forked from boostorg/preprocessor
70 lines
2.9 KiB
HTML
70 lines
2.9 KiB
HTML
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<html>
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<head>
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<title>BOOST_PP_SET_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_SET_NIL</b> macro is a placeholder macro for an empty <i>set</i>.
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It is only valid if it is elements are appended to the end of this empty "set."
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</div>
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<h4>Usage</h4>
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<div class="code">
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<b>BOOST_PP_SET_NIL</b>
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</div>
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<h4>Remarks</h4>
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<div>
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This macro is a utility macro intended as a empty starting point for appending to the tail.
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It is <i>not</i> a nil <i>set</i>.
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When an element is appended to this macro, it expands on the element and to the element--thereby removing itself.
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For example, both <b>BOOST_PP_SET_NIL</b>(<i>x</i>)
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and <b>BOOST_PP_SET_PUSH_BACK</b>(<b>BOOST_PP_SET_NIL</b>, <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_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>,
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the behavior 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_SET_NIL</b> for appending to the head is <b>BOOST_PP_EMPTY</b>.
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After all the elements have been 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_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.
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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 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 arguments.
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</div>
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<h4>See Also</h4>
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<ul>
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<li><a href="empty.html">BOOST_PP_EMPTY</a></li>
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</ul>
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<h4>Requirements</h4>
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<div>
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<b>Header:</b> <a href="../headers/set/set.hpp.html"><boost/preprocessor/set/set.hpp></a>
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</div>
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<h4>Sample Code</h4>
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<div><pre>
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#include <<a href="../headers/facilities/empty.hpp.html">boost/preprocessor/facilities/empty.hpp</a>>
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#include <<a href="../headers/set/push_back.hpp.html">boost/preprocessor/set/push_back.hpp</a>>
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#include <<a href="../headers/set/push_front.hpp.html">boost/preprocessor/set/push_front.hpp</a>>
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#include <<a href="../headers/set/set.hpp.html">boost/preprocessor/set/set.hpp</a>>
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#define SET_L <a href="set_nil.html">BOOST_PP_SET_NIL</a>
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#define SET_R <a href="empty.html">BOOST_PP_EMPTY</a>
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<a href="set_push_back.html">BOOST_PP_SET_PUSH_BACK</a>(
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<a href="set_push_back.html">BOOST_PP_SET_PUSH_BACK</a>(SET_L, a), b
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)
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// expands to (a)(b)
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<a href="set_push_front.html">BOOST_PP_SET_PUSH_FRONT</a>(
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<a href="set_push_front.html">BOOST_PP_SET_PUSH_FRONT</a>(SET_R, a), b
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)()
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// expands to (b)(a)
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</pre></div>
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</body>
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</html>
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