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> 82