[boost] [preprocessor] use case: named parameters for macros

From: Tobias Schwinger (tschwinger_at_[hidden])
Date: 2006-10-24 16:24:41

Hi,

some of you might like this pattern - it might be even practical at places...
Maybe it's new, at least I couldn't find anything about it on the web.

Regards,

Tobias

// read bottom up

// ---- "library" code

#include <boost/preprocessor/comparison/equal.hpp>
#include <boost/preprocessor/control/iif.hpp>
#include <boost/preprocessor/tuple/elem.hpp>
#include <boost/preprocessor/cat.hpp>

// public

#define ARG(i,args,deflt) \
    BOOST_PP_TUPLE_ELEM(2,1,BOOST_PP_CAT(SEARCH_A_ ## i args (i,deflt),_END))

// private

#define SEARCH_A_0(k,v) BOOST_PP_IIF(BOOST_PP_EQUAL(0,k),(k,v) SKIP_A, SEARCH_B_0)
#define SEARCH_B_0(k,v) BOOST_PP_IIF(BOOST_PP_EQUAL(0,k),(k,v) SKIP_A, SEARCH_A_0)
#define SEARCH_A_1(k,v) BOOST_PP_IIF(BOOST_PP_EQUAL(1,k),(k,v) SKIP_A, SEARCH_B_1)
#define SEARCH_B_1(k,v) BOOST_PP_IIF(BOOST_PP_EQUAL(1,k),(k,v) SKIP_A, SEARCH_A_1)
#define SEARCH_A_2(k,v) BOOST_PP_IIF(BOOST_PP_EQUAL(2,k),(k,v) SKIP_A, SEARCH_B_2)
#define SEARCH_B_2(k,v) BOOST_PP_IIF(BOOST_PP_EQUAL(2,k),(k,v) SKIP_A, SEARCH_A_2)
// ...

#define SKIP_A(k,v) SKIP_B
#define SKIP_B(k,v) SKIP_A
#define SKIP_A_END
#define SKIP_B_END

// ---- macro with named parameters example

// public

#define MY_FIRST_ARG(value)  (0,value)
#define MY_SECOND_ARG(value) (1,value)
#define MY_THIRD_ARG(value)  (2,value)

#define MY_MACRO(args) \
    MY_MACRO_IMPL( ARG(0,args,1st_default), ARG(1,args,2nd_default), ARG(2,args,3rd_default) )

// private

#define MY_MACRO_IMPL(arg1,arg2,arg3) \
    arg1 arg2 arg3

// note: parameter names can be reused among macros

// ---- client code

MY_MACRO(
    MY_FIRST_ARG(yaba)
    MY_THIRD_ARG(doo)
    MY_SECOND_ARG(daba)
)
// expands to: yaba daba doo

MY_MACRO(
    MY_SECOND_ARG(bubu)
)
// expands to: 1st_default bubu 3rd_default.