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preliminary set support

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[SVN r15908]
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Paul Mensonides
2002-10-13 05:47:23 +00:00
parent b2bd498a4f
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<html>
<head>
<title>BOOST_PP_SET_NIL</title>
<link rel="stylesheet" type="text/css" href="../styles.css">
</head>
<body>
<div style="margin-left: 0px;">
The <b>BOOST_PP_SET_NIL</b> macro is a placeholder macro for an empty <i>set</i>.&nbsp;
It is only valid if it is elements are appended to the end of this empty "set."&nbsp;
</div>
<h4>Usage</h4>
<div class="code">
<b>BOOST_PP_SET_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>set</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_SET_NIL</b>(<i>x</i>)
and <b>BOOST_PP_SET_PUSH_BACK</b>(<b>BOOST_PP_SET_NIL</b>, <i>x</i>) expand to <i>x</i>.
</div>
<div>
If any <b>BOOST_PP_SET_</b>* macro (other than <b>BOOST_PP_SET_PUSH_BACK</b>) is invoked with an argument that contains <b>BOOST_PP_SET_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_SET_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_SET_NIL</b>, passing an argument that contains <b>BOOST_PP_EMPTY</b> to any <b>BOOST_PP_SET_</b>* macro (other than <b>BOOST_PP_SET_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/set/set.hpp.html">&lt;boost/preprocessor/set/set.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/set/push_back.hpp.html">boost/preprocessor/set/push_back.hpp</a>&gt;
#include &lt;<a href="../headers/set/push_front.hpp.html">boost/preprocessor/set/push_front.hpp</a>&gt;
#include &lt;<a href="../headers/set/set.hpp.html">boost/preprocessor/set/set.hpp</a>&gt;
#define SET_L <a href="set_nil.html">BOOST_PP_SET_NIL</a>
#define SET_R <a href="empty.html">BOOST_PP_EMPTY</a>
<a href="set_push_back.html">BOOST_PP_SET_PUSH_BACK</a>(
<a href="set_push_back.html">BOOST_PP_SET_PUSH_BACK</a>(SET_L, a), b
)
// expands to (a)(b)
<a href="set_push_front.html">BOOST_PP_SET_PUSH_FRONT</a>(
<a href="set_push_front.html">BOOST_PP_SET_PUSH_FRONT</a>(SET_R, a), b
)()
// expands to (b)(a)
</pre></div>
</body>
</html>