###

# Boost.Preprocessor

## Reference

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# #include <boost/preprocessor/for.hpp>

## #define BOOST_PP_FOR(STATE,PRED,OP,MACRO)

Repeats `MACRO(R,STATE)` and iterates `OP(R,STATE)` while `PRED(R,STATE)` is true.

In other words, expands to the sequence:

      MACRO(R,STATE)
      MACRO(R,OP(R,STATE))
      MACRO(R,OP(R,OP(R,STATE)))
      ...
      MACRO(R,OP(R,OP(...OP(R,STATE)...)))

The length of the sequence is determined by `PRED(R,STATE)`.

For example,

      #define PRED(R,STATE) BOOST_PP_LESS(BOOST_PP_TUPLE_ELEM(2,0,STATE),BOOST_PP_TUPLE_ELEM(2,1,STATE))
      #define OP(R,STATE) (BOOST_PP_INC(BOOST_PP_TUPLE_ELEM(2,0,STATE)),BOOST_PP_TUPLE_ELEM(2,1,STATE))
      #define MACRO(R,STATE) BOOST_PP_TUPLE_ELEM(2,0,STATE)
      BOOST_PP_FOR((0,3),PRED,OP,MACRO)

expands to:

      0 1 2

### Legend

- STATE is the current state of iteration. The state is usually a tuple.
- PRED is the condition for iteration. It must expand to a decimal integer literal.
- OP is the iterated macro. Note that if the state is a tuple, then OP(R,STATE) usually expands to a tuple of the same number of elements.
- MACRO is the state instantiation macro.
- R is the recursion depth and should only be used as a parameter to other macros using BOOST_PP_FOR() or for invoking BOOST_PP_FOR##R() directly. For each macro using BOOST_PP_FOR(), e.g. BOOST_PP_LIST_FOR_EACH(), there is a version of the macro, e.g. BOOST_PP_LIST_FOR_EACH_R(), distinguished by the R suffix, that accepts an additional recursion depth as the first parameter. This technique is necessary to avoid recursively expanding the same macro again, which is not permitted by the C++ preprocessor.

### BOOST_PP_REPEAT() vs BOOST_PP_FOR()

BOOST_PP_FOR() is a generalization of BOOST_PP_REPEAT(). This means that BOOST_PP_REPEAT() can be implemented using BOOST_PP_FOR(). However, BOOST_PP_REPEAT() was introduced earlier, is generally easier to use, and is still quite useful on its own.

### 2D and 3D repetition

BOOST_PP_FOR() can be used for multidimensional repetition simply by invoking BOOST_PP_FOR##R() directly.

### Test

- for_test.cpp

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© Copyright Housemarque Oy 2002

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