On 8 May 2012 10:24, Daniel Santos <danielfsantos@att.net> wrote:
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.
There's BOOST_PP_IS_EMPTY in <boost/preprocessor/facilities/is_empty.hpp> although it isn't publicized as it won't work on some preprocessors. I haven't look closely enough to see how it compares.