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

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

On 4/30/26 6:49 AM, Tom de Vries wrote:
> 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.


I've written a next_iterator unit test, and added (part of) this 
demonstrator ( 
https://sourceware.org/pipermail/gdb-patches/2026-May/227065.html ).

Thanks,
- Tom