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[SVN r16152]
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Paul Mensonides
2002-11-08 02:12:35 +00:00
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<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>.&nbsp;
It is only valid if it is elements are appended to the end of this empty
"seq."&nbsp;
</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.&nbsp; It is <i>not</i> a nil <i>seq</i>.&nbsp; When an element is
appended to this macro, it expands on the element and to the element--thereby
removing itself.&nbsp; 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>.&nbsp; After all the elements have been
prepended, empty parenthesis can be invoked on the tail to remove the <b>BOOST_PP_EMPTY</b>.&nbsp;
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.&nbsp;
</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> &nbsp;<a href="../headers/seq/seq.hpp.html">&lt;boost/preprocessor/seq/seq.hpp&gt;</a>
</div>
<h4>
Sample Code
</h4>
<div>
<pre>
#include &lt;<a href="../headers/facilities/empty.hpp.html">boost/preprocessor/facilities/empty.hpp</a>&gt;
#include &lt;<a href="../headers/seq/push_back.hpp.html">boost/preprocessor/seq/push_back.hpp</a>&gt;
#include &lt;<a href="../headers/seq/push_front.hpp.html">boost/preprocessor/seq/push_front.hpp</a>&gt;
#include &lt;<a href="../headers/seq/seq.hpp.html">boost/preprocessor/seq/seq.hpp</a>&gt;
#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>