Re: [Boost-Users] RE: PP Library...

6 May 2003

      Also, I was asked by a coworker if the following would be std C++:

BOOST_PP_LIST_FOR_EACH( DO_IT, BOOST_PP_EMPTY(), ARG_LIST )

I said that it would not because BOOST_PP_EMPTY would be expanded first to
nothing, which would be the same as
BOOST_PP_LIST_FOR_EACH( DO_IT, , ARG_LIST )
which Paul explicitly said below was non-conformant.

Is this correct?
----- Original Message ----- 
From: "Eric Robert" 
To: 
Sent: Tuesday, May 06, 2003 2:19 PM
Subject: [Boost-Users] RE: PP Library...
...
Ok,
I see that there are 3 ways of doing this now. What is the
recommendation for something small like 2 or 3 lines for each native
types i.e. more or less 12 iterations?
Eric
"Paul Mensonides"  wrote
...
Tanton Gibbs wrote:
...
You can use BOOST_PP_LIST_FOR_EACH
#define ARG_LIST (char, (int, (unsigned int, (long, ...))))
#define DO_IT(r, data, elem) void foo( elem );
BOOST_PP_LIST_FOR_EACH( DO_IT, , ARG_LIST )
                                ^
This empty argument is undefined in C++.  In C99, it is allowed and
called
a
"placemarker."  Instead of passing nothing, just pass anything and
ignore
it.
// ----- //
#include 
#define ARG_LIST \
    (char, (int, (unsigned, (long, BOOST_PP_NIL)))) \
    /**/
#define DO_IT(r, ignored, elem) \
    void foo( elem ); \
    /**/
BOOST_PP_LIST_FOR_EACH( DO_IT, ?, ARG_LIST )
#undef ARG_LIST
#undef DO_IT
// ----- //
Note that there are other ways to do this also.  For instance, use of
"sequences" makes the definition of ARG_LIST easier:
// ----- //
#include 
#define ARGS \
    (char)(int)(unsigned)(long) \
    /**/
#define DO_IT(r, ignored, elem) \
    void foo( elem ); \
    /**/
BOOST_PP_SEQ_FOR_EACH( DO_IT, ?, ARGS )
#undef ARGS
#undef DO_IT
// ----- //
Of course, if what you want to generate is much larger, you might want
to
use
some form of vertical repetition:
// ----- //
#include 
#include 
#define ARGS \
    (char)(int)(unsigned)(long) \
    /**/
#define BOOST_PP_LOCAL_MACRO(n) \
    void foo( BOOST_PP_SEQ_ELEM(n, ARGS) ); \
    /**/
#define BOOST_PP_LOCAL_LIMITS \
    (0, BOOST_PP_SEQ_SIZE(ARGS) - 1) \
    /**/
#include BOOST_PP_LOCAL_ITERATE()
#undef ARGS
// ----- //
Regards,
Paul Mensonides
[Non-text portions of this message have been removed]
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