Subject: Re: [PATCH 2/3] convert "enum date_mode" into a struct

From: John Keeping <[email-masked]>
Date: Thu, 25 Jun 2015 18:03:28 +0100

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.