Re: [PATCH] Specialize std::hash for ptid_t

[Simon Marchi <simon.marchi@polymtl.ca>]

On 9/11/23 10:47, Tom Tromey via Gdb-patches wrote:
> This changes hash_ptid to instead be a specialization of std::hash.
> This makes it a little easier to use with standard containers.
> ---
>  gdb/inferior.h    | 2 +-
>  gdb/regcache.c    | 2 +-
>  gdbsupport/ptid.h | 8 +++++---
>  3 files changed, 7 insertions(+), 5 deletions(-)
> 
> diff --git a/gdb/inferior.h b/gdb/inferior.h
> index 04672582984..29c90d15efa 100644
> --- a/gdb/inferior.h
> +++ b/gdb/inferior.h
> @@ -462,7 +462,7 @@ class inferior : public refcounted_object,
>  
>    /* A map of ptid_t to thread_info*, for average O(1) ptid_t lookup.
>       Exited threads do not appear in the map.  */
> -  std::unordered_map<ptid_t, thread_info *, hash_ptid> ptid_thread_map;
> +  std::unordered_map<ptid_t, thread_info *> ptid_thread_map;
>  
>    /* Returns a range adapter covering the inferior's threads,
>       including exited threads.  Used like this:
> diff --git a/gdb/regcache.c b/gdb/regcache.c
> index 9b71931bb0b..91b20b7a2a2 100644
> --- a/gdb/regcache.c
> +++ b/gdb/regcache.c
> @@ -329,7 +329,7 @@ reg_buffer::assert_regnum (int regnum) const
>     regcaches, associated to different gdbarches).  */
>  
>  using ptid_regcache_map
> -  = std::unordered_multimap<ptid_t, regcache_up, hash_ptid>;
> +  = std::unordered_multimap<ptid_t, regcache_up>;
>  
>  /* Type holding regcaches for a given pid.  */
>  
> diff --git a/gdbsupport/ptid.h b/gdbsupport/ptid.h
> index aa296b83181..96c7d9c8bfd 100644
> --- a/gdbsupport/ptid.h
> +++ b/gdbsupport/ptid.h
> @@ -157,9 +157,10 @@ class ptid_t
>    tid_type m_tid;
>  };
>  
> -/* Functor to hash a ptid.  */
> -
> -struct hash_ptid
> +namespace std
> +{
> +template<>
> +struct hash<ptid_t>
>  {
>    size_t operator() (const ptid_t &ptid) const
>    {
> @@ -170,6 +171,7 @@ struct hash_ptid
>  	    + long_hash (ptid.tid ()));
>    }
>  };
> +}

When searching on that topic, I found some sources advising against
opening namespace std, like this one:

https://quuxplusone.github.io/blog/2021/10/27/dont-reopen-namespace-std/

It can instead be written as:

template<>
struct std::hash<ptid_t>
{
  ...
};

Not sure if it matters here.

Simon