The BOOST_PP_FOR_r macro represents a reentry into the BOOST_PP_FOR repetition construct.

Usage

BOOST_PP_FOR_ ## r(state, pred, op, macro)

Arguments

r
The next available BOOST_PP_FOR repetition.
state
The initial state.
pred
A binary predicate of the form pred(r, state).  This macro must expand to an integer in the range of 0 to BOOST_PP_LIMIT_MAG.  BOOST_PP_FOR repeatedly expands macro while this predicate returns non-zero.  This macro is called with the next available BOOST_PP_FOR repetition and the current state.
op
A binary operation of the form op(r, state).  This operation is expanded by BOOST_PP_FOR with the next available BOOST_PP_FOR repetition and the current state.  This macro is repeatedly applied to the state, each time producing a new state, until pred returns 0.
macro
A binary macro of the form macro(r, state).  This macro is expanded by BOOST_PP_FOR with the next available BOOST_PP_FOR repetition and the current state.  This macro is is repeated by BOOST_PP_FOR until pred returns 0.

Remarks

This macro expands to the sequence:
macro(r, state) macro(r, op(r, state)) ... macro(r, op(r, ... op(r, 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 r 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_FOR_, r)(state, pred, op, macro)

See Also

Requirements

Header:  <boost/preprocessor/repetition/for.hpp>

Sample Code

#include <boost/preprocessor/arithmetic/dec.hpp>
#include <boost/preprocessor/arithmetic/inc.hpp>
#include <boost/preprocessor/comparison/not_equal.hpp>
#include <boost/preprocessor/punctuation/comma_if.hpp>
#include <boost/preprocessor/repetition/for.hpp>
#include <boost/preprocessor/tuple/elem.hpp>

#define PRED(r, state) /* ............ */ \
   BOOST_PP_NOT_EQUAL(                    \
      BOOST_PP_TUPLE_ELEM(4, 0, state),   \
      BOOST_PP_INC(                       \
         BOOST_PP_TUPLE_ELEM(4, 1, state) \
      )                                   \
   )                                      \
   /**/

#define OP(r, state) /* .............. */ \
   (                                      \
      BOOST_PP_INC(                       \
         BOOST_PP_TUPLE_ELEM(4, 0, state) \
      ),                                  \
      BOOST_PP_TUPLE_ELEM(4, 1, state),   \
      BOOST_PP_TUPLE_ELEM(4, 2, state),   \
      BOOST_PP_INC(                       \
         BOOST_PP_TUPLE_ELEM(4, 3, state) \
      )                                   \
   )                                      \
   /**/

#define MACRO(r, state) /* ....................... */ \
   BOOST_PP_COMMA_IF(                                 \
      BOOST_PP_TUPLE_ELEM(4, 3, state)                \
   ) template<                                        \
      BOOST_PP_FOR_ ## r(                             \
         (0, BOOST_PP_TUPLE_ELEM(4, 0, state), _, 0), \
         PRED_2, OP, MACRO_2                          \
      )                                               \
   > class BOOST_PP_CAT(                              \
      BOOST_PP_TUPLE_ELEM(4, 2, state),               \
      BOOST_PP_TUPLE_ELEM(4, 0, state)                \
   )                                                  \
   /**/

#define PRED_2(r, state) /* ........ */ \
   BOOST_PP_NOT_EQUAL(                  \
      BOOST_PP_TUPLE_ELEM(4, 0, state), \
      BOOST_PP_TUPLE_ELEM(4, 1, state)  \
   )                                    \
   /**/

#define MACRO_2(r, state) /* ...... */ \
   BOOST_PP_COMMA_IF(                  \
      BOOST_PP_TUPLE_ELEM(4, 0, state) \
   ) class                             \
   /**/

#define TEMPLATE_TEMPLATE(low, high, name) /* ... */ \
   BOOST_PP_FOR(                                     \
      (low, high, name, 0),                          \
      PRED, OP, MACRO                                \
   )                                                 \
   /**/

TEMPLATE_TEMPLATE(2, 4, T)
/*
   expands to...
   template<class, class> class T2,
   template<class, class, class> class T3,
   template<class, class, class, class> class T4
*/