1<html> 2<head> 3<meta http-equiv="Content-Type" content="text/html; charset=UTF-8"> 4<title>Expectation Operator (a > b)</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="../operator.html" title="Parser Operators"> 9<link rel="prev" href="difference.html" title="Difference Parser (a - b)"> 10<link rel="next" href="kleene.html" title="Kleene Parser (*a)"> 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="difference.html"><img src="../../../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../operator.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="kleene.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.operator.expect"></a><a class="link" href="expect.html" title="Expectation Operator (a > b)">Expectation 28 Operator (<code class="computeroutput"><span class="identifier">a</span> <span class="special">></span> 29 <span class="identifier">b</span></code>)</a> 30</h5></div></div></div> 31<h6> 32<a name="spirit.qi.reference.operator.expect.h0"></a> 33 <span class="phrase"><a name="spirit.qi.reference.operator.expect.description"></a></span><a class="link" href="expect.html#spirit.qi.reference.operator.expect.description">Description</a> 34 </h6> 35<p> 36 There are occasions in which it is expected that the input must match 37 a particular parser or the input is invalid. Such cases generally arise 38 after matching a portion of a grammar, such that the context is fully 39 known. In such a situation, failure to match should result in an exception. 40 For example, when parsing an e-mail address, after matching a name and 41 "@" there must be a domain name or the address is invalid. 42 </p> 43<p> 44 The expectation operator (>) requires that the following parser match 45 the input or an exception is emitted. Using on_error(), that exception 46 can be handled by calling a handler with the context at which the parsing 47 failed can be reported. 48 </p> 49<p> 50 By contrast, the follows operator (>>) does not require that the 51 following parser match the input, which allows for backtracking or simply 52 returning false from the parse() function with no exceptions. 53 </p> 54<p> 55 Like the <a class="link" href="sequence.html" title="Sequence Parser (a >> b)">Sequence</a>, 56 the expectation operator, <code class="computeroutput"><span class="identifier">a</span> 57 <span class="special">></span> <span class="identifier">b</span></code>, 58 parses two or more operands (<code class="computeroutput"><span class="identifier">a</span></code>, 59 <code class="computeroutput"><span class="identifier">b</span></code>, ... etc.), in sequence: 60 </p> 61<pre class="programlisting"><span class="identifier">a</span> <span class="special">></span> <span class="identifier">b</span> <span class="special">></span> <span class="special">...</span> 62</pre> 63<p> 64 However, while the plain <a class="link" href="sequence.html" title="Sequence Parser (a >> b)">Sequence</a> 65 simply returns a no-match (returns <code class="computeroutput"><span class="keyword">false</span></code>) 66 when one of the elements fail, the expectation: <code class="computeroutput"><span class="special">></span></code> 67 operator throws an <a class="link" href="expect.html#spirit.qi.reference.operator.expect.expectation_failure"><code class="computeroutput"><span class="identifier">expectation_failure</span></code></a><code class="computeroutput"><span class="special"><</span><span class="identifier">Iter</span><span class="special">></span></code> when the second or succeeding operands 68 (all operands except the first) fail to match. 69 </p> 70<h6> 71<a name="spirit.qi.reference.operator.expect.h1"></a> 72 <span class="phrase"><a name="spirit.qi.reference.operator.expect.header"></a></span><a class="link" href="expect.html#spirit.qi.reference.operator.expect.header">Header</a> 73 </h6> 74<pre class="programlisting"><span class="comment">// forwards to <boost/spirit/home/qi/operator/expect.hpp></span> 75<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_expect</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> 76</pre> 77<p> 78 Also, see <a class="link" href="../../../structure/include.html" title="Include">Include Structure</a>. 79 </p> 80<h6> 81<a name="spirit.qi.reference.operator.expect.h2"></a> 82 <span class="phrase"><a name="spirit.qi.reference.operator.expect.model_of"></a></span><a class="link" href="expect.html#spirit.qi.reference.operator.expect.model_of">Model 83 of</a> 84 </h6> 85<div class="blockquote"><blockquote class="blockquote"><p> 86 <a class="link" href="../parser_concepts/naryparser.html" title="NaryParser"><code class="computeroutput"><span class="identifier">NaryParser</span></code></a> 87 </p></blockquote></div> 88<div class="variablelist"> 89<p class="title"><b>Notation</b></p> 90<dl class="variablelist"> 91<dt><span class="term"><code class="computeroutput"><span class="identifier">a</span></code>, <code class="computeroutput"><span class="identifier">b</span></code></span></dt> 92<dd><p> 93 A <a class="link" href="../parser_concepts/parser.html" title="Parser"><code class="computeroutput"><span class="identifier">Parser</span></code></a> 94 </p></dd> 95<dt><span class="term"><code class="computeroutput"><span class="identifier">Iter</span></code></span></dt> 96<dd><p> 97 A <a href="http://en.cppreference.com/w/cpp/named_req/ForwardIterator" target="_top"><code class="computeroutput"><span class="identifier">ForwardIterator</span></code></a> type 98 </p></dd> 99</dl> 100</div> 101<h6> 102<a name="spirit.qi.reference.operator.expect.h3"></a> 103 <span class="phrase"><a name="spirit.qi.reference.operator.expect.expectation_failure"></a></span><a class="link" href="expect.html#spirit.qi.reference.operator.expect.expectation_failure">Expectation 104 Failure</a> 105 </h6> 106<p> 107 When any operand, except the first, fail to match an <code class="computeroutput"><span class="identifier">expectation_failure</span><span class="special"><</span><span class="identifier">Iter</span><span class="special">></span></code> is thrown: 108 </p> 109<pre class="programlisting"><span class="keyword">template</span> <span class="special"><</span><span class="keyword">typename</span> <span class="identifier">Iter</span><span class="special">></span> 110<span class="keyword">struct</span> <span class="identifier">expectation_failure</span> <span class="special">:</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">runtime_error</span> 111<span class="special">{</span> 112 <span class="identifier">Iter</span> <span class="identifier">first</span><span class="special">;</span> <span class="comment">// [first, last) iterator pointing</span> 113 <span class="identifier">Iter</span> <span class="identifier">last</span><span class="special">;</span> <span class="comment">// to the error position in the input.</span> 114 <code class="computeroutput"><span class="identifier">info</span></code> <span class="identifier">what_</span><span class="special">;</span> <span class="comment">// Information about the nature of the error.</span> 115<span class="special">};</span> 116</pre> 117<h6> 118<a name="spirit.qi.reference.operator.expect.h4"></a> 119 <span class="phrase"><a name="spirit.qi.reference.operator.expect.expression_semantics"></a></span><a class="link" href="expect.html#spirit.qi.reference.operator.expect.expression_semantics">Expression 120 Semantics</a> 121 </h6> 122<p> 123 Semantics of an expression is defined only where it differs from, or 124 is not defined in <a class="link" href="../parser_concepts/naryparser.html" title="NaryParser"><code class="computeroutput"><span class="identifier">NaryParser</span></code></a>. 125 </p> 126<div class="informaltable"><table class="table"> 127<colgroup> 128<col> 129<col> 130</colgroup> 131<thead><tr> 132<th> 133 <p> 134 Expression 135 </p> 136 </th> 137<th> 138 <p> 139 Semantics 140 </p> 141 </th> 142</tr></thead> 143<tbody><tr> 144<td> 145 <p> 146 <code class="computeroutput"><span class="identifier">a</span> <span class="special">></span> 147 <span class="identifier">b</span></code> 148 </p> 149 </td> 150<td> 151 <p> 152 Match <code class="computeroutput"><span class="identifier">a</span></code> followed 153 by <code class="computeroutput"><span class="identifier">b</span></code>. If <code class="computeroutput"><span class="identifier">a</span></code> fails, no-match. If <code class="computeroutput"><span class="identifier">b</span></code> fails, throw an <code class="computeroutput"><span class="identifier">expectation_failure</span><span class="special"><</span><span class="identifier">Iter</span><span class="special">></span></code> 154 </p> 155 </td> 156</tr></tbody> 157</table></div> 158<div class="note"><table border="0" summary="Note"> 159<tr> 160<td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../../images/note.png"></td> 161<th align="left">Note</th> 162</tr> 163<tr><td align="left" valign="top"><p> 164 If you need an exception to be thrown if <code class="computeroutput"><span class="identifier">a</span></code> 165 fails, use the <a class="link" href="../directive/expect.html" title="Expectation Directive (expect[])"><code class="computeroutput"><span class="identifier">expect</span></code></a> directive instead of 166 the expectation operator. 167 </p></td></tr> 168</table></div> 169<h6> 170<a name="spirit.qi.reference.operator.expect.h5"></a> 171 <span class="phrase"><a name="spirit.qi.reference.operator.expect.attributes"></a></span><a class="link" href="expect.html#spirit.qi.reference.operator.expect.attributes">Attributes</a> 172 </h6> 173<p> 174 See <a class="link" href="../../quick_reference/compound_attribute_rules.html#spirit.qi.quick_reference.compound_attribute_rules.notation">Compound 175 Attribute Notation</a>. 176 </p> 177<div class="informaltable"><table class="table"> 178<colgroup> 179<col> 180<col> 181</colgroup> 182<thead><tr> 183<th> 184 <p> 185 Expression 186 </p> 187 </th> 188<th> 189 <p> 190 Attribute 191 </p> 192 </th> 193</tr></thead> 194<tbody><tr> 195<td> 196 <p> 197 <code class="computeroutput"><span class="identifier">a</span> <span class="special">></span> 198 <span class="identifier">b</span></code> 199 </p> 200 </td> 201<td> 202 <p> 203</p> 204<pre xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" class="table-programlisting"><span class="identifier">a</span><span class="special">:</span> <span class="identifier">A</span><span class="special">,</span> <span class="identifier">b</span><span class="special">:</span> <span class="identifier">B</span> <span class="special">--></span> <span class="special">(</span><span class="identifier">a</span> <span class="special">></span> <span class="identifier">b</span><span class="special">):</span> <span class="identifier">tuple</span><span class="special"><</span><span class="identifier">A</span><span class="special">,</span> <span class="identifier">B</span><span class="special">></span> 205<span class="identifier">a</span><span class="special">:</span> <span class="identifier">A</span><span class="special">,</span> <span class="identifier">b</span><span class="special">:</span> <span class="identifier">Unused</span> <span class="special">--></span> <span class="special">(</span><span class="identifier">a</span> <span class="special">></span> <span class="identifier">b</span><span class="special">):</span> <span class="identifier">A</span> 206<span class="identifier">a</span><span class="special">:</span> <span class="identifier">Unused</span><span class="special">,</span> <span class="identifier">b</span><span class="special">:</span> <span class="identifier">B</span> <span class="special">--></span> <span class="special">(</span><span class="identifier">a</span> <span class="special">></span> <span class="identifier">b</span><span class="special">):</span> <span class="identifier">B</span> 207<span class="identifier">a</span><span class="special">:</span> <span class="identifier">Unused</span><span class="special">,</span> <span class="identifier">b</span><span class="special">:</span> <span class="identifier">Unused</span> <span class="special">--></span> <span class="special">(</span><span class="identifier">a</span> <span class="special">></span> <span class="identifier">b</span><span class="special">):</span> <span class="identifier">Unused</span> 208 209<span class="identifier">a</span><span class="special">:</span> <span class="identifier">A</span><span class="special">,</span> <span class="identifier">b</span><span class="special">:</span> <span class="identifier">A</span> <span class="special">--></span> <span class="special">(</span><span class="identifier">a</span> <span class="special">></span> <span class="identifier">b</span><span class="special">):</span> <span class="identifier">vector</span><span class="special"><</span><span class="identifier">A</span><span class="special">></span> 210<span class="identifier">a</span><span class="special">:</span> <span class="identifier">vector</span><span class="special"><</span><span class="identifier">A</span><span class="special">>,</span> <span class="identifier">b</span><span class="special">:</span> <span class="identifier">A</span> <span class="special">--></span> <span class="special">(</span><span class="identifier">a</span> <span class="special">></span> <span class="identifier">b</span><span class="special">):</span> <span class="identifier">vector</span><span class="special"><</span><span class="identifier">A</span><span class="special">></span> 211<span class="identifier">a</span><span class="special">:</span> <span class="identifier">A</span><span class="special">,</span> <span class="identifier">b</span><span class="special">:</span> <span class="identifier">vector</span><span class="special"><</span><span class="identifier">A</span><span class="special">></span> <span class="special">--></span> <span class="special">(</span><span class="identifier">a</span> <span class="special">></span> <span class="identifier">b</span><span class="special">):</span> <span class="identifier">vector</span><span class="special"><</span><span class="identifier">A</span><span class="special">></span> 212<span class="identifier">a</span><span class="special">:</span> <span class="identifier">vector</span><span class="special"><</span><span class="identifier">A</span><span class="special">>,</span> <span class="identifier">b</span><span class="special">:</span> <span class="identifier">vector</span><span class="special"><</span><span class="identifier">A</span><span class="special">></span> <span class="special">--></span> <span class="special">(</span><span class="identifier">a</span> <span class="special">></span> <span class="identifier">b</span><span class="special">):</span> <span class="identifier">vector</span><span class="special"><</span><span class="identifier">A</span><span class="special">></span></pre> 213<p> 214 </p> 215 </td> 216</tr></tbody> 217</table></div> 218<h6> 219<a name="spirit.qi.reference.operator.expect.h6"></a> 220 <span class="phrase"><a name="spirit.qi.reference.operator.expect.complexity"></a></span><a class="link" href="expect.html#spirit.qi.reference.operator.expect.complexity">Complexity</a> 221 </h6> 222<div class="blockquote"><blockquote class="blockquote"><p> 223 The overall complexity of the expectation parser is defined by the 224 sum of the complexities of its elements. The complexity of the expectation 225 operator itself is O(N), where N is the number of elements in the sequence. 226 </p></blockquote></div> 227<h6> 228<a name="spirit.qi.reference.operator.expect.h7"></a> 229 <span class="phrase"><a name="spirit.qi.reference.operator.expect.example"></a></span><a class="link" href="expect.html#spirit.qi.reference.operator.expect.example">Example</a> 230 </h6> 231<div class="note"><table border="0" summary="Note"> 232<tr> 233<td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../../images/note.png"></td> 234<th align="left">Note</th> 235</tr> 236<tr><td align="left" valign="top"><p> 237 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> 238 section. 239 </p></td></tr> 240</table></div> 241<p> 242 Some using declarations: 243 </p> 244<p> 245</p> 246<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">ascii</span><span class="special">::</span><span class="identifier">char_</span><span class="special">;</span> 247<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">expectation_failure</span><span class="special">;</span> 248</pre> 249<p> 250 </p> 251<p> 252 The code below uses an expectation operator to throw an <a class="link" href="expect.html#spirit.qi.reference.operator.expect.expectation_failure"><code class="computeroutput"><span class="identifier">expectation_failure</span></code></a> with a deliberate 253 parsing error when <code class="computeroutput"><span class="string">"o"</span></code> 254 is expected and <code class="computeroutput"><span class="string">"i"</span></code> 255 is what is found in the input. The <code class="computeroutput"><span class="keyword">catch</span></code> 256 block prints the information related to the error. Note: This is low 257 level code that demonstrates the <span class="emphasis"><em>bare-metal</em></span>. Typically, 258 you use an Error Handler to deal with the error. 259 </p> 260<p> 261</p> 262<pre class="programlisting"><span class="keyword">try</span> 263<span class="special">{</span> 264 <span class="identifier">test_parser</span><span class="special">(</span><span class="string">"xi"</span><span class="special">,</span> <span class="identifier">char_</span><span class="special">(</span><span class="char">'x'</span><span class="special">)</span> <span class="special">></span> <span class="identifier">char_</span><span class="special">(</span><span class="char">'o'</span><span class="special">));</span> <span class="comment">// should throw an exception</span> 265<span class="special">}</span> 266<span class="keyword">catch</span> <span class="special">(</span><span class="identifier">expectation_failure</span><span class="special"><</span><span class="keyword">char</span> <span class="keyword">const</span><span class="special">*></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">x</span><span class="special">)</span> 267<span class="special">{</span> 268 <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="string">"expected: "</span><span class="special">;</span> <span class="identifier">print_info</span><span class="special">(</span><span class="identifier">x</span><span class="special">.</span><span class="identifier">what_</span><span class="special">);</span> 269 <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="string">"got: \""</span> <span class="special"><<</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">(</span><span class="identifier">x</span><span class="special">.</span><span class="identifier">first</span><span class="special">,</span> <span class="identifier">x</span><span class="special">.</span><span class="identifier">last</span><span class="special">)</span> <span class="special"><<</span> <span class="char">'"'</span> <span class="special"><<</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span> 270<span class="special">}</span> 271</pre> 272<p> 273 </p> 274<p> 275 The code above will print: 276 </p> 277<pre class="programlisting">expected: tag: literal-char, value: o 278got: "i" 279</pre> 280</div> 281<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> 282<td align="left"></td> 283<td align="right"><div class="copyright-footer">Copyright © 2001-2011 Joel de Guzman, Hartmut Kaiser<p> 284 Distributed under the Boost Software License, Version 1.0. (See accompanying 285 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>) 286 </p> 287</div></td> 288</tr></table> 289<hr> 290<div class="spirit-nav"> 291<a accesskey="p" href="difference.html"><img src="../../../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../operator.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="kleene.html"><img src="../../../../../../../../doc/src/images/next.png" alt="Next"></a> 292</div> 293</body> 294</html> 295
