Re: interpret this code?

George <gbeccles@xxxxxxxxxxx> writes:

I'm working on an old (comment-free) program, and finding some of the
code is above my pay grade. I'd appreciate insight into what's going on
with the segment below.

-----------------
#define SCRN_LIST \
SCRN(Null) \
SCRN(RYG) \
SCRN(Fr00) \
SCRN(LastN) \
SCRN(DateTime) \

#define INIT_LIST \
INIT(Default) \
INIT(LastN) \
INIT(EvtLog) \
INIT(EvtLogDetail) \

typedef void (*VOID_FNC)(void);

#define SCRN(symm) (const VOID_FNC)symm##ScrnFnc,
#define INIT(symm) (const VOID_FNC)symm##InitFnc,
const VOID_FNC ScrnFnc[] = {
SCRN_LIST
INIT_LIST
};
#undef SCRN
#undef INIT

// typical fnc

void
NullScrnFnc(void) { ... }
-----------------

I think the final result wants to be ...
const void (*ScrnFnc[])() = {

The type is actually

void (*const ScrnFnc[])(void)

Your () is not quite the same as (void), and the const refers to the
whole pointer type -- i.e. the elements of the array are read-only. In
your version, what is const is the returned type of the functions
pointer to by the array elements and one never writes such things.
Writing

const int f(void);

is pointless. Of course one often sees

const char *f(void);

or

char const *f(void);

but here what is const is the pointed-to type: f is a function returning
a pointer to const char (or char const). A function returning a const pointer
type is never written:

char *const f(void);

Once the programmer has defined the "pointer to function type" called
VOID_FNC, making *that* const applies to the pointers in the array.

Adding the the fact that ScrnFnc is an array makes the type harder to
write but all C types are easy to readif you know the trick. This is
such a common issue that I've put a page up about this:

http://bsb.me.uk/c-types

NullScrnFnc,
RYGScrnFnc,
...
DefaultInitFnc,
LastNInitFnc,
...
};

The result is actually

const VOID_FNC ScrnFnc[] = {
(const VOID_FNC)NullScrnFnc,
(const VOID_FNC)RYGScrnFnc,
...
(const VOID_FNC)DefaultInitFnc,
(const VOID_FNC)LastNInitFnc,
...
};

because VOID_FNC is a typedef and so not "expanded", but I know what you
meant.

Even assuming that's correct, I don't see how it gets there. I see that
SCRN_LIST/INIT_LIST will cause ea of their listed arguments (Null, RYG,
etc) to instantiate the 'SCRN'/'INIT' macros in the ScrnFnc[]
declaration, But, ISTM, this would create something like (eg) ...
void (*NullFnc)()

Hmmm... not sure why you say that so I can't readily help. The typedef
is not expanded, of course. Maybe that's part of the confusion?

Bonus points: I don't know how to 'read' the VOID_FNC typedef. I think
that's a standard construct (ptr to void fnc?), but the logic of it
eludes me.

Does my linked page help with that?

--
Ben.
.

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