The BOOST_PP_WHILE_d macro represents a reentry into the BOOST_PP_WHILE looping construct.

Usage

BOOST_PP_WHILE_ ## d(pred, op, state)

Arguments

d
The next available BOOST_PP_WHILE iteration.
pred
A binary predicate of the form pred(d, state).  This predicate is expanded by BOOST_PP_WHILE with the next available iteration d and the current state.  This predicate must expand to value in the range of 0 to BOOST_PP_LIMIT_MAG. The construct continues to loop until this predicate returns 0.  When this predicate returns 0, BOOST_PP_WHILE returns the current state.
op
A binary operation of the form op(d, state).  This operation is expanded by BOOST_PP_WHILE with the next available iteration d and the current state.  This macro is repeatedly applied to the state, each time producing a new state, until pred returns 0.
state
The initial state.  Often this argument is a tuple.

Remarks

This macro iterates op(d, state) while pred(d, state) is non-zero.  In other words expands to:
op(d, ... op(d, op(d, state)) ... ).
At certain times, it may be necessary to perform the concatenation with BOOST_PP_CAT rather than the preprocessor token-pasting operator.  This happens when the d value is a macro invocation itself.  It needs a delay to allow it to expand.  The syntax in such a scenario becomes:
BOOST_PP_CAT(BOOST_PP_WHILE_, d)(pred, op, state).
Previously, it was possible to concatenate d directly to BOOST_PP_WHILE (without the trailing underscore).  This is no longer supported.

See Also

Requirements

Header:  <boost/preprocessor/control/while.hpp>

Sample Code

#include <boost/preprocessor/arithmetic/add.hpp>
#include <boost/preprocessor/arithmetic/dec.hpp>
#include <boost/preprocessor/array/elem.hpp>
#include <boost/preprocessor/array/size.hpp>
#include <boost/preprocessor/control/while.hpp>
#include <boost/preprocessor/tuple/elem.hpp>

#define PRED(d, data) BOOST_PP_TUPLE_ELEM(3, 1, data)

#define OP(d, data) /* ............ */ \
   OP_D(                               \
      d,                               \
      BOOST_PP_TUPLE_ELEM(3, 0, data), \
      BOOST_PP_TUPLE_ELEM(3, 1, data), \
      BOOST_PP_TUPLE_ELEM(3, 2, data)  \
   )                                   \
   /**/

#define OP_D(d, res, i, array) /* ................. */ \
   (                                                   \
      BOOST_PP_ADD_D(                                  \
         d, res,                                       \
         BOOST_PP_ARRAY_ELEM(BOOST_PP_DEC(i), array)), \
      BOOST_PP_DEC(i),                                 \
      array                                            \
   )                                                   \
   /**/

#define ACCUMULATE_D(d, array) /* .......... */ \
   BOOST_PP_TUPLE_ELEM(                         \
      3, 0,                                     \
      BOOST_PP_WHILE_ ## d(                     \
         PRED, OP,                              \
         (0, BOOST_PP_ARRAY_SIZE(array), array) \
      )                                         \
   )                                            \
   /**/

#define ARRAY (4, (1, 2, 3, 4))

ACCUMULATE_D(1, ARRAY)// expands to 10