2002-10-03 22:13:37 +00:00
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<head>
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<title>BOOST_PP_LIST_FOLD_LEFT</title>
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<link rel="stylesheet" type="text/css" href="../styles.css">
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</head>
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<body>
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<div style="margin-left: 0px;">
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The <b>BOOST_PP_LIST_FOLD_LEFT</b> macro folds (or accumulates) the elements of a <i>list</i> left-to-right.
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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_LIST_FOLD_LEFT</b>(<i>op</i>, <i>state</i>, <i>list</i>)
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</div>
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<h4>Arguments</h4>
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<dl>
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<dt>op</dt>
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<dd>
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A ternary operation of the form <i>op</i>(<i>d</i>, <i>state</i>, <i>elem</i>).
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This macro is called for each element in <i>list</i>--each time returning a new <i>state</i>.
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This operation is expanded by <b>BOOST_PP_LIST_FOLD_LEFT</b> with the next available <b>BOOST_PP_WHILE</b> iteration,
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the current <i>state</i>, and the current element.
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</dd>
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<dt>state</dt>
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<dd>
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The initial state of the fold.
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</dd>
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<dt>list</dt>
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<dd>
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The <i>list</i> to be folded.
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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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For the <i>list</i>, (<i>0</i>, (<i>1</i>, (<i>2</i>, <b>BOOST_PP_NIL</b>))), this macro 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>, <i>0</i>), <i>1</i>), <i>2</i>)
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</div>
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</div>
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<div>
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Previously, this macro could not be used inside <b>BOOST_PP_WHILE</b>.
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There is no longer any such restriction.
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It is more efficient, however, to use <b>BOOST_PP_LIST_FOLD_LEFT_<i>d</i></b> in such a situation.
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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="list_fold_left_d.html">BOOST_PP_LIST_FOLD_LEFT_<i>d</i></a></li>
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</ul>
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<h4>Requirements</h4>
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<div>
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2004-10-11 06:58:45 +00:00
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<b>Header:</b> <a href="../headers/list/fold_left.html"><boost/preprocessor/list/fold_left.hpp></a>
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2002-10-03 22:13:37 +00:00
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</div>
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<h4>Sample Code</h4>
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<div><pre>
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2004-10-11 06:58:45 +00:00
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#include <<a href="../headers/cat.html">boost/preprocessor/cat.hpp</a>>
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#include <<a href="../headers/list/fold_left.html">boost/preprocessor/list/fold_left.hpp</a>>
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2002-10-03 22:13:37 +00:00
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#define LIST (a, (b, (c, <a href="nil.html">BOOST_PP_NIL</a>)))
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#define OP(d, state, x) <a href="cat.html">BOOST_PP_CAT</a>(state, x)
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<a href="list_fold_left.html">BOOST_PP_LIST_FOLD_LEFT</a>(OP, _, LIST) // expands to _abc
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</pre></div>
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</body>
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</html>
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