The BOOST_PP_WHILE macro represents a looping construct.
Usage
BOOST_PP_WHILE(pred, op, state)
Arguments
- 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)) ... ).
The d value that is passed to both pred and op represents the next available iteration.
Other macros that have _D suffix variants internally use BOOST_PP_WHILE--for example, BOOST_PP_ADD and BOOST_PP_ADD_D.
Using these _D versions is not strictly necessary, but passing the d value (that passed to pred or op) to these macros allows them to reenter BOOST_PP_WHILE with maximum efficiency.
To directly use this d value, rather than simply passing it to another macro, see BOOST_PP_WHILE_d.
Previously, this macro could not be used recursively inside BOOST_PP_WHILE.
This limitation no longer exists, as the library can automatically detect the next available iteration.
See Also
Requirements
Sample Code
#include <boost/preprocessor/arithmetic/add.hpp>
#include <boost/preprocessor/control/while.hpp>
#include <boost/preprocessor/tuple/elem.hpp>
#define PRED(n, state) BOOST_PP_TUPLE_ELEM(2, 1, state)
#define OP(d, state) \
OP_D( \
d, \
BOOST_PP_TUPLE_ELEM(2, 0, state), \
BOOST_PP_TUPLE_ELEM(2, 1, state) \
) \
/**/
#define OP_D(d, res, c) \
( \
BOOST_PP_ADD_D( \
d, \
res, \
BOOST_PP_DEC(c) \
), \
BOOST_PP_DEC(c) \
) \
/**/
#define SUMMATION(n) \
BOOST_PP_TUPLE_ELEM( \
2, 0, \
BOOST_PP_WHILE(PRED, OP, (n, n)) \
) \
/**/
SUMMATION(3) // expands to 6
SUMMATION(4) // expands to 10