<html> <head> <title>BOOST_PP_SEQ_NIL</title> <link rel="stylesheet" type="text/css" href="../styles.css"> </head> <body> <div style="margin-left: 0px;"> 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." </div> <h4> Usage </h4> <div class="code"> <b>BOOST_PP_SEQ_NIL</b> </div> <h4> Remarks </h4> <div> 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>. </div> <div> 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. </div> <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. </div> <div> (It is also possible to start with an extra element and pop it off when you have finished appending to it.) </div> <div> In C99, neither of these macros are necessary since it is legal to pass empty arguments. </div> <h4> See Also </h4> <ul> <li> <a href="empty.html">BOOST_PP_EMPTY</a></li> </ul> <h4> Requirements </h4> <div> <b>Header:</b> <a href="../headers/seq/seq.hpp.html"><boost/preprocessor/seq/seq.hpp></a> </div> <h4> Sample Code </h4> <div> <pre> #include <<a href="../headers/facilities/empty.hpp.html">boost/preprocessor/facilities/empty.hpp</a>> #include <<a href="../headers/seq/push_back.hpp.html">boost/preprocessor/seq/push_back.hpp</a>> #include <<a href="../headers/seq/push_front.hpp.html">boost/preprocessor/seq/push_front.hpp</a>> #include <<a href="../headers/seq/seq.hpp.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> </body> </html>
