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<html>
<head>
<title>BOOST_PP_WHILE_d</title>
<link rel="stylesheet" type="text/css" href="../styles.css">
</head>
<body>
<div style="margin-left: 0px;">
The <b>BOOST_PP_WHILE_<i>d</i></b> macro represents a reentry into the <b>BOOST_PP_WHILE</b> looping construct.
</div>
<h4>Usage</h4>
<div class="code">
<b>BOOST_PP_WHILE_</b> ## <i>d</i>(<i>pred</i>, <i>op</i>, <i>state</i>)
</div>
<h4>Arguments</h4>
<dl>
<dt>d</dt>
<dd>
The next available <b>BOOST_PP_WHILE</b> iteration.
</dd>
<dt>pred</dt>
<dd>
A binary predicate of the form <i>pred</i>(<i>d</i>, <i>state</i>).
This predicate is expanded by <b>BOOST_PP_WHILE</b> with the next available
iteration <i>d</i> and the current <i>state</i>.
This predicate must expand to value in the range of <i>0</i> to <b>BOOST_PP_LIMIT_MAG</b>.
The construct continues to loop until this predicate returns <i>0</i>.
When this predicate returns <i>0</i>, <b>BOOST_PP_WHILE</b> returns the current <i>state</i>.
</dd>
<dt>op</dt>
<dd>
A binary operation of the form <i>op</i>(<i>d</i>, <i>state</i>).
This operation is expanded by <b>BOOST_PP_WHILE</b> with the next available
iteration <i>d</i> and the current <i>state</i>.
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>.
</dd>
<dt>state</dt>
<dd>
The initial state.
Often this argument is a <i>tuple</i>.
</dd>
</dl>
<h4>Remarks</h4>
<div>
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.
In other words expands to:
<div>
<i>op</i>(<i>d</i>, ... <i>op</i>(<i>d</i>, <i>op</i>(<i>d</i>, <i>state</i>)) ... ).
</div>
</div>
<div>
At certain times, it may be necessary to perform the concatenation with <b>BOOST_PP_CAT</b> rather than the preprocessor token-pasting operator.
This happens when the <i>d</i> value is a macro invocation itself.
It needs a delay to allow it to expand.
The syntax in such a scenario becomes:
<div>
<b>BOOST_PP_CAT</b>(<b>BOOST_PP_WHILE_</b>, <i>d</i>)(<i>pred</i>, <i>op</i>, <i>state</i>).
</div>
</div>
<div>
Previously, it was possible to concatenate <i>d</i> directly to <b>BOOST_PP_WHILE</b> (without the trailing underscore).
This is no longer supported.
</div>
<h4>See Also</h4>
<ul>
<li><a href="cat.html">BOOST_PP_CAT</a></li>
<li><a href="limit_mag.html">BOOST_PP_LIMIT_MAG</a></li>
<li><a href="while.html">BOOST_PP_WHILE</a></li>
</ul>
<h4>Requirements</h4>
<div>
<b>Header:</b> <a href="../headers/control/while.html"><boost/preprocessor/control/while.hpp></a>
</div>
<h4>Sample Code</h4>
<div><pre>
#include <<a href="../headers/arithmetic/add.html">boost/preprocessor/arithmetic/add.hpp</a>>
#include <<a href="../headers/arithmetic/dec.html">boost/preprocessor/arithmetic/dec.hpp</a>>
#include <<a href="../headers/array/elem.html">boost/preprocessor/array/elem.hpp</a>>
#include <<a href="../headers/array/size.html">boost/preprocessor/array/size.hpp</a>>
#include <<a href="../headers/control/while.html">boost/preprocessor/control/while.hpp</a>>
#include <<a href="../headers/tuple/elem.html">boost/preprocessor/tuple/elem.hpp</a>>
#define PRED(d, data) <a href="tuple_elem.html">BOOST_PP_TUPLE_ELEM</a>(3, 1, data)
#define OP(d, data) \
OP_D( \
d, \
<a href="tuple_elem.html">BOOST_PP_TUPLE_ELEM</a>(3, 0, data), \
<a href="tuple_elem.html">BOOST_PP_TUPLE_ELEM</a>(3, 1, data), \
<a href="tuple_elem.html">BOOST_PP_TUPLE_ELEM</a>(3, 2, data) \
) \
/**/
#define OP_D(d, res, i, array) \
( \
<a href="add_d.html">BOOST_PP_ADD_D</a>( \
d, res, \
<a href="array_elem.html">BOOST_PP_ARRAY_ELEM</a>(<a href="dec.html">BOOST_PP_DEC</a>(i), array)), \
<a href="dec.html">BOOST_PP_DEC</a>(i), \
array \
) \
/**/
#define ACCUMULATE_D(d, array) \
<a href="tuple_elem.html">BOOST_PP_TUPLE_ELEM</a>( \
3, 0, \
<a href="while_d.html">BOOST_PP_WHILE_</a> ## d( \
PRED, OP, \
(0, <a href="array_size.html">BOOST_PP_ARRAY_SIZE</a>(array), array) \
) \
) \
/**/
#define ARRAY (4, (1, 2, 3, 4))
ACCUMULATE_D(1, ARRAY)// expands to 10
</pre></div>
<hr size="1">
<div style="margin-left: 0px;">
<i> Copyright <a href="http://www.housemarque.com" target="_top">Housemarque Oy</a> 2002</i>
</br><i> Copyright Paul Mensonides 2002</i>
</div>
<div style="margin-left: 0px;">
<p><small>Distributed under the Boost Software License, Version 1.0. (See
accompanying file <a href="../../../../LICENSE_1_0.txt">LICENSE_1_0.txt</a> or
copy at <a href=
"http://www.boost.org/LICENSE_1_0.txt">www.boost.org/LICENSE_1_0.txt</a>)</small></p>
</div>
</body>
</html>
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