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From: John Keeping <john@keeping.me.uk>
To: Jeff King <peff@peff.net>
Cc: "H.Merijn Brand" <h.m.brand@xs4all.nl>, git@vger.kernel.org
Subject: Re: [PATCH 2/3] convert "enum date_mode" into a struct
Date: Thu, 25 Jun 2015 18:03:28 +0100	[thread overview]
Message-ID: <20150625170328.GV18226@serenity.lan> (raw)
In-Reply-To: <20150625165501.GB23503@peff.net>

On Thu, Jun 25, 2015 at 12:55:02PM -0400, Jeff King wrote:
> In preparation for adding date modes that may carry extra
> information beyond the mode itself, this patch converts the
> date_mode enum into a struct.
> 
> Most of the conversion is fairly straightforward; we pass
> the struct as a pointer and dereference the type field where
> necessary. Locations that declare a date_mode can use a "{}"
> constructor.  However, the tricky case is where we use the
> enum labels as constants, like:
> 
>   show_date(t, tz, DATE_NORMAL);
> 
> Ideally we could say:
> 
>   show_date(t, tz, &{ DATE_NORMAL });
> 
> but of course C does not allow that.

Yes it does, e.g. in 6.5.2.5 of C11, example 3 shows:

	drawline(&(struct point){.x=1, .y=1},
		&(struct point){.x=3, .y=4});

The cast is required, but if the argument is pointer-to-const you can
construct a temporary in the function call.

Of course, whether all of the compilers we target support it is a
different question.  If they do, perhaps something like:

#define SIMPLE_DATE(f)		&(struct date_mode) { DATE_NORMAL }

would allow the callers to remain reasonably sane.

>                                      Likewise, we cannot
> cast the constant to a struct, because we need to pass an
> actual address. Our options are basically:
> 
>   1. Manually add a "struct date_mode d = { DATE_NORMAL }"
>      definition to each caller, and pass "&d". This makes
>      the callers uglier, because they sometimes do not even
>      have their own scope (e.g., they are inside a switch
>      statement).
> 
>   2. Provide a pre-made global "date_normal" struct that can
>      be passed by address. We'd also need "date_rfc2822",
>      "date_iso8601", and so forth. But at least the ugliness
>      is defined in one place.
> 
>   3. Provide a wrapper that generates the correct struct on
>      the fly. The big downside is that we end up pointing to
>      a single global, which makes our wrapper non-reentrant.
>      But show_date is already not reentrant, so it does not
>      matter.
> 
> This patch implements 3, along with a minor macro to keep
> the size of the callers sane.

  reply	other threads:[~2015-06-25 17:04 UTC|newest]

Thread overview: 31+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2015-06-25 11:19 several date related issues H.Merijn Brand
2015-06-25 12:44 ` Jeff King
2015-06-25 12:56   ` H.Merijn Brand
2015-06-25 16:53     ` [PATCH 0/3] localized date format Jeff King
2015-06-25 16:54       ` [PATCH 1/3] show-branch: use DATE_RELATIVE instead of magic number Jeff King
2015-06-25 16:55       ` [PATCH 2/3] convert "enum date_mode" into a struct Jeff King
2015-06-25 17:03         ` John Keeping [this message]
2015-06-25 17:22           ` Jeff King
2015-07-07 20:37         ` Junio C Hamano
2015-07-07 20:48           ` Jeff King
2015-07-07 21:05             ` Junio C Hamano
2015-07-07 21:13               ` Jeff King
2015-07-07 21:19                 ` Junio C Hamano
2015-06-25 16:55       ` [PATCH 3/3] introduce "format" date-mode Jeff King
2015-06-29 22:22         ` Eric Sunshine
2015-06-30 10:20           ` Jeff King
2015-06-30 16:22             ` Junio C Hamano
2015-06-30 17:50               ` Jeff King
2015-06-30 19:23                 ` Junio C Hamano
2015-06-30 19:33                   ` Jeff King
2015-06-30 16:58             ` Eric Sunshine
2015-06-30 17:58               ` Jeff King
2015-06-30 18:13                 ` Eric Sunshine
2015-06-30 19:22                   ` Jeff King
2015-06-30 17:05             ` Junio C Hamano
2015-06-30 17:48               ` Eric Sunshine
2015-06-30 19:17               ` Jeff King
2015-06-30 13:26           ` Jeff King
2015-06-30 17:05             ` Eric Sunshine
2015-07-21  0:41             ` Eric Sunshine
2015-07-21  1:19               ` Jeff King

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