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.
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