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107 lines
4.8 KiB
HTML
<html>
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<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1">
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<link rel="stylesheet" type="text/css" href="../../../../boost.css">
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<title>Boost.Preprocessor - Reference</title>
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</head>
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<body link="#0000ff" vlink="#800080">
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<table border="0" cellpadding="7" cellspacing="0" width="100%" summary=
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"header">
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<tr>
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<td valign="top" width="300">
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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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</td>
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<td valign="top">
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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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</td>
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</tr>
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</table>
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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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<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
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<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)
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MACRO(R,OP(R,STATE))
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MACRO(R,OP(R,OP(R,STATE)))
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...
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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>
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<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))
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#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))
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#define MACRO(R,STATE) <a href="tuple_elem.htm#BOOST_PP_TUPLE_ELEM">BOOST_PP_TUPLE_ELEM</a>(2,0,STATE)
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<a href="for.htm#BOOST_PP_FOR">BOOST_PP_FOR</a>((0,3),PRED,OP,MACRO)
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</pre>
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<p>expands to:</p>
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<pre>
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0 1 2
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</pre>
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<h3>Legend</h3>
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<ul>
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<li><b>STATE</b> is the current state of iteration. The state is usually a tuple.</li>
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<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
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OP(R,STATE) usually expands to a tuple of the same number of elements.</li>
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<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>(),
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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
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by the R suffix, that accepts an additional recursion depth as the first
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parameter. This technique is necessary to avoid recursively expanding the same
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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
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invoking <a href="for.htm#BOOST_PP_FOR">BOOST_PP_FOR</a>##R() directly.</p>
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<h3>Test</h3>
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<ul>
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<li><a href="../../test/for_test.cpp">for_test.cpp</a></li>
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</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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<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
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"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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</body></html>
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