From: Aleksey Gurtovoy (agurtovoy_at_[hidden])
Date: 2002-04-12 04:06:53
Vesa Karvonen wrote:
> >MPL calls these BOOST_MPL_PP_PROJECT1ST and BOOST_MPL_PP_PROJECT2ND
> >(taking the STL terminology).
>
> The preprocessor library actually contains equivalent macros, but they are
> not documented and are currently considered a private implementation
detail.
> The BOOST_PP_TUPLE_ELEM() macro is implemented using macros that look like
> this:
>
> // ...
> #define BOOST_PP_TUPLE2_ELEM0(A,B) A
> #define BOOST_PP_TUPLE2_ELEM0(A,B) B
>
> #define BOOST_PP_TUPLE3_ELEM0(A,B,C) A
> #define BOOST_PP_TUPLE3_ELEM1(A,B,C) B
> #define BOOST_PP_TUPLE3_ELEM2(A,B,C) C
> // ...
>
> It seems unlikely that the implementation would change, because the
> alternative would be a slower implementation of BOOST_PP_TUPLE_ELEM(). So,
> perhaps these macros should be documented.
I would much prefer if the public macros had more generic names (preferably
STL-ish ones, something like I've cited above - BOOST_PP_PROJECT1ST,
BOOST_PP_PROJECT2ND :).
3 (and more)-argument forms don't seem necessary because the need for
PROJECT1ST/2ND arises only when you want to pass the macro to an iteration
primitive, and all of those (to my best knowledge) work only with binary
macros.
Aleksey