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2002-11-07 23:39:29 +00:00
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<head>
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<title>BOOST_PP_SEQ_NIL</title>
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<link rel="stylesheet" type="text/css" href="../styles.css">
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</head>
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<body>
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<div style="margin-left: 0px;">
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The <b>BOOST_PP_SEQ_NIL</b> macro is a placeholder macro for an empty <i>seq</i>.
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It is only valid if it is elements are appended to the end of this empty
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"seq."
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</div>
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<h4>
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Usage
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</h4>
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<div class="code">
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<b>BOOST_PP_SEQ_NIL</b>
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</div>
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<h4>
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Remarks
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</h4>
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2002-10-13 05:47:23 +00:00
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<div>
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This macro is a utility macro intended as a empty starting point for appending
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to the tail. It is <i>not</i> a nil <i>seq</i>. When an element is
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appended to this macro, it expands on the element and to the element--thereby
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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>,
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<i>x</i>) expand to <i>x</i>.
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</div>
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<div>
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If any <b>BOOST_PP_SEQ_</b>* macro (other than <b>BOOST_PP_SEQ_PUSH_BACK</b>)
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is invoked with an argument that contains <b>BOOST_PP_SEQ_NIL</b>, the behavior
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is undefined and in most cases will result in obscure errors.
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</div>
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<div>
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The closest thing available to <b>BOOST_PP_SEQ_NIL</b> for appending to the
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head is <b>BOOST_PP_EMPTY</b>. After all the elements have been
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prepended, empty parenthesis can be invoked on the tail to remove the <b>BOOST_PP_EMPTY</b>.
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As with <b>BOOST_PP_SEQ_NIL</b>, passing an argument that contains <b>BOOST_PP_EMPTY</b>
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to any <b>BOOST_PP_SEQ_</b>* macro (other than <b>BOOST_PP_SEQ_PUSH_FRONT</b>)
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is undefined.
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</div>
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<div>
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(It is also possible to start with an extra element and pop it off when you
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have finished appending to it.)
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</div>
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<div>
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In C99, neither of these macros are necessary since it is legal to pass empty
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arguments.
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</div>
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<h4>
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See Also
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</h4>
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<ul>
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<li>
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<a href="empty.html">BOOST_PP_EMPTY</a></li>
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</ul>
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<h4>
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Requirements
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</h4>
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<div>
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2004-10-11 06:58:45 +00:00
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<b>Header:</b> <a href="../headers/seq/seq.html"><boost/preprocessor/seq/seq.hpp></a>
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</div>
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<h4>
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Sample Code
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</h4>
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<div>
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<pre>
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2004-10-11 06:58:45 +00:00
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#include <<a href="../headers/facilities/empty.html">boost/preprocessor/facilities/empty.hpp</a>>
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#include <<a href="../headers/seq/push_back.html">boost/preprocessor/seq/push_back.hpp</a>>
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#include <<a href="../headers/seq/push_front.html">boost/preprocessor/seq/push_front.hpp</a>>
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#include <<a href="../headers/seq/seq.html">boost/preprocessor/seq/seq.hpp</a>>
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#define SEQ_L <a href="seq_nil.html">BOOST_PP_SEQ_NIL</a>
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#define SEQ_R <a href="empty.html">BOOST_PP_EMPTY</a>
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<a href="seq_push_back.html">BOOST_PP_SEQ_PUSH_BACK</a>(
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<a href="seq_push_back.html">BOOST_PP_SEQ_PUSH_BACK</a>(SEQ_L, a), b
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)
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// expands to (a)(b)
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2002-11-07 23:39:29 +00:00
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<a href="seq_push_front.html">BOOST_PP_SEQ_PUSH_FRONT</a>(
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<a href="seq_push_front.html">BOOST_PP_SEQ_PUSH_FRONT</a>(SEQ_R, a), b
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)()
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// expands to (b)(a)
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</pre>
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</div>
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</body>
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2002-10-13 05:47:23 +00:00
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</html>
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