1<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"> 2<html> 3<head> 4<meta http-equiv="Content-Type" content="text/html; charset=UTF-8"> 5<title>Function template optional</title> 6<link rel="stylesheet" href="../../../../doc/src/boostbook.css" type="text/css"> 7<meta name="generator" content="DocBook XSL Stylesheets V1.79.1"> 8<link rel="home" href="../../index.html" title="The Boost C++ Libraries BoostBook Documentation Subset"> 9<link rel="up" href="../../xpressive/reference.html#header.boost.xpressive.regex_primitives_hpp" title="Header <boost/xpressive/regex_primitives.hpp>"> 10<link rel="prev" href="range.html" title="Function template range"> 11<link rel="next" href="repeat.html" title="Function repeat"> 12</head> 13<body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"> 14<table cellpadding="2" width="100%"><tr> 15<td valign="top"><img alt="Boost C++ Libraries" width="277" height="86" src="../../../../boost.png"></td> 16<td align="center"><a href="../../../../index.html">Home</a></td> 17<td align="center"><a href="../../../../libs/libraries.htm">Libraries</a></td> 18<td align="center"><a href="http://www.boost.org/users/people.html">People</a></td> 19<td align="center"><a href="http://www.boost.org/users/faq.html">FAQ</a></td> 20<td align="center"><a href="../../../../more/index.htm">More</a></td> 21</tr></table> 22<hr> 23<div class="spirit-nav"> 24<a accesskey="p" href="range.html"><img src="../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../../xpressive/reference.html#header.boost.xpressive.regex_primitives_hpp"><img src="../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../../index.html"><img src="../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="repeat.html"><img src="../../../../doc/src/images/next.png" alt="Next"></a> 25</div> 26<div class="refentry"> 27<a name="boost.xpressive.optional"></a><div class="titlepage"></div> 28<div class="refnamediv"> 29<h2><span class="refentrytitle">Function template optional</span></h2> 30<p>boost::xpressive::optional — Make a sub-expression optional. Equivalent to !as_xpr(expr). </p> 31</div> 32<h2 xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" class="refsynopsisdiv-title">Synopsis</h2> 33<div xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" class="refsynopsisdiv"><pre class="synopsis"><span class="comment">// In header: <<a class="link" href="../../xpressive/reference.html#header.boost.xpressive.regex_primitives_hpp" title="Header <boost/xpressive/regex_primitives.hpp>">boost/xpressive/regex_primitives.hpp</a>> 34 35</span> 36<span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <a class="link" href="../../Expr.html" title="Concept Expr">Expr</a><span class="special">></span> 37 <span class="identifier">proto</span><span class="special">::</span><span class="identifier">result_of</span><span class="special">::</span><span class="identifier">make_expr</span><span class="special"><</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">logical_not</span><span class="special">,</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">default_domain</span><span class="special">,</span> <span class="identifier">Expr</span> <span class="keyword">const</span> <span class="special">&</span> <span class="special">></span><span class="special">::</span><span class="identifier">type</span> <span class="keyword">const</span> 38 <span class="identifier">optional</span><span class="special">(</span><span class="identifier">Expr</span> <span class="keyword">const</span> <span class="special">&</span> expr<span class="special">)</span><span class="special">;</span></pre></div> 39<div class="refsect1"> 40<a name="id-1.3.47.5.10.57.4"></a><h2>Description</h2> 41<p> 42</p> 43<div class="variablelist"><table border="0" class="variablelist compact"> 44<colgroup> 45<col align="left" valign="top"> 46<col> 47</colgroup> 48<tbody><tr> 49<td><p><span class="term">Parameters:</span></p></td> 50<td><div class="variablelist"><table border="0" class="variablelist compact"> 51<colgroup> 52<col align="left" valign="top"> 53<col> 54</colgroup> 55<tbody><tr> 56<td><p><span class="term"><code class="computeroutput">expr</code></span></p></td> 57<td><p>The sub-expression to make optional. </p></td> 58</tr></tbody> 59</table></div></td> 60</tr></tbody> 61</table></div> 62</div> 63</div> 64<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> 65<td align="left"></td> 66<td align="right"><div class="copyright-footer">Copyright © 2007 Eric Niebler<p> 67 Distributed under the Boost Software License, Version 1.0. (See accompanying 68 file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>) 69 </p> 70</div></td> 71</tr></table> 72<hr> 73<div class="spirit-nav"> 74<a accesskey="p" href="range.html"><img src="../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../../xpressive/reference.html#header.boost.xpressive.regex_primitives_hpp"><img src="../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../../index.html"><img src="../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="repeat.html"><img src="../../../../doc/src/images/next.png" alt="Next"></a> 75</div> 76</body> 77</html> 78
