Re: [PATCH 3/7] [gdbsupport] Factor out base_next_iterator

[Tom de Vries <tdevries@suse.de>]

On 4/24/26 6:21 PM, Tom Tromey wrote:
> Tom> +   Instead of factoring out a base class, we could use something like this:
> Tom> +
> Tom> +     template<typename T, auto F = &T::next>
> Tom> +     struct next_iterator
> Tom> +     {
> Tom> +       ...
> Tom> +       self_type &operator++ ()
> Tom> +       {
> Tom> +	 m_item = m_item->*F;
> Tom> +	 return *this;
> Tom> +       }
> Tom> +       ...
> Tom> +     }
> Tom> +
> Tom> +  but that has the drawback that it doesn't work with incomplete T.  */
>   
> I am curious about this because I wonder how the current code could work
> with an incomplete T -- since the code references T::next directly.

Yeah, that's a good question.

I wrote the following stand-alone demonstrator:
...
$ cat test.c
#include <cstddef>
#include <utility>
#include <iterator>
#include <cstdio>

#include "gdbsupport/next-iterator.h"

struct list;

void
print_pointer (list *l)
{
   auto begin = next_iterator<struct list> (l);
   for (auto elem : next_range<struct list> (begin))
     printf ("%p\n", elem);
}

struct list
{
   int i;
   struct list *next;
};

void
print_int (list *l)
{
   auto begin = next_iterator<struct list> (l);
   for (auto elem : next_range<struct list> (begin))
     printf ("%d\n", elem->i);
}

struct list a = { 1, nullptr };
struct list b = { 2, &a };
struct list c = { 3, &b };

int
main ()
{
   print_pointer (&c);
   print_int (&c);

   return 0;
}
$ g++ test.c -I .
$ ./a.out
0x404040
0x404030
0x404020
3
2
1
$
...

I asked an LLM about this, and it explained that this is deferred or 
delayed template instantiation at work.

Basically the operator++ is used in print_pointer, but not yet 
instantiated, so we don't have to know the details of the incomplete 
type at that point.

Thanks,
- Tom