Re: ui_out format strings for fields and styles (Re: [PATCH] Style "pwd" output)

[Pedro Alves <palves@redhat.com>]

On 7/1/19 1:25 PM, Tom Tromey wrote:
>>>>>> "Pedro" == Pedro Alves <palves@redhat.com> writes:
> 
> Pedro> Do you still see value in keeping %pN?  If we make nullptr mean
> Pedro> "keep the default", as you mentioned earlier, then the above can be
> Pedro> rewritten as:
> 
> Pedro> 	  fprintf_filtered (stream, " %pS<repeats %u times>%pS",
> Pedro>                             metadata_style.style ().ptr (), reps, nullptr);
> 
> I was thinking that we'd change the spelling to some form of brackets,
> in which case it would be good.  So like "%p[<repeats %u times>%p]".
> We discussed this before but I don't recall whether there was some
> counter-argument to it.
> 
> If we're just using letters, then there doesn't seem to be a reason to
> have two %p suffixes.

Ah, my counter-argument which I guess I may have never expressed was that I
liked the idea of making %pS+nullptr mean "default", for eliminating
the having to explain/think about the "must pass a nullptr to %pN" wart.

The visual balance of brackets is appealing as well, so I can't say
I have a strong preference either way.  If you've been converting things
already, you'll have a better judgment, so I'll defer to you.
(And it should go without saying, but, to everyone else, you're more
than welcome to voice your opinions too, of course.)

If we went the always-%pS way, I had written the patch and it'd look like this:

From 74899ceab18b23e45225d5560033993596c2a942 Mon Sep 17 00:00:00 2001
From: Pedro Alves <palves@redhat.com>
Date: Fri, 7 Jun 2019 22:42:57 +0100
Subject: [PATCH] Eliminate %pN

---
 gdb/common/format.c |  1 -
 gdb/ui-out.c        | 13 ++++++++-----
 2 files changed, 8 insertions(+), 6 deletions(-)

diff --git a/gdb/common/format.c b/gdb/common/format.c
index 177f79afee3..6fcbaba290d 100644
--- a/gdb/common/format.c
+++ b/gdb/common/format.c
@@ -259,7 +259,6 @@ format_pieces::format_pieces (const char **arg, bool gdb_extensions)
 		  case 's':
 		  case 'S':
 		  case 'F':
-		  case 'N':
 		    f++;
 		    break;
 		  }
diff --git a/gdb/ui-out.c b/gdb/ui-out.c
index d50fb3a3e3f..25efa98d56a 100644
--- a/gdb/ui-out.c
+++ b/gdb/ui-out.c
@@ -618,11 +618,14 @@ ui_out::vmessage (const char *format, va_list args)
 	      }
 	      break;
 	    case 'S':
-	      style = *va_arg (args, const ui_file_style *);
-	      break;
-	    case 'N':
-	      va_arg (args, void *);
-	      style = {};
+	      {
+		const ui_file_style *style_p
+		  = va_arg (args, const ui_file_style *);
+		if (style_p != nullptr)
+		  style = *style_p;
+		else
+		  style = {};
+	      }
 	      break;
 	    default:
 	      call_do_message (style, current_substring, va_arg (args, void *));
-- 
2.14.5