###

# Boost.Preprocessor

## Reference

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

## #define BOOST_PP_WHILE(PRED,OP,STATE)

Iterates `OP(D,STATE)` while `PRED(D,STATE)` is true.

In other words, expands to:

      OP(D, ... OP(D, OP(D,STATE) ) ... )

The depth of iteration is determined by `PRED(D,STATE)`.

For example,

      #define PRED(D,STATE) BOOST_PP_LESS_D(D,BOOST_PP_TUPLE_ELEM(2,0,STATE),BOOST_PP_TUPLE_ELEM(2,1,STATE))
      #define OP(D,STATE) (BOOST_PP_INC(BOOST_PP_TUPLE_ELEM(2,0,STATE)),BOOST_PP_TUPLE_ELEM(2,1,STATE))
      BOOST_PP_WHILE(PRED,OP,(0,3))

expands to:

      (3,3)

### 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(D,STATE) usually expands to a tuple of the same number of elements.
- D is the recursion depth and should only be used as a parameter to other macros using BOOST_PP_WHILE(). Such macros include BOOST_PP_ADD() and other arithmetic operations. For each macro using BOOST_PP_WHILE(), there is a version of the macro, distinguished by the D suffix (e.g. BOOST_PP_ADD_D()), 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.

### Note

- The value of the D parameter may exceed BOOST_PP_LIMIT_MAG.
- Using BOOST_PP_WHILE() is a bit tricky. This is due to the C++ preprocessor limitations. It is recommended to take a look at the implementations of the various PREPROCESSOR library primitives such as BOOST_PP_ADD() for additional examples.

### Example

- count_down.c
- linear_fib.c
- delay.c

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