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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 > <EFBFBD> Copyright < a href = "http://www.housemarque.com" target = "_top" > Housemarque Oy< / a > 2002< / i >
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< / br > < i > <EFBFBD> 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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