[boost] For Boost PP: IF_NONEMPTY(), IS_NONEMPTY

Subject: [boost] For Boost PP: IF_NONEMPTY(), IS_NONEMPTY
From: Daniel Santos (danielfsantos_at_[hidden])
Date: 2012-05-08 05:24:01

I came up with some macros the other day trying to solve a kernel
problem and I haven't seen them anywhere else and if I found one, I sure
would have stolen it. :)  I used this to have a single function-like
macro service different numbers of parameters, essentially adding
"optional parameter" support so I didn't have to have different version
of the macro.

IF_NONEMPTY(test, result) expands to result if test is non-empty, empty
otherwise.
IS_NONEMPTY(arg) expands to 0 if arg is empty, 0x1 if it isn't.
OPT_OFFSETOF(type, member) isn't very useful in C++, but it is an
example of how the above can be used. It expands to a compiler-time
constant that is the offset of the member in the struct or union type,
unless member isn't specified, in which case it's zero.

#define __JUNK junk,
#define _if_nonempty(arg1_or_junk, result) __if_nonempty(arg1_or_junk,
result)
#define __if_nonempty(__ignored, result, ...) result
#define CONCAT_TOKEN(a, b)    _concat_token(a, b)
#define _concat_token(a, b)    a ## b

#define IF_NONEMPTY(test, result) _if_nonempty(__JUNK##test, result)
#define IS_NONEMPTY(arg)    CONCAT_TOKEN(0, IF_NONEMPTY(arg, x1))
#define OPT_OFFSETOF(type, m) ((size_t) &((*((type *)0)) IF_NONEMPTY(m,.)m))