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- <html>
- <head>
- <title>BOOST_PP_WHILE</title>
- <link rel="stylesheet" type="text/css" href="../styles.css">
- </head>
- <body>
- <div style="margin-left: 0px;">
- The <b>BOOST_PP_WHILE</b> macro represents a looping construct.
- </div>
- <h4>Usage</h4>
- <div class="code">
- <b>BOOST_PP_WHILE</b>(<i>pred</i>, <i>op</i>, <i>state</i>)
- </div>
- <h4>Arguments</h4>
- <dl>
- <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>
- The <i>d</i> value that is passed to both <i>pred</i> and <i>op</i> represents the next available iteration.
- Other macros that have <b>_D</b> suffix variants internally use <b>BOOST_PP_WHILE</b>--for example, <b>BOOST_PP_ADD</b> and <b>BOOST_PP_ADD_D</b>.
- Using these <b>_D</b> versions is not strictly necessary, but passing the <i>d</i> value (that passed to <i>pred</i> or <i>op</i>) to these macros allows them to reenter <b>BOOST_PP_WHILE</b> with maximum efficiency.
- </div>
- <div>
- To directly use this <i>d</i> value, rather than simply passing it to another macro, see <b>BOOST_PP_WHILE_<i>d</i></b>.
- </div>
- <div>
- Previously, this macro could not be used recursively inside <b>BOOST_PP_WHILE</b>.
- This limitation no longer exists, as the library can automatically detect the next available iteration.
- </div>
- <h4>See Also</h4>
- <ul>
- <li><a href="limit_mag.html">BOOST_PP_LIMIT_MAG</a></li>
- <li><a href="while_d.html">BOOST_PP_WHILE_<i>d</i></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/control/while.html">boost/preprocessor/control/while.hpp</a>>
- #include <<a href="../headers/tuple/elem.html">boost/preprocessor/tuple/elem.hpp</a>>
- #define PRED(n, state) <a href="tuple_elem.html">BOOST_PP_TUPLE_ELEM</a>(2, 1, state)
- #define OP(d, state) \
- OP_D( \
- d, \
- <a href="tuple_elem.html">BOOST_PP_TUPLE_ELEM</a>(2, 0, state), \
- <a href="tuple_elem.html">BOOST_PP_TUPLE_ELEM</a>(2, 1, state) \
- ) \
- /**/
- #define OP_D(d, res, c) \
- ( \
- <a href="add_d.html">BOOST_PP_ADD_D</a>( \
- d, \
- res, \
- <a href="dec.html">BOOST_PP_DEC</a>(c) \
- ), \
- <a href="dec.html">BOOST_PP_DEC</a>(c) \
- ) \
- /**/
- #define SUMMATION(n) \
- <a href="tuple_elem.html">BOOST_PP_TUPLE_ELEM</a>( \
- 2, 0, \
- <a href="while.html">BOOST_PP_WHILE</a>(PRED, OP, (n, n)) \
- ) \
- /**/
- SUMMATION(3) // expands to 6
- SUMMATION(4) // 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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