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< title > BOOST_PP_WHILE</ title >
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The < b > BOOST_PP_WHILE</ b > macro represents a looping construct.
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< h4 > Usage</ h4 >
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< b > BOOST_PP_WHILE</ b > (< i > pred</ i > , < i > op</ i > , < i > state</ i > )
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< h4 > Arguments</ h4 >
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< dt > pred</ dt >
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A binary predicate of the form < i > pred</ i > (< i > d</ i > , < i > state</ i > ).
This predicate is expanded by < b > BOOST_PP_WHILE</ b > with the next available
iteration < i > d</ i > and the current < i > state</ i > .
This predicate must expand to value in the range of < i > 0</ i > to < b > BOOST_PP_LIMIT_MAG</ b > .
The construct continues to loop until this predicate returns < i > 0</ i > .
When this predicate returns < i > 0</ i > , < b > BOOST_PP_WHILE</ b > returns the current < i > state</ i > .
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< dt > op</ dt >
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A binary operation of the form < i > op</ i > (< i > d</ i > , < i > state</ i > ).
This operation is expanded by < b > BOOST_PP_WHILE</ b > with the next available
iteration < i > d</ i > and the current < i > state</ i > .
This macro is repeatedly applied to the < i > state</ i > , each time producing a new < i > state</ i > , until < i > pred</ i > returns < i > 0</ i > .
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< dt > state</ dt >
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The initial state.
Often this argument is a < i > tuple</ i > .
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< h4 > Remarks</ h4 >
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This macro iterates < i > op</ i > (< i > d</ i > , < i > state</ i > ) while < i > pred</ i > (< i > d</ i > , < i > state</ i > ) is non-zero.
In other words expands to:
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< i > op</ i > (< i > d</ i > , ... < i > op</ i > (< i > d</ i > , < i > op</ i > (< i > d</ i > , < i > state</ i > )) ... ).
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The < i > d</ i > value that is passed to both < i > pred</ i > and < i > op</ i > represents the next available iteration.
Other macros that have < b > _D</ b > suffix variants internally use < b > BOOST_PP_WHILE</ b > --for example, < b > BOOST_PP_ADD</ b > and < b > BOOST_PP_ADD_D</ b > .
Using these < b > _D</ b > versions is not strictly necessary, but passing the < i > d</ i > value (that passed to < i > pred</ i > or < i > op</ i > ) to these macros allows them to reenter < b > BOOST_PP_WHILE</ b > with maximum efficiency.
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To directly use this < i > d</ i > value, rather than simply passing it to another macro, see < b > BOOST_PP_WHILE_< i > d</ i ></ b > .
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Previously, this macro could not be used recursively inside < b > BOOST_PP_WHILE</ b > .
This limitation no longer exists, as the library can automatically detect the next available iteration.
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< h4 > See Also</ h4 >
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< li >< a href = "limit_mag.html" > BOOST_PP_LIMIT_MAG</ a ></ li >
< li >< a href = "while_d.html" > BOOST_PP_WHILE_< i > d</ i ></ a ></ li >
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< h4 > Requirements</ h4 >
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< b > Header:</ b > < a href = "../headers/control/while.html" > < boost/preprocessor/control/while.hpp> </ a >
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< h4 > Sample Code</ h4 >
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#include < < a href = "../headers/arithmetic/add.html" > boost/preprocessor/arithmetic/add.hpp</ a > >
#include < < a href = "../headers/control/while.html" > boost/preprocessor/control/while.hpp</ a > >
#include < < a href = "../headers/tuple/elem.html" > boost/preprocessor/tuple/elem.hpp</ a > >
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#define PRED(n, state) < a href = "tuple_elem.html" > BOOST_PP_TUPLE_ELEM</ a > (2, 1, state)
#define OP(d, state) \
OP_D( \
d, \
< a href = "tuple_elem.html" > BOOST_PP_TUPLE_ELEM</ a > (2, 0, state), \
< a href = "tuple_elem.html" > BOOST_PP_TUPLE_ELEM</ a > (2, 1, state) \
) \
/**/
#define OP_D(d, res, c) \
( \
< a href = "add_d.html" > BOOST_PP_ADD_D</ a > ( \
d, \
res, \
< a href = "dec.html" > BOOST_PP_DEC</ a > (c) \
), \
< a href = "dec.html" > BOOST_PP_DEC</ a > (c) \
) \
/**/
#define SUMMATION(n) \
< a href = "tuple_elem.html" > BOOST_PP_TUPLE_ELEM</ a > ( \
2, 0, \
< a href = "while.html" > BOOST_PP_WHILE</ a > (PRED, OP, (n, n)) \
) \
/**/
SUMMATION(3) // expands to 6
SUMMATION(4) // expands to 10
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< i > © Copyright < a href = "http://www.housemarque.com" target = "_top" > Housemarque Oy</ a > 2002</ i >
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</ br >< i > © Copyright Paul Mensonides 2002</ i >
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< p >< small > Distributed under the Boost Software License, Version 1.0. (See
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accompanying file < a href = "../../../../LICENSE_1_0.txt" > LICENSE_1_0.txt</ a > or
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copy at < a href =
"http://www.boost.org/LICENSE_1_0.txt" > www.boost.org/LICENSE_1_0.txt</ a > )</ small ></ p >
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