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< title > Boost.Preprocessor - Reference</ title >
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< h1 align = "center" > Boost.Preprocessor</ h1 >
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< h2 align = "center" > Header < < a href = "../../../../boost/preprocessor/for.hpp" > boost/preprocessor/for.hpp</ a > > </ h2 >
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< a href = "expr_if.htm" > Prev</ a > < a href = "identity.htm" > Next</ a > < a href = "index.htm#Macros" > Macros</ a > < a href = "index.htm#Headers" > Headers</ a >
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< h3 >< a name = "BOOST_PP_FOR" > #define BOOST_PP_FOR</ a > (STATE,PRED,OP,MACRO)</ h3 >
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< p > Repeats < code > MACRO(R,STATE)</ code > and iterates < code > OP(R,STATE)</ code > while
< code > PRED(R,STATE)</ code > is true.</ p >
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< p > In other words, expands to the sequence:</ p >
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< pre >
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MACRO(R,STATE)
MACRO(R,OP(R,STATE))
MACRO(R,OP(R,OP(R,STATE)))
...
MACRO(R,OP(R,OP(...OP(R,STATE)...)))
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</ pre >
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< p > The length of the sequence is determined by < code > PRED(R,STATE)</ code > .</ p >
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< p > For example,</ p >
< pre >
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#define PRED(R,STATE) < a href = "comparison_less.htm#BOOST_PP_LESS" > BOOST_PP_LESS</ a > (< a href = "tuple_elem.htm#BOOST_PP_TUPLE_ELEM" > BOOST_PP_TUPLE_ELEM</ a > (2,0,STATE),< a href = "tuple_elem.htm#BOOST_PP_TUPLE_ELEM" > BOOST_PP_TUPLE_ELEM</ a > (2,1,STATE))
#define OP(R,STATE) (< a href = "inc.htm#BOOST_PP_INC" > BOOST_PP_INC</ a > (< a href = "tuple_elem.htm#BOOST_PP_TUPLE_ELEM" > BOOST_PP_TUPLE_ELEM</ a > (2,0,STATE)),< a href = "tuple_elem.htm#BOOST_PP_TUPLE_ELEM" > BOOST_PP_TUPLE_ELEM</ a > (2,1,STATE))
#define MACRO(R,STATE) < a href = "tuple_elem.htm#BOOST_PP_TUPLE_ELEM" > BOOST_PP_TUPLE_ELEM</ a > (2,0,STATE)
< a href = "for.htm#BOOST_PP_FOR" > BOOST_PP_FOR</ a > ((0,3),PRED,OP,MACRO)
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</ pre >
< p > expands to:</ p >
< pre >
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0 1 2
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</ pre >
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< h3 > Legend</ h3 >
< ul >
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< li >< b > STATE</ b > is the current state of iteration. The state is usually a tuple.</ li >
< li >< b > PRED</ b > is the condition for iteration. It must expand to a decimal
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integer literal.</ li >
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< li >< b > OP</ b > 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.</ li >
< li >< b > MACRO</ b > is the state instantiation macro.</ li >
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< li >< b > R</ b > is the recursion depth and should only be used as a parameter to
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other macros using < a href = "for.htm#BOOST_PP_FOR" > BOOST_PP_FOR</ a > () or for invoking < a href = "for.htm#BOOST_PP_FOR" > BOOST_PP_FOR</ a > ##R()
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directly. For each macro using < a href = "for.htm#BOOST_PP_FOR" > BOOST_PP_FOR</ a > (), e.g. < a href = "list_for_each.htm#BOOST_PP_LIST_FOR_EACH" > BOOST_PP_LIST_FOR_EACH</ a > (),
there is a version of the macro, e.g. < a href = "list_for_each.htm#BOOST_PP_LIST_FOR_EACH_R" > BOOST_PP_LIST_FOR_EACH_R</ a > (), 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.</ li >
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</ ul >
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< h3 >< a href = "repeat.htm#BOOST_PP_REPEAT" > BOOST_PP_REPEAT</ a > () vs < a href = "for.htm#BOOST_PP_FOR" > BOOST_PP_FOR</ a > ()</ h3 >
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< p >< a href = "for.htm#BOOST_PP_FOR" > BOOST_PP_FOR</ a > () is a generalization of < a href = "repeat.htm#BOOST_PP_REPEAT" > BOOST_PP_REPEAT</ a > (). This means that
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< a href = "repeat.htm#BOOST_PP_REPEAT" > BOOST_PP_REPEAT</ a > () can be implemented using < a href = "for.htm#BOOST_PP_FOR" > BOOST_PP_FOR</ a > (). However,
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< a href = "repeat.htm#BOOST_PP_REPEAT" > BOOST_PP_REPEAT</ a > () was introduced earlier, is generally easier to use, and is
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still quite useful on its own.</ p >
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< h3 > 2D and 3D repetition</ h3 >
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< p >< a href = "for.htm#BOOST_PP_FOR" > BOOST_PP_FOR</ a > () can be used for multidimensional repetition simply by
invoking < a href = "for.htm#BOOST_PP_FOR" > BOOST_PP_FOR</ a > ##R() directly.</ p >
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< h3 > Test</ h3 >
< ul >
< li >< a href = "../../test/for_test.cpp" > for_test.cpp</ a ></ li >
</ ul >
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< hr >
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< p > Revised <!--webbot bot="Timestamp" s-type="EDITED" s-format="%d %b %Y" startspan --><!--webbot bot="Timestamp" endspan i-checksum="15246" --> </ p >
< p >< i > © Copyright < a href = "http://www.housemarque.com" > Housemarque Oy</ a > 2002</ i ></ p >
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< p > Permission to copy, use, modify, sell and distribute this document is granted
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provided this copyright notice appears in all copies. This document is provided
"as is" without express or implied warranty, and with no claim as to its suitability
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for any purpose.</ p >
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