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| <title>BOOST_PP_SEQ_NIL</title> |
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| The <b>BOOST_PP_SEQ_NIL</b> macro is a placeholder macro for an empty <i>seq</i>. |
| It is only valid if it is elements are appended to the end of this empty |
| "seq." |
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| <h4> |
| Usage |
| </h4> |
| <div class="code"> |
| <b>BOOST_PP_SEQ_NIL</b> |
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| <h4> |
| Remarks |
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| This macro is a utility macro intended as a empty starting point for appending |
| to the tail. It is <i>not</i> a nil <i>seq</i>. When an element is |
| appended to this macro, it expands on the element and to the element--thereby |
| 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>, |
| <i>x</i>) expand to <i>x</i>. |
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| If any <b>BOOST_PP_SEQ_</b>* macro (other than <b>BOOST_PP_SEQ_PUSH_BACK</b>) |
| is invoked with an argument that contains <b>BOOST_PP_SEQ_NIL</b>, the behavior |
| is undefined and in most cases will result in obscure errors. |
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| <div> |
| The closest thing available to <b>BOOST_PP_SEQ_NIL</b> for appending to the |
| head is <b>BOOST_PP_EMPTY</b>. After all the elements have been |
| prepended, empty parenthesis can be invoked on the tail to remove the <b>BOOST_PP_EMPTY</b>. |
| As with <b>BOOST_PP_SEQ_NIL</b>, passing an argument that contains <b>BOOST_PP_EMPTY</b> |
| to any <b>BOOST_PP_SEQ_</b>* macro (other than <b>BOOST_PP_SEQ_PUSH_FRONT</b>) |
| is undefined. |
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| (It is also possible to start with an extra element and pop it off when you |
| have finished appending to it.) |
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| In C99, neither of these macros are necessary since it is legal to pass empty |
| arguments. |
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| <h4> |
| See Also |
| </h4> |
| <ul> |
| <li> |
| <a href="empty.html">BOOST_PP_EMPTY</a></li> |
| </ul> |
| <h4> |
| Requirements |
| </h4> |
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| <b>Header:</b> <a href="../headers/seq/seq.html"><boost/preprocessor/seq/seq.hpp></a> |
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| <h4> |
| Sample Code |
| </h4> |
| <div> |
| <pre> |
| #include <<a href="../headers/facilities/empty.html">boost/preprocessor/facilities/empty.hpp</a>> |
| #include <<a href="../headers/seq/push_back.html">boost/preprocessor/seq/push_back.hpp</a>> |
| #include <<a href="../headers/seq/push_front.html">boost/preprocessor/seq/push_front.hpp</a>> |
| #include <<a href="../headers/seq/seq.html">boost/preprocessor/seq/seq.hpp</a>> |
| |
| #define SEQ_L <a href="seq_nil.html">BOOST_PP_SEQ_NIL</a> |
| #define SEQ_R <a href="empty.html">BOOST_PP_EMPTY</a> |
| |
| <a href="seq_push_back.html">BOOST_PP_SEQ_PUSH_BACK</a>( |
| <a href="seq_push_back.html">BOOST_PP_SEQ_PUSH_BACK</a>(SEQ_L, a), b |
| ) |
| |
| // expands to (a)(b) |
| |
| <a href="seq_push_front.html">BOOST_PP_SEQ_PUSH_FRONT</a>( |
| <a href="seq_push_front.html">BOOST_PP_SEQ_PUSH_FRONT</a>(SEQ_R, a), b |
| )() |
| |
| // expands to (b)(a) |
| </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> |
