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< title > Boost.Preprocessor - Reference< / title >
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< h3 > < a href = "../../../../index.htm" > < img height = "86" width = "277" alt = "C++ Boost" src = "../../../../c++boost.gif" border = "0" > < / a > < / h3 >
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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 >
< 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 > Automatic recursion?< / h3 >
< p > < a href = "for.htm#BOOST_PP_FOR" > BOOST_PP_FOR< / a > () currently does not implement automatic recursion. The reason
for this is that it would lead to very poor performance. The automatic recursion
technique takes O(N) steps just to find out that the Nth recursion should be used.
This would dramatically effect the time complexity of macros using automatic
recursion.< / 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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< 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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< 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 >
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< p > < i > © Copyright < a href = "http://www.housemarque.com" > Housemarque Oy< / a > 2002< / i > < / p >
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for any purpose.< / p >
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