]

On Wednesday, 26 May 2021 at 16:35:36 UTC, Ali Çehreli wrote:
> On 5/25/21 9:00 PM, Виталий Фадеев wrote:
>
> >      immutable(char)* toStringz( ref string s )
> >      {
> >          if ( s.capacity <= s.length )
> >              s.reserve( s.length + 1 );
> >
> >          char* cptr = cast( char* ) s.ptr; // C ptr
> >          char* zptr = cptr + s.length;     // zero ptr
> >          *zptr = '\0';
>
> That's undefined behavior because that location does not belong 
> to the string. Here is an example that defeats the proposed 
> toStringz:
>
> void main()
> {
>     string s;
>     s = "D string";
>
>     auto c_string = toStringz( s );
>
>     auto other = s;
>     other ~= 'X';    // <-- Seemingly unrelated operation
>
> // ...
> }
>
> puts accesses that unrelated 'X' and more bytes after that:
>
>   C string: D stringX1^

>
> Ali

Yes. True.

reserve/capacity - not for all cases.