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| <title>BOOST_PP_WHILE</title> |
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| The <b>BOOST_PP_WHILE</b> macro represents a looping construct. |
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| <h4>Usage</h4> |
| <div class="code"> |
| <b>BOOST_PP_WHILE</b>(<i>pred</i>, <i>op</i>, <i>state</i>) |
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| <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> |
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| 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: |
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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>)) ... ). |
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| </div> |
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| 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. |
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| 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>. |
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| 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> |
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