1<html> 2<head> 3<meta http-equiv="Content-Type" content="text/html; charset=UTF-8"> 4<title>Lazy Generator (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 Generators"> 9<link rel="prev" href="eps.html" title="Epsilon Generator (eps)"> 10<link rel="next" href="../binary.html" title="Binary Generators"> 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.karma.reference.auxiliary.lazy"></a><a class="link" href="lazy.html" title="Lazy Generator (lazy)">Lazy Generator 28 (<code class="computeroutput"><span class="identifier">lazy</span></code>)</a> 29</h5></div></div></div> 30<h6> 31<a name="spirit.karma.reference.auxiliary.lazy.h0"></a> 32 <span class="phrase"><a name="spirit.karma.reference.auxiliary.lazy.description"></a></span><a class="link" href="lazy.html#spirit.karma.reference.auxiliary.lazy.description">Description</a> 33 </h6> 34<p> 35 The family of <code class="computeroutput"><span class="identifier">lazy</span></code> components 36 allows to use a dynamically returned generator component for output generation. 37 It calls the provided function or function object at generate time using 38 its return value as the actual generator to produce the output. 39 </p> 40<h6> 41<a name="spirit.karma.reference.auxiliary.lazy.h1"></a> 42 <span class="phrase"><a name="spirit.karma.reference.auxiliary.lazy.header"></a></span><a class="link" href="lazy.html#spirit.karma.reference.auxiliary.lazy.header">Header</a> 43 </h6> 44<pre class="programlisting"><span class="comment">// forwards to <boost/spirit/home/karma/auxiliary/lazy.hpp></span> 45<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">karma_lazy</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> 46</pre> 47<p> 48 Also, see <a class="link" href="../../../structure/include.html" title="Include">Include Structure</a>. 49 </p> 50<h6> 51<a name="spirit.karma.reference.auxiliary.lazy.h2"></a> 52 <span class="phrase"><a name="spirit.karma.reference.auxiliary.lazy.namespace"></a></span><a class="link" href="lazy.html#spirit.karma.reference.auxiliary.lazy.namespace">Namespace</a> 53 </h6> 54<div class="informaltable"><table class="table"> 55<colgroup><col></colgroup> 56<thead><tr><th> 57 <p> 58 Name 59 </p> 60 </th></tr></thead> 61<tbody><tr><td> 62 <p> 63 <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: 64 boost::spirit::karma::lazy</span></code> 65 </p> 66 </td></tr></tbody> 67</table></div> 68<h6> 69<a name="spirit.karma.reference.auxiliary.lazy.h3"></a> 70 <span class="phrase"><a name="spirit.karma.reference.auxiliary.lazy.model_of"></a></span><a class="link" href="lazy.html#spirit.karma.reference.auxiliary.lazy.model_of">Model 71 of</a> 72 </h6> 73<div class="blockquote"><blockquote class="blockquote"><p> 74 <a class="link" href="../generator_concepts/generator.html" title="Generator"><code class="computeroutput"><span class="identifier">Generator</span></code></a> 75 </p></blockquote></div> 76<div class="variablelist"> 77<p class="title"><b>Notation</b></p> 78<dl class="variablelist"> 79<dt><span class="term"><code class="computeroutput"><span class="identifier">fg</span></code></span></dt> 80<dd><p> 81 A function or function object that evaluates to a generator object 82 (an object exposing the <a class="link" href="../generator_concepts/generator.html" title="Generator"><code class="computeroutput"><span class="identifier">Generator</span></code></a>). This function 83 will be invoked at generate time. 84 </p></dd> 85</dl> 86</div> 87<p> 88 The signature of <code class="computeroutput"><span class="identifier">fg</span></code> is 89 expected to be 90 </p> 91<pre class="programlisting"><span class="identifier">G</span> <span class="identifier">f</span><span class="special">(</span><span class="identifier">Unused</span><span class="special">,</span> <span class="identifier">Context</span><span class="special">)</span> 92</pre> 93<p> 94 where <code class="computeroutput"><span class="identifier">G</span></code>, the function's 95 return value, is the type of the generator to be invoked, and <code class="computeroutput"><span class="identifier">Context</span></code> is the generator's Context 96 type (The first argument is <code class="computeroutput"><span class="identifier">unused</span></code> 97 to make the <code class="computeroutput"><span class="identifier">Context</span></code> the 98 second argument. This is done for uniformity with <a class="link" href="../../tutorials/semantic_actions.html" title="Generator Semantic Actions">Semantic 99 Actions</a>). 100 </p> 101<h6> 102<a name="spirit.karma.reference.auxiliary.lazy.h4"></a> 103 <span class="phrase"><a name="spirit.karma.reference.auxiliary.lazy.expression_semantics"></a></span><a class="link" href="lazy.html#spirit.karma.reference.auxiliary.lazy.expression_semantics">Expression 104 Semantics</a> 105 </h6> 106<p> 107 Semantics of an expression is defined only where it differs from, or 108 is not defined in <a class="link" href="../generator_concepts/generator.html" title="Generator"><code class="computeroutput"><span class="identifier">Generator</span></code></a>. 109 </p> 110<div class="informaltable"><table class="table"> 111<colgroup> 112<col> 113<col> 114</colgroup> 115<thead><tr> 116<th> 117 <p> 118 Expression 119 </p> 120 </th> 121<th> 122 <p> 123 Semantics 124 </p> 125 </th> 126</tr></thead> 127<tbody> 128<tr> 129<td> 130 <p> 131 <code class="computeroutput"><span class="identifier">fg</span></code> 132 </p> 133 </td> 134<td> 135 <p> 136 The <a href="../../../../../../../../libs/phoenix/doc/html/index.html" target="_top">Boost.Phoenix</a> 137 function object <code class="computeroutput"><span class="identifier">fg</span></code> 138 will be invoked at generate time. It is expected to return 139 a generator instance. This generator is then invoked in order 140 to generate the output. This generator will succeed as long 141 as the invoked generated succeeds as well (unless the underlying 142 output stream reports an error). 143 </p> 144 </td> 145</tr> 146<tr> 147<td> 148 <p> 149 <code class="computeroutput"><span class="identifier">lazy</span><span class="special">(</span><span class="identifier">fg</span><span class="special">)</span></code> 150 </p> 151 </td> 152<td> 153 <p> 154 The function or function object will be invoked at generate 155 time. It is expected to return a generator instance (note this 156 version of <code class="computeroutput"><span class="identifier">lazy</span></code> 157 does not require <code class="computeroutput"><span class="identifier">fg</span></code> 158 to be a <a href="../../../../../../../../libs/phoenix/doc/html/index.html" target="_top">Boost.Phoenix</a> 159 function object). This generator is then invoked in order to 160 generate the output. This generator will succeed as long as 161 the invoked generated succeeds as well (except if the underlying 162 output stream reports an error). 163 </p> 164 </td> 165</tr> 166</tbody> 167</table></div> 168<h6> 169<a name="spirit.karma.reference.auxiliary.lazy.h5"></a> 170 <span class="phrase"><a name="spirit.karma.reference.auxiliary.lazy.attributes"></a></span><a class="link" href="lazy.html#spirit.karma.reference.auxiliary.lazy.attributes">Attributes</a> 171 </h6> 172<div class="informaltable"><table class="table"> 173<colgroup> 174<col> 175<col> 176</colgroup> 177<thead><tr> 178<th> 179 <p> 180 Expression 181 </p> 182 </th> 183<th> 184 <p> 185 Attribute 186 </p> 187 </th> 188</tr></thead> 189<tbody> 190<tr> 191<td> 192 <p> 193 <code class="computeroutput"><span class="identifier">fg</span></code> 194 </p> 195 </td> 196<td> 197 <p> 198 The attribute type <code class="computeroutput"><span class="identifier">G</span></code> 199 as exposed by the generator <code class="computeroutput"><span class="identifier">g</span></code> 200 returned from <code class="computeroutput"><span class="identifier">fg</span></code>. 201 </p> 202 </td> 203</tr> 204<tr> 205<td> 206 <p> 207 <code class="computeroutput"><span class="identifier">lazy</span><span class="special">(</span><span class="identifier">fg</span><span class="special">)</span></code> 208 </p> 209 </td> 210<td> 211 <p> 212 The attribute type <code class="computeroutput"><span class="identifier">G</span></code> 213 as exposed by the generator <code class="computeroutput"><span class="identifier">g</span></code> 214 returned from <code class="computeroutput"><span class="identifier">fg</span></code>. 215 </p> 216 </td> 217</tr> 218</tbody> 219</table></div> 220<h6> 221<a name="spirit.karma.reference.auxiliary.lazy.h6"></a> 222 <span class="phrase"><a name="spirit.karma.reference.auxiliary.lazy.complexity"></a></span><a class="link" href="lazy.html#spirit.karma.reference.auxiliary.lazy.complexity">Complexity</a> 223 </h6> 224<p> 225 The complexity of the <code class="computeroutput"><span class="identifier">lazy</span></code> 226 component is determined by the complexity of the generator returned from 227 <code class="computeroutput"><span class="identifier">fg</span></code>. 228 </p> 229<h6> 230<a name="spirit.karma.reference.auxiliary.lazy.h7"></a> 231 <span class="phrase"><a name="spirit.karma.reference.auxiliary.lazy.example"></a></span><a class="link" href="lazy.html#spirit.karma.reference.auxiliary.lazy.example">Example</a> 232 </h6> 233<div class="note"><table border="0" summary="Note"> 234<tr> 235<td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../../images/note.png"></td> 236<th align="left">Note</th> 237</tr> 238<tr><td align="left" valign="top"><p> 239 The test harness for the example(s) below is presented in the <a class="link" href="../basics.html#spirit.karma.reference.basics.examples">Basics Examples</a> 240 section. 241 </p></td></tr> 242</table></div> 243<p> 244 Some includes: 245 </p> 246<p> 247</p> 248<pre class="programlisting"><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">karma</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> 249<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">support_utree</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> 250<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">phoenix_core</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> 251<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">phoenix_operator</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> 252<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">fusion</span><span class="special">/</span><span class="identifier">include</span><span class="special">/</span><span class="identifier">std_pair</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> 253<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">iostream</span><span class="special">></span> 254<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">string</span><span class="special">></span> 255</pre> 256<p> 257 </p> 258<p> 259 Some using declarations: 260 </p> 261<p> 262</p> 263<pre class="programlisting"><span class="keyword">namespace</span> <span class="identifier">karma</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">karma</span><span class="special">;</span> 264<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">karma</span><span class="special">::</span><span class="identifier">_1</span><span class="special">;</span> 265<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> 266<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> 267</pre> 268<p> 269 </p> 270<p> 271 Basic usage of the <code class="computeroutput"><span class="identifier">lazy</span></code> 272 generator: 273 </p> 274<p> 275</p> 276<pre class="programlisting"><span class="identifier">test_generator_attr</span><span class="special">(</span><span class="string">"abc"</span><span class="special">,</span> <span class="identifier">karma</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">"abc"</span><span class="special">);</span> 277<span class="identifier">test_generator</span><span class="special">(</span><span class="string">"abc"</span><span class="special">,</span> <span class="identifier">karma</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="identifier">_1</span> <span class="special">=</span> <span class="string">"abc"</span><span class="special">]);</span> 278</pre> 279<p> 280 </p> 281</div> 282<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> 283<td align="left"></td> 284<td align="right"><div class="copyright-footer">Copyright © 2001-2011 Joel de Guzman, Hartmut Kaiser<p> 285 Distributed under the Boost Software License, Version 1.0. (See accompanying 286 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>) 287 </p> 288</div></td> 289</tr></table> 290<hr> 291<div class="spirit-nav"> 292<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> 293</div> 294</body> 295</html> 296
