forked from boostorg/preprocessor
120 lines
4.6 KiB
HTML
120 lines
4.6 KiB
HTML
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<html>
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<head>
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<title>BOOST_PP_WHILE_d</title>
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<link rel="stylesheet" type="text/css" href="../styles.css">
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<script language="javascript" type="text/javascript" src="../scripts.js"></script>
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</head>
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<body onload="init('sample');">
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<div style="margin-left: 0px;">
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The <b>BOOST_PP_WHILE_<i>d</i></b> macro represents a reentry into the <b>BOOST_PP_WHILE</b> looping construct.
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</div>
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<h4>Usage</h4>
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<div class="code">
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<b>BOOST_PP_WHILE_</b> ## <i>d</i>(<i>pred</i>, <i>op</i>, <i>state</i>)
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</div>
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<h4>Arguments</h4>
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<dl>
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<dt>d</dt>
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<dd>
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The next available <b>BOOST_PP_WHILE</b> iteration.
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</dd>
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<dt>pred</dt>
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<dd>
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A binary predicate of the form <i>pred</i>(<i>d</i>, <i>state</i>).
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This predicate is expanded by <b>BOOST_PP_WHILE</b> with the next available
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iteration <i>d</i> and the current <i>state</i>.
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This predicate must expand to value in the range of <i>0</i> to <b>BOOST_PP_LIMIT_MAG</b>.
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The construct continues to loop until this predicate returns <i>0</i>.
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When this predicate returns <i>0</i>, <b>BOOST_PP_WHILE</b> returns the current <i>state</i>.
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</dd>
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<dt>op</dt>
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<dd>
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A binary operation of the form <i>op</i>(<i>d</i>, <i>state</i>).
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This operation is expanded by <b>BOOST_PP_WHILE</b> with the next available
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iteration <i>d</i> and the current <i>state</i>.
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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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</dd>
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<dt>state</dt>
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<dd>
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The initial state.
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Often this argument is a <i>tuple</i>.
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</dd>
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</dl>
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<h4>Remarks</h4>
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<div>
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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.
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In other words expands to:
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<div>
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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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</div>
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</div>
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<div>
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At certain times, it may be necessary to perform the concatenation with <b>BOOST_PP_CAT</b> rather than the preprocessor token-pasting operator.
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This happens when the <i>d</i> value is a macro invocation itself.
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It needs a delay to allow it to expand.
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The syntax in such a scenario becomes:
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<div>
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<b>BOOST_PP_CAT</b>(<b>BOOST_PP_WHILE_</b>, <i>d</i>)(<i>pred</i>, <i>op</i>, <i>state</i>).
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</div>
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</div>
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<div class="incompatibility">
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Previously, it was possible to concatenate <i>d</i> directly to <b>BOOST_PP_WHILE</b> (without the trailing underscore).
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This is no longer supported.
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</div>
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<h4>See Also</h4>
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<ul>
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<li><a href="cat.html">BOOST_PP_CAT</a></li>
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<li><a href="limit_mag.html">BOOST_PP_LIMIT_MAG</a></li>
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<li><a href="while.html">BOOST_PP_WHILE</a></li>
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</ul>
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<h4>Requirements</h4>
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<div>
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<b>Header:</b> <a href="../headers/control/while.hpp.html"><boost/preprocessor/control/while.hpp></a>
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</div>
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<h4><a class="local" onclick="toggle('sample');" onmouseover="change(this);" onmouseout="revert(this);">Sample Code</a></h4>
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<div id="sample"><pre>
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#include <boost/preprocessor/arithmetic/add.hpp>
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#include <boost/preprocessor/arithmetic/dec.hpp>
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#include <boost/preprocessor/array/elem.hpp>
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#include <boost/preprocessor/array/size.hpp>
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#include <boost/preprocessor/control/while.hpp>
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#include <boost/preprocessor/tuple/elem.hpp>
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#define PRED(d, data) BOOST_PP_TUPLE_ELEM(3, 1, data)
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#define OP(d, data) /* ............ */ \
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OP_D( \
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d, \
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BOOST_PP_TUPLE_ELEM(3, 0, data), \
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BOOST_PP_TUPLE_ELEM(3, 1, data), \
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BOOST_PP_TUPLE_ELEM(3, 2, data) \
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) \
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/**/
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#define OP_D(d, res, i, array) /* ................. */ \
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( \
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BOOST_PP_ADD_D( \
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d, res, \
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BOOST_PP_ARRAY_ELEM(BOOST_PP_DEC(i), array)), \
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BOOST_PP_DEC(i), \
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array \
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) \
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/**/
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#define ACCUMULATE_D(d, array) /* .......... */ \
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BOOST_PP_TUPLE_ELEM( \
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3, 0, \
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BOOST_PP_WHILE_ ## d( \
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PRED, OP, \
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(0, BOOST_PP_ARRAY_SIZE(array), array) \
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) \
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) \
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/**/
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#define ARRAY (4, (1, 2, 3, 4))
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ACCUMULATE_D(1, ARRAY)// expands to 10
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<pre></div>
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</body>
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</html>
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