<html> <head> <meta http-equiv="Content-Type" content="text/html; charset=US-ASCII"> <title>Expectation (a > b)</title> <link rel="stylesheet" href="../../../../../../../../doc/src/boostbook.css" type="text/css"> <meta name="generator" content="DocBook XSL Stylesheets V1.75.0"> <link rel="home" href="../../../../index.html" title="Spirit 2.4"> <link rel="up" href="../operator.html" title="Operator"> <link rel="prev" href="difference.html" title="Difference (a - b)"> <link rel="next" href="kleene.html" title="Kleene (*a)"> </head> <body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"> <table cellpadding="2" width="100%"><tr> <td valign="top"><img alt="Boost C++ Libraries" width="277" height="86" src="../../../../../../../../boost.png"></td> <td align="center"><a href="../../../../../../../../index.html">Home</a></td> <td align="center"><a href="../../../../../../../../libs/libraries.htm">Libraries</a></td> <td align="center"><a href="http://www.boost.org/users/people.html">People</a></td> <td align="center"><a href="http://www.boost.org/users/faq.html">FAQ</a></td> <td align="center"><a href="../../../../../../../../more/index.htm">More</a></td> </tr></table> <hr> <div class="spirit-nav"> <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> </div> <div class="section"> <div class="titlepage"><div><div><h5 class="title"> <a name="spirit.qi.reference.operator.expect"></a><a class="link" href="expect.html" title="Expectation (a > b)"> Expectation (<code class="computeroutput"><span class="identifier">a</span> <span class="special">></span> <span class="identifier">b</span></code>)</a> </h5></div></div></div> <a name="spirit.qi.reference.operator.expect.description"></a><h6> <a name="id972910"></a> <a class="link" href="expect.html#spirit.qi.reference.operator.expect.description">Description</a> </h6> <p> Like the <a class="link" href="sequence.html" title="Sequence (a >> b)">Sequence</a>, the expectation operator, <code class="computeroutput"><span class="identifier">a</span> <span class="special">></span> <span class="identifier">b</span></code>, parses two or more operands (<code class="computeroutput"><span class="identifier">a</span></code>, <code class="computeroutput"><span class="identifier">b</span></code>, ... etc.), in sequence: </p> <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> </pre> <p> However, while the plain <a class="link" href="sequence.html" title="Sequence (a >> b)">Sequence</a> simply returns a no-match (returns <code class="computeroutput"><span class="keyword">false</span></code>) when one of the elements fail, the expectation: <code class="computeroutput"><span class="special">></span></code> 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 (all operands except the first) fail to match. </p> <a name="spirit.qi.reference.operator.expect.header"></a><h6> <a name="id973037"></a> <a class="link" href="expect.html#spirit.qi.reference.operator.expect.header">Header</a> </h6> <pre class="programlisting"><span class="comment">// forwards to <boost/spirit/home/qi/operator/expect.hpp> </span><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> </pre> <p> Also, see <a class="link" href="../../../structure/include.html" title="Include">Include Structure</a>. </p> <a name="spirit.qi.reference.operator.expect.model_of"></a><h6> <a name="id973112"></a> <a class="link" href="expect.html#spirit.qi.reference.operator.expect.model_of">Model of</a> </h6> <div class="blockquote"><blockquote class="blockquote"><p> <a class="link" href="../parser_concepts/naryparser.html" title="NaryParser"><code class="computeroutput"><span class="identifier">NaryParser</span></code></a> </p></blockquote></div> <div class="variablelist"> <p class="title"><b>Notation</b></p> <dl> <dt><span class="term"><code class="computeroutput"><span class="identifier">a</span></code>, <code class="computeroutput"><span class="identifier">b</span></code></span></dt> <dd><p> A <a class="link" href="../parser_concepts/parser.html" title="Parser"><code class="computeroutput"><span class="identifier">Parser</span></code></a> </p></dd> <dt><span class="term"><code class="computeroutput"><span class="identifier">Iter</span></code></span></dt> <dd><p> A <a href="http://www.sgi.com/tech/stl/ForwardIterator.html" target="_top"><code class="computeroutput"><span class="identifier">ForwardIterator</span></code></a> type </p></dd> </dl> </div> <a name="spirit.qi.reference.operator.expect.expectation_failure"></a><h6> <a name="id973214"></a> <a class="link" href="expect.html#spirit.qi.reference.operator.expect.expectation_failure">Expectation Failure</a> </h6> <p> 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: </p> <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> <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> <span class="special">{</span> <span class="identifier">Iter</span> <span class="identifier">first</span><span class="special">;</span> <span class="comment">// [first, last) iterator pointing </span> <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> <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><span class="special">};</span> </pre> <a name="spirit.qi.reference.operator.expect.expression_semantics"></a><h6> <a name="id973367"></a> <a class="link" href="expect.html#spirit.qi.reference.operator.expect.expression_semantics">Expression Semantics</a> </h6> <p> Semantics of an expression is defined only where it differs from, or is not defined in <a class="link" href="../parser_concepts/naryparser.html" title="NaryParser"><code class="computeroutput"><span class="identifier">NaryParser</span></code></a>. </p> <div class="informaltable"><table class="table"> <colgroup> <col> <col> </colgroup> <thead><tr> <th> <p> Expression </p> </th> <th> <p> Semantics </p> </th> </tr></thead> <tbody><tr> <td> <p> <code class="computeroutput"><span class="identifier">a</span> <span class="special">></span> <span class="identifier">b</span></code> </p> </td> <td> <p> Match <code class="computeroutput"><span class="identifier">a</span></code> followed 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> </p> </td> </tr></tbody> </table></div> <a name="spirit.qi.reference.operator.expect.attributes"></a><h6> <a name="id973512"></a> <a class="link" href="expect.html#spirit.qi.reference.operator.expect.attributes">Attributes</a> </h6> <p> See <a class="link" href="../../quick_reference/compound_attribute_rules.html#spirit.qi.quick_reference.compound_attribute_rules.notation">Compound Attribute Notation</a>. </p> <div class="informaltable"><table class="table"> <colgroup> <col> <col> </colgroup> <thead><tr> <th> <p> Expression </p> </th> <th> <p> Attribute </p> </th> </tr></thead> <tbody><tr> <td> <p> <code class="computeroutput"><span class="identifier">a</span> <span class="special">></span> <span class="identifier">b</span></code> </p> </td> <td> <p> </p> <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> <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> <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> <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> <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> <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> <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> <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> <p> </p> </td> </tr></tbody> </table></div> <a name="spirit.qi.reference.operator.expect.complexity"></a><h6> <a name="id974196"></a> <a class="link" href="expect.html#spirit.qi.reference.operator.expect.complexity">Complexity</a> </h6> <div class="blockquote"><blockquote class="blockquote"><p> The overall complexity of the expectation parser is defined by the sum of the complexities of its elements. The complexity of the expectation operator itself is O(N), where N is the number of elements in the sequence. </p></blockquote></div> <a name="spirit.qi.reference.operator.expect.example"></a><h6> <a name="id974217"></a> <a class="link" href="expect.html#spirit.qi.reference.operator.expect.example">Example</a> </h6> <div class="note"><table border="0" summary="Note"> <tr> <td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../../images/note.png"></td> <th align="left">Note</th> </tr> <tr><td align="left" valign="top"><p> 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> section. </p></td></tr> </table></div> <p> Some using declarations: </p> <p> </p> <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> <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> </pre> <p> </p> <p> 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 parsing error when <code class="computeroutput"><span class="string">"o"</span></code> is expected and <code class="computeroutput"><span class="string">"i"</span></code> is what is found in the input. The <code class="computeroutput"><span class="keyword">catch</span></code> block prints the information related to the error. Note: This is low level code that demonstrates the <span class="emphasis"><em>bare-metal</em></span>. Typically, you use an Error Handler to deal with the error. </p> <p> </p> <pre class="programlisting"><span class="keyword">try</span> <span class="special">{</span> <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><span class="special">}</span> <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> <span class="special">{</span> <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> <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> <span class="special">}</span></pre> <p> </p> <p> The code above will print: </p> <pre class="programlisting">expected: tag: literal-char, value: o got: "i" </pre> </div> <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> <td align="left"></td> <td align="right"><div class="copyright-footer">Copyright © 2001-2010 Joel de Guzman, Hartmut Kaiser<p> Distributed under the Boost Software License, Version 1.0. (See accompanying 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>) </p> </div></td> </tr></table> <hr> <div class="spirit-nav"> <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> </div> </body> </html>