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Paul Mensonides
2002-09-08 09:40:56 +00:00
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<html>
<head>
<title>BOOST_PP_LIST_FOLD_LEFT</title>
<link rel="stylesheet" type="text/css" href="../styles.css">
<script language="javascript" type="text/javascript" src="../scripts.js"></script>
</head>
<body onload="init('sample');">
<div style="margin-left: 0px;">
The <b>BOOST_PP_LIST_FOLD_LEFT</b> macro folds (or accumulates) the elements of a <i>list</i> left-to-right.
</div>
<h4>Usage</h4>
<div class="code">
<b>BOOST_PP_LIST_FOLD_LEFT</b>(<i>op</i>, <i>state</i>, <i>list</i>)
</div>
<h4>Arguments</h4>
<dl>
<dt>op</dt>
<dd>
A ternary operation of the form <i>op</i>(<i>d</i>, <i>state</i>, <i>elem</i>).&nbsp;
This macro is called for each element in <i>list</i>--each time returning a new <i>state</i>.&nbsp;
This operation is expanded by <b>BOOST_PP_LIST_FOLD_LEFT</b> with the next available <b>BOOST_PP_WHILE</b> iteration,
the current <i>state</i>, and the current element.
</dd>
<dt>state</dt>
<dd>
The initial state of the fold.
</dd>
<dt>list</dt>
<dd>
The <i>list</i> to be folded.
</dd>
</dl>
<h4>Remarks</h4>
<div>
For the <i>list</i>, (<i>0</i>, (<i>1</i>, (<i>2</i>, <b>BOOST_PP_NIL</b>))), this macro expands to:
<div>
<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>)
</div>
</div>
<div>
Previously, this macro could not be used inside <b>BOOST_PP_WHILE</b>.&nbsp;
There is no longer any such restriction.&nbsp;
It is more efficient, however, to use <b>BOOST_PP_LIST_FOLD_LEFT_<i>d</i></b> in such a situation.
</div>
<h4>See Also</h4>
<ul>
<li><a href="list_fold_left_d.html">BOOST_PP_LIST_FOLD_LEFT_<i>d</i></a></li>
</ul>
<h4>Requirements</h4>
<div>
<b>Header:</b> &nbsp;<a href="../headers/list/fold_left.hpp.html">&lt;boost/preprocessor/list/fold_left.hpp&gt;</a>
</div>
<h4><a class="local" onclick="toggle('sample');" onmouseover="change(this);" onmouseout="revert(this);">Sample Code</a></h4>
<div id="sample"><pre>
#include &lt;boost/preprocessor/cat.hpp&gt;
#include &lt;boost/preprocessor/list/fold_left.hpp&gt;
#define LIST (a, (b, (c, BOOST_PP_NIL)))
#define OP(d, state, x) BOOST_PP_CAT(state, x)
BOOST_PP_LIST_FOLD_LEFT(OP, _, LIST) // expands to _abc
<pre></div>
</body>
</html>