| 1 | <html> |
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| 2 | <head> |
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| 3 | <title>BOOST_PP_WHILE</title> |
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| 4 | <link rel="stylesheet" type="text/css" href="../styles.css"> |
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| 5 | </head> |
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| 6 | <body> |
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| 7 | <div style="margin-left: 0px;"> |
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| 8 | The <b>BOOST_PP_WHILE</b> macro represents a looping construct. |
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| 9 | </div> |
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| 10 | <h4>Usage</h4> |
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| 11 | <div class="code"> |
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| 12 | <b>BOOST_PP_WHILE</b>(<i>pred</i>, <i>op</i>, <i>state</i>) |
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| 13 | </div> |
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| 14 | <h4>Arguments</h4> |
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| 15 | <dl> |
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| 16 | <dt>pred</dt> |
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| 17 | <dd> |
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| 18 | A binary predicate of the form <i>pred</i>(<i>d</i>, <i>state</i>). |
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| 19 | This predicate is expanded by <b>BOOST_PP_WHILE</b> with the next available |
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| 20 | iteration <i>d</i> and the current <i>state</i>. |
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| 21 | This predicate must expand to value in the range of <i>0</i> to <b>BOOST_PP_LIMIT_MAG</b>. |
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| 22 | The construct continues to loop until this predicate returns <i>0</i>. |
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| 23 | When this predicate returns <i>0</i>, <b>BOOST_PP_WHILE</b> returns the current <i>state</i>. |
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| 24 | </dd> |
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| 25 | <dt>op</dt> |
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| 26 | <dd> |
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| 27 | A binary operation of the form <i>op</i>(<i>d</i>, <i>state</i>). |
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| 28 | This operation is expanded by <b>BOOST_PP_WHILE</b> with the next available |
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| 29 | iteration <i>d</i> and the current <i>state</i>. |
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| 30 | 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>. |
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| 31 | </dd> |
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| 32 | <dt>state</dt> |
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| 33 | <dd> |
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| 34 | The initial state. |
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| 35 | Often this argument is a <i>tuple</i>. |
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| 36 | </dd> |
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| 37 | </dl> |
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| 38 | <h4>Remarks</h4> |
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| 39 | <div> |
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| 40 | 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. |
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| 41 | In other words expands to: |
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| 42 | <div> |
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| 43 | <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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| 44 | </div> |
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| 45 | </div> |
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| 46 | <div> |
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| 47 | The <i>d</i> value that is passed to both <i>pred</i> and <i>op</i> represents the next available iteration. |
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| 48 | 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>. |
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| 49 | 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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| 50 | </div> |
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| 51 | <div> |
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| 52 | 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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| 53 | </div> |
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| 54 | <div> |
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| 55 | Previously, this macro could not be used recursively inside <b>BOOST_PP_WHILE</b>. |
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| 56 | This limitation no longer exists, as the library can automatically detect the next available iteration. |
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| 57 | </div> |
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| 58 | <h4>See Also</h4> |
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| 59 | <ul> |
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| 60 | <li><a href="limit_mag.html">BOOST_PP_LIMIT_MAG</a></li> |
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| 61 | <li><a href="while_d.html">BOOST_PP_WHILE_<i>d</i></a></li> |
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| 62 | </ul> |
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| 63 | <h4>Requirements</h4> |
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| 64 | <div> |
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| 65 | <b>Header:</b> <a href="../headers/control/while.html"><boost/preprocessor/control/while.hpp></a> |
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| 66 | </div> |
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| 67 | <h4>Sample Code</h4> |
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| 68 | <div><pre> |
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| 69 | #include <<a href="../headers/arithmetic/add.html">boost/preprocessor/arithmetic/add.hpp</a>> |
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| 70 | #include <<a href="../headers/control/while.html">boost/preprocessor/control/while.hpp</a>> |
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| 71 | #include <<a href="../headers/tuple/elem.html">boost/preprocessor/tuple/elem.hpp</a>> |
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| 72 | |
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| 73 | #define PRED(n, state) <a href="tuple_elem.html">BOOST_PP_TUPLE_ELEM</a>(2, 1, state) |
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| 74 | |
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| 75 | #define OP(d, state) \ |
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| 76 | OP_D( \ |
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| 77 | d, \ |
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| 78 | <a href="tuple_elem.html">BOOST_PP_TUPLE_ELEM</a>(2, 0, state), \ |
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| 79 | <a href="tuple_elem.html">BOOST_PP_TUPLE_ELEM</a>(2, 1, state) \ |
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| 80 | ) \ |
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| 81 | /**/ |
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| 82 | |
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| 83 | #define OP_D(d, res, c) \ |
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| 84 | ( \ |
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| 85 | <a href="add_d.html">BOOST_PP_ADD_D</a>( \ |
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| 86 | d, \ |
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| 87 | res, \ |
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| 88 | <a href="dec.html">BOOST_PP_DEC</a>(c) \ |
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| 89 | ), \ |
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| 90 | <a href="dec.html">BOOST_PP_DEC</a>(c) \ |
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| 91 | ) \ |
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| 92 | /**/ |
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| 93 | |
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| 94 | #define SUMMATION(n) \ |
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| 95 | <a href="tuple_elem.html">BOOST_PP_TUPLE_ELEM</a>( \ |
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| 96 | 2, 0, \ |
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| 97 | <a href="while.html">BOOST_PP_WHILE</a>(PRED, OP, (n, n)) \ |
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| 98 | ) \ |
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| 99 | /**/ |
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| 100 | |
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| 101 | SUMMATION(3) // expands to 6 |
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| 102 | SUMMATION(4) // expands to 10 |
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| 103 | </pre></div> |
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| 104 | </body> |
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| 105 | </html> |
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