| Brian Silverman | f27e085 | 2018-08-04 23:56:45 -0700 | [diff] [blame^] | 1 | <html> |
| 2 | <head> |
| 3 | <title>BOOST_PP_SEQ_NIL</title> |
| 4 | <link rel="stylesheet" type="text/css" href="../styles.css"> |
| 5 | </head> |
| 6 | <body> |
| 7 | <div style="margin-left: 0px;"> |
| 8 | The <b>BOOST_PP_SEQ_NIL</b> macro is a placeholder macro for an empty <i>seq</i>. |
| 9 | It is only valid if it's elements are appended to the end of this empty |
| 10 | "seq." |
| 11 | </div> |
| 12 | <h4> |
| 13 | Usage |
| 14 | </h4> |
| 15 | <div class="code"> |
| 16 | <b>BOOST_PP_SEQ_NIL</b> |
| 17 | </div> |
| 18 | <h4> |
| 19 | Remarks |
| 20 | </h4> |
| 21 | <div> |
| 22 | This macro is a utility macro intended as a empty starting point for appending |
| 23 | to the tail. It is <i>not</i> a nil <i>seq</i>. When an element is |
| 24 | appended to this macro, it expands on the element and to the element--thereby |
| 25 | removing itself. For example, both <b>BOOST_PP_SEQ_NIL</b>(<i>x</i>) and <b>BOOST_PP_SEQ_PUSH_BACK</b>(<b>BOOST_PP_SEQ_NIL</b>, |
| 26 | <i>x</i>) expand to <i>x</i>. |
| 27 | </div> |
| 28 | <div> |
| 29 | If any <b>BOOST_PP_SEQ_</b>* macro (other than <b>BOOST_PP_SEQ_PUSH_BACK</b>) |
| 30 | is invoked with an argument that contains <b>BOOST_PP_SEQ_NIL</b>, the behavior |
| 31 | is undefined and in most cases will result in obscure errors. |
| 32 | </div> |
| 33 | <div> |
| 34 | The closest thing available to <b>BOOST_PP_SEQ_NIL</b> for appending to the |
| 35 | head is <b>BOOST_PP_EMPTY</b>. After all the elements have been |
| 36 | prepended, empty parenthesis can be invoked on the tail to remove the <b>BOOST_PP_EMPTY</b>. |
| 37 | As with <b>BOOST_PP_SEQ_NIL</b>, passing an argument that contains <b>BOOST_PP_EMPTY</b> |
| 38 | to any <b>BOOST_PP_SEQ_</b>* macro (other than <b>BOOST_PP_SEQ_PUSH_FRONT</b>) |
| 39 | is undefined. |
| 40 | </div> |
| 41 | <div> |
| 42 | (It is also possible to start with an extra element and pop it off when you |
| 43 | have finished appending to it.) |
| 44 | </div> |
| 45 | <div> |
| 46 | In C99, neither of these macros are necessary since it is legal to pass empty |
| 47 | arguments. |
| 48 | </div> |
| 49 | <h4> |
| 50 | See Also |
| 51 | </h4> |
| 52 | <ul> |
| 53 | <li> |
| 54 | <a href="empty.html">BOOST_PP_EMPTY</a></li> |
| 55 | </ul> |
| 56 | <h4> |
| 57 | Requirements |
| 58 | </h4> |
| 59 | <div> |
| 60 | <b>Header:</b> <a href="../headers/seq/seq.html"><boost/preprocessor/seq/seq.hpp></a> |
| 61 | </div> |
| 62 | <h4> |
| 63 | Sample Code |
| 64 | </h4> |
| 65 | <div> |
| 66 | <pre>#include <<a href="../headers/facilities/empty.html">boost/preprocessor/facilities/empty.hpp</a>><br>#include <<a href="../headers/seq/push_back.html">boost/preprocessor/seq/push_back.hpp</a>><br>#include <<a href="../headers/seq/push_front.html">boost/preprocessor/seq/push_front.hpp</a>><br>#include <<a href="../headers/seq/seq.html">boost/preprocessor/seq/seq.hpp</a>><br><br>#define SEQ_L <a href="seq_nil.html">BOOST_PP_SEQ_NIL</a> |
| 67 | #define SEQ_R <a href="empty.html">BOOST_PP_EMPTY</a> |
| 68 | <a href="seq_push_back.html">BOOST_PP_SEQ_PUSH_BACK</a>(<br> <a href="seq_push_back.html">BOOST_PP_SEQ_PUSH_BACK</a>(SEQ_L, a), b<br>)<br><br>// expands to (a)(b)<br><br><a href="seq_push_front.html">BOOST_PP_SEQ_PUSH_FRONT</a>(<br> <a href="seq_push_front.html">BOOST_PP_SEQ_PUSH_FRONT</a>(SEQ_R, a), b<br>)()<br><br>// expands to (b)(a)<br></pre> |
| 69 | </div> |
| 70 | <hr size="1"> |
| 71 | <div style="margin-left: 0px;"> |
| 72 | <i>© Copyright <a href="http://www.housemarque.com" target="_top">Housemarque Oy</a> 2002</i> |
| 73 | <br><i>© Copyright Paul Mensonides 2002</i> |
| 74 | </div> |
| 75 | <div style="margin-left: 0px;"> |
| 76 | <p><small>Distributed under the Boost Software License, Version 1.0. (See |
| 77 | accompanying file <a href="../../../../LICENSE_1_0.txt">LICENSE_1_0.txt</a> or |
| 78 | copy at <a href="http://www.boost.org/LICENSE_1_0.txt">www.boost.org/LICENSE_1_0.txt</a>)</small></p> |
| 79 | </div> |
| 80 | </body> |
| 81 | </html> |