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Paul Mensonides
2002-09-08 09:40:56 +00:00
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<html>
<head>
<title>BOOST_PP_EXPR_IF</title>
<link rel="stylesheet" type="text/css" href="../styles.css">
<script language="javascript" type="text/javascript" src="../scripts.js"></script>
</head>
<body onload="init('sample');">
<div style="margin-left: 0px;">
The <b>BOOST_PP_EXPR_IF</b> macro expands to its second argument if its first argument is non-zero or expands to nothing otherwise.
</div>
<h4>Usage</h4>
<div class="code">
<b>BOOST_PP_EXPR_IF</b>(<i>cond</i>, <i>expr</i>)
</div>
<h4>Arguments</h4>
<dl>
<dt>cond</dt>
<dd>
The condition that determines if the result is <i>expr</i> or nothing.&nbsp;
Valid values range from <i>0</i> to <b>BOOST_PP_LIMIT_MAG</b>.
</dd>
<dt>expr</dt>
<dd>
The result of the expansion if <i>cond</i> is non-zero.
</dd>
</dl>
<h4>Remarks</h4>
<div>
This macro performs a boolean conversion on its first argument.&nbsp;
If that conversion is unnecessary, use <b>BOOST_PP_EXPR_IIF</b> instead.
</div>
<h4>See Also</h4>
<ul>
<li><a href="expr_iif.html">BOOST_PP_EXPR_IIF</a></li>
<li><a href="limit_mag.html">BOOST_PP_LIMIT_MAG</a></li>
</ul>
<h4>Requirements</h4>
<div>
<b>Header:</b> &nbsp;<a href="../headers/control/expr_if.hpp.html">&lt;boost/preprocessor/control/expr_if.hpp&gt;</a>
</div>
<h4><a class="local" onclick="toggle('sample');" onmouseover="change(this);" onmouseout="revert(this);">Sample Code</a></h4>
<div id="sample"><pre>
#include &lt;boost/preprocessor/control/expr_if.hpp&gt;
#include &lt;boost/preprocessor/tuple/elem.hpp&gt;
#define CV(n) /* ........................... */ \
BOOST_PP_EXPR_IF( \
n, \
BOOST_PP_TUPLE_ELEM( \
4, n, \
(..., const, volatile, const volatile) \
) \
) \
/**/
CV(0) // expands to nothing
CV(1) // expands to const
<pre></div>
</body>
</html>