1<html> 2<head> 3<meta http-equiv="Content-Type" content="text/html; charset=UTF-8"> 4<title>Lazy Parser (lazy)</title> 5<link rel="stylesheet" href="../../../../../../../../doc/src/boostbook.css" type="text/css"> 6<meta name="generator" content="DocBook XSL Stylesheets V1.79.1"> 7<link rel="home" href="../../../../index.html" title="Spirit 2.5.8"> 8<link rel="up" href="../auxiliary.html" title="Auxiliary Parsers"> 9<link rel="prev" href="eps.html" title="Epsilon Parser (eps)"> 10<link rel="next" href="../binary.html" title="Binary Parsers"> 11</head> 12<body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"> 13<table cellpadding="2" width="100%"><tr> 14<td valign="top"><img alt="Boost C++ Libraries" width="277" height="86" src="../../../../../../../../boost.png"></td> 15<td align="center"><a href="../../../../../../../../index.html">Home</a></td> 16<td align="center"><a href="../../../../../../../../libs/libraries.htm">Libraries</a></td> 17<td align="center"><a href="http://www.boost.org/users/people.html">People</a></td> 18<td align="center"><a href="http://www.boost.org/users/faq.html">FAQ</a></td> 19<td align="center"><a href="../../../../../../../../more/index.htm">More</a></td> 20</tr></table> 21<hr> 22<div class="spirit-nav"> 23<a accesskey="p" href="eps.html"><img src="../../../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../auxiliary.html"><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="../binary.html"><img src="../../../../../../../../doc/src/images/next.png" alt="Next"></a> 24</div> 25<div class="section"> 26<div class="titlepage"><div><div><h5 class="title"> 27<a name="spirit.qi.reference.auxiliary.lazy"></a><a class="link" href="lazy.html" title="Lazy Parser (lazy)">Lazy Parser (<code class="computeroutput"><span class="identifier">lazy</span></code>)</a> 28</h5></div></div></div> 29<h6> 30<a name="spirit.qi.reference.auxiliary.lazy.h0"></a> 31 <span class="phrase"><a name="spirit.qi.reference.auxiliary.lazy.description"></a></span><a class="link" href="lazy.html#spirit.qi.reference.auxiliary.lazy.description">Description</a> 32 </h6> 33<p> 34 The <code class="computeroutput"><span class="identifier">lazy</span></code> parser, as its 35 name suggests, invokes a lazy <a href="../../../../../../../../libs/phoenix/doc/html/index.html" target="_top">Boost.Phoenix</a> 36 function that returns a parser at parse time. This parser will be used 37 once it is created to continue the parse. 38 </p> 39<h6> 40<a name="spirit.qi.reference.auxiliary.lazy.h1"></a> 41 <span class="phrase"><a name="spirit.qi.reference.auxiliary.lazy.header"></a></span><a class="link" href="lazy.html#spirit.qi.reference.auxiliary.lazy.header">Header</a> 42 </h6> 43<pre class="programlisting"><span class="comment">// forwards to <boost/spirit/home/qi/auxiliary/lazy.hpp></span> 44<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">spirit</span><span class="special">/</span><span class="identifier">include</span><span class="special">/</span><span class="identifier">qi_lazy</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> 45</pre> 46<p> 47 Also, see <a class="link" href="../../../structure/include.html" title="Include">Include Structure</a>. 48 </p> 49<h6> 50<a name="spirit.qi.reference.auxiliary.lazy.h2"></a> 51 <span class="phrase"><a name="spirit.qi.reference.auxiliary.lazy.namespace"></a></span><a class="link" href="lazy.html#spirit.qi.reference.auxiliary.lazy.namespace">Namespace</a> 52 </h6> 53<div class="informaltable"><table class="table"> 54<colgroup><col></colgroup> 55<thead><tr><th> 56 <p> 57 Name 58 </p> 59 </th></tr></thead> 60<tbody><tr><td> 61 <p> 62 <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">spirit</span><span class="special">::</span><span class="identifier">lazy</span> <span class="comment">// alias: 63 boost::spirit::qi::lazy</span></code> 64 </p> 65 </td></tr></tbody> 66</table></div> 67<h6> 68<a name="spirit.qi.reference.auxiliary.lazy.h3"></a> 69 <span class="phrase"><a name="spirit.qi.reference.auxiliary.lazy.model_of"></a></span><a class="link" href="lazy.html#spirit.qi.reference.auxiliary.lazy.model_of">Model 70 of</a> 71 </h6> 72<div class="blockquote"><blockquote class="blockquote"><p> 73 <a class="link" href="../parser_concepts/parser.html" title="Parser"><code class="computeroutput"><span class="identifier">Parser</span></code></a> 74 </p></blockquote></div> 75<div class="variablelist"> 76<p class="title"><b>Notation</b></p> 77<dl class="variablelist"> 78<dt><span class="term"><code class="computeroutput"><span class="identifier">fp</span></code></span></dt> 79<dd><p> 80 A <a class="link" href="../basics.html#spirit.qi.reference.basics.lazy_argument">Lazy 81 Argument</a> that evaluates to a <a class="link" href="../parser_concepts/parser.html" title="Parser"><code class="computeroutput"><span class="identifier">Parser</span></code></a>. 82 </p></dd> 83</dl> 84</div> 85<h6> 86<a name="spirit.qi.reference.auxiliary.lazy.h4"></a> 87 <span class="phrase"><a name="spirit.qi.reference.auxiliary.lazy.expression_semantics"></a></span><a class="link" href="lazy.html#spirit.qi.reference.auxiliary.lazy.expression_semantics">Expression 88 Semantics</a> 89 </h6> 90<p> 91 Semantics of an expression is defined only where it differs from, or 92 is not defined in <a class="link" href="../parser_concepts/parser.html" title="Parser"><code class="computeroutput"><span class="identifier">Parser</span></code></a>. 93 </p> 94<div class="informaltable"><table class="table"> 95<colgroup> 96<col> 97<col> 98</colgroup> 99<thead><tr> 100<th> 101 <p> 102 Expression 103 </p> 104 </th> 105<th> 106 <p> 107 Semantics 108 </p> 109 </th> 110</tr></thead> 111<tbody> 112<tr> 113<td> 114 <p> 115 <code class="computeroutput"><span class="identifier">fp</span></code> 116 </p> 117 </td> 118<td> 119 <p> 120 Create a lazy-parser from a <a class="link" href="../basics.html#spirit.qi.reference.basics.lazy_argument">Lazy 121 Argument</a>, <code class="computeroutput"><span class="identifier">fp</span></code>. 122 <code class="computeroutput"><span class="identifier">fp</span></code> will be 123 invoked at parse time. <code class="computeroutput"><span class="identifier">fp</span></code> 124 is expected to return a <a class="link" href="../parser_concepts/parser.html" title="Parser"><code class="computeroutput"><span class="identifier">Parser</span></code></a> object. This 125 parser is then invoked in order to parse the input. 126 </p> 127 </td> 128</tr> 129<tr> 130<td> 131 <p> 132 <code class="computeroutput"><span class="identifier">lazy</span><span class="special">(</span><span class="identifier">fp</span><span class="special">)</span></code> 133 </p> 134 </td> 135<td> 136 <p> 137 Create a lazy-parser from a <a class="link" href="../basics.html#spirit.qi.reference.basics.lazy_argument">Lazy 138 Argument</a>, <code class="computeroutput"><span class="identifier">fp</span></code>. 139 <code class="computeroutput"><span class="identifier">fp</span></code> will be 140 invoked at parse time. <code class="computeroutput"><span class="identifier">fp</span></code> 141 is expected to return a <a class="link" href="../parser_concepts/parser.html" title="Parser"><code class="computeroutput"><span class="identifier">Parser</span></code></a> object. This 142 parser is then invoked in order to parse the input. 143 </p> 144 </td> 145</tr> 146</tbody> 147</table></div> 148<h6> 149<a name="spirit.qi.reference.auxiliary.lazy.h5"></a> 150 <span class="phrase"><a name="spirit.qi.reference.auxiliary.lazy.attributes"></a></span><a class="link" href="lazy.html#spirit.qi.reference.auxiliary.lazy.attributes">Attributes</a> 151 </h6> 152<div class="informaltable"><table class="table"> 153<colgroup> 154<col> 155<col> 156</colgroup> 157<thead><tr> 158<th> 159 <p> 160 Expression 161 </p> 162 </th> 163<th> 164 <p> 165 Attribute 166 </p> 167 </th> 168</tr></thead> 169<tbody> 170<tr> 171<td> 172 <p> 173 <code class="computeroutput"><span class="identifier">fp</span></code> 174 </p> 175 </td> 176<td> 177 <p> 178 The attribute type of the return type of <code class="computeroutput"><span class="identifier">fp</span></code>. 179 </p> 180 </td> 181</tr> 182<tr> 183<td> 184 <p> 185 <code class="computeroutput"><span class="identifier">lazy</span><span class="special">(</span><span class="identifier">fp</span><span class="special">)</span></code> 186 </p> 187 </td> 188<td> 189 <p> 190 The attribute type of the return type of <code class="computeroutput"><span class="identifier">fp</span></code>. 191 </p> 192 </td> 193</tr> 194</tbody> 195</table></div> 196<h6> 197<a name="spirit.qi.reference.auxiliary.lazy.h6"></a> 198 <span class="phrase"><a name="spirit.qi.reference.auxiliary.lazy.complexity"></a></span><a class="link" href="lazy.html#spirit.qi.reference.auxiliary.lazy.complexity">Complexity</a> 199 </h6> 200<p> 201 The complexity of the <code class="computeroutput"><span class="identifier">lazy</span></code> 202 parser is determined by the complexity of the parser returned from <code class="computeroutput"><span class="identifier">fp</span></code>. 203 </p> 204<h6> 205<a name="spirit.qi.reference.auxiliary.lazy.h7"></a> 206 <span class="phrase"><a name="spirit.qi.reference.auxiliary.lazy.example"></a></span><a class="link" href="lazy.html#spirit.qi.reference.auxiliary.lazy.example">Example</a> 207 </h6> 208<div class="note"><table border="0" summary="Note"> 209<tr> 210<td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../../images/note.png"></td> 211<th align="left">Note</th> 212</tr> 213<tr><td align="left" valign="top"><p> 214 The test harness for the example(s) below is presented in the <a class="link" href="../basics.html#spirit.qi.reference.basics.examples">Basics Examples</a> 215 section. 216 </p></td></tr> 217</table></div> 218<p> 219 Some using declarations: 220 </p> 221<p> 222</p> 223<pre class="programlisting"><span class="keyword">using</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">spirit</span><span class="special">::</span><span class="identifier">qi</span><span class="special">::</span><span class="identifier">lazy</span><span class="special">;</span> 224<span class="keyword">using</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">spirit</span><span class="special">::</span><span class="identifier">ascii</span><span class="special">::</span><span class="identifier">string</span><span class="special">;</span> 225<span class="keyword">using</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">phoenix</span><span class="special">::</span><span class="identifier">val</span><span class="special">;</span> 226</pre> 227<p> 228 </p> 229<p> 230 Using <code class="computeroutput"><span class="identifier">lazy</span></code>: 231 </p> 232<p> 233 Here, the phoenix::val expression creates a function that returns its 234 argument when invoked. The lazy expression defers the invocation of this 235 function at parse time. Then, this parser (string parser) is called into 236 action. All this takes place at parse time. 237 </p> 238<p> 239</p> 240<pre class="programlisting"><span class="identifier">test_parser</span><span class="special">(</span><span class="string">"Hello"</span><span class="special">,</span> <span class="identifier">lazy</span><span class="special">(</span><span class="identifier">val</span><span class="special">(</span><span class="identifier">string</span><span class="special">(</span><span class="string">"Hello"</span><span class="special">))));</span> 241</pre> 242<p> 243 </p> 244<p> 245 The above is equivalent to: 246 </p> 247<p> 248</p> 249<pre class="programlisting"><span class="identifier">test_parser</span><span class="special">(</span><span class="string">"Hello"</span><span class="special">,</span> <span class="identifier">string</span><span class="special">(</span><span class="string">"Hello"</span><span class="special">));</span> 250</pre> 251<p> 252 </p> 253</div> 254<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> 255<td align="left"></td> 256<td align="right"><div class="copyright-footer">Copyright © 2001-2011 Joel de Guzman, Hartmut Kaiser<p> 257 Distributed under the Boost Software License, Version 1.0. (See accompanying 258 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>) 259 </p> 260</div></td> 261</tr></table> 262<hr> 263<div class="spirit-nav"> 264<a accesskey="p" href="eps.html"><img src="../../../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../auxiliary.html"><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="../binary.html"><img src="../../../../../../../../doc/src/images/next.png" alt="Next"></a> 265</div> 266</body> 267</html> 268
