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    <title>BOOST_PP_WHILE_d</title>
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    <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>).&nbsp;
        This predicate is expanded by <b>BOOST_PP_WHILE</b> with the next
        available iteration <i>d</i> and the current <i>state</i>.&nbsp; 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>.&nbsp;
        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>).&nbsp;
        This operation is expanded by <b>BOOST_PP_WHILE</b> with the next
        available iteration <i>d</i> and the current <i>state</i>.&nbsp; 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.&nbsp; 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.&nbsp; 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.&nbsp; This happens when the <i>d</i> value is a macro
      invocation itself.&nbsp; It needs a delay to allow it to expand.&nbsp; 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).&nbsp; 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>
      <li><a href="while_d_macros.html"><span style="color: gray;">Macros with D
            re-entrancy forms</span></a></li>
    </ul>
    <h4>Requirements</h4>
    <div> <b>Header:</b> &nbsp;<a href="../headers/control/while.html">&lt;boost/preprocessor/control/while.hpp&gt;</a>
    </div>
    <h4>Sample Code</h4>
    <div>
      <pre>#include &lt;<a href="../headers/arithmetic/add.html">boost/preprocessor/arithmetic/add.hpp</a>&gt;
#include &lt;<a href="../headers/arithmetic/dec.html">boost/preprocessor/arithmetic/dec.hpp</a>&gt;
#include &lt;<a href="../headers/array/elem.html">boost/preprocessor/array/elem.hpp</a>&gt;
#include &lt;<a href="../headers/array/size.html">boost/preprocessor/array/size.hpp</a>&gt;
#include &lt;<a href="../headers/control/while.html">boost/preprocessor/control/while.hpp</a>&gt;
#include &lt;<a href="../headers/tuple/elem.html">boost/preprocessor/tuple/elem.hpp</a>&gt;

#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>
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    <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<br>
      </i><i>© Copyright Edward Diener 2014</i><br>
    </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>
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