From: Darren Cook (darren_at_[hidden])
Date: 2003-10-16 04:28:14
Hi Paul,
I'm trying to use your example code now, and also to understand it, so I've
got a few questions.
1. For "#define params(z, n, _)" is "_" a convention meaning a parameter
that is not used? Is 'z' being used behind the scenese somewhere, or could I
also write:
#define params(_, n, _)
or
#define params(_1, n, _2)
2. What is the meaning of "~" here:
void func(BOOST_PP_ENUM(arity, params, ~))
I couldn't see this used in any of the examples, nor mentioned in
BOOST_PP_ENUM reference page.
Thanks,
Darren
> Hi Darren, the following implements the overloads up to ARITY pairs of
> parameters:
>
> #include <boost/preprocessor/cat.hpp>
> #include <boost/preprocessor/iteration/local.hpp>
> #include <boost/preprocessor/repetition/enum.hpp>
> #include <boost/preprocessor/repetition/repeat.hpp>
>
> #define ARITY 3
>
> class some_class {
> public:
> void add_ifthere(int d, int v) { /**/ }
> void add_normal(int d, int v) { /**/ }
> void add_only(int d, int v) { /**/ }
>
> // generates n-th parameter pair, as in:
> // int d0, int v0
>
> #define params(z, n, _) \
> int BOOST_PP_CAT(d, n), int BOOST_PP_CAT(v, n) \
> /**/
>
> // generates the n-th call to "func", as in:
> // add_ifthere(d0, v0);
>
> #define call(z, n, func) \
> func(BOOST_PP_CAT(d, n), BOOST_PP_CAT(v, n)); \
> /**/
>
> // generates a function definition of "func"
> // with "arity" parameter pairs, as in:
> // void add_ifthere(int d0, int v0, int d1, int v1) {
> // add_ifthere(d0, v0);
> // add_ifthere(d1, v1);
> // }
>
> #define make_function(arity, func) \
> void func(BOOST_PP_ENUM(arity, params, ~)) { \
> BOOST_PP_REPEAT(arity, call, func); \
> } \
> /**/
>
> // [macro repeated by local iteration]
> // makes a copy of each function for logical arity n
>
> #define BOOST_PP_LOCAL_MACRO(n) \
> make_function(n, add_ifthere) \
> make_function(n, add_normal) \
> make_function(n, add_only) \
> /**/
>
> // [boundaries of local iteration]
>
> #define BOOST_PP_LOCAL_LIMITS \
> (2, (ARITY) < 2 ? 2 : (ARITY)) \
> /**/
>
> // repeat BOOST_PP_LOCAL_MACRO with 'n' ranging
> // from 2 to ARITY
>
> #include BOOST_PP_LOCAL_ITERATE()
>
> // undefine temporary macros
> // (BOOST_PP_ macros are automatically undefined)
>
> #undef params
> #undef call
> #undef make_function
> };
>
> Regards,
> Paul Mensonides
>
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