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<title>Expectation (a &gt; b)</title>
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<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 &gt; b)">Expectation
(<code class="computeroutput"><span class="identifier">a</span> <span class="special">&gt;</span>
<span class="identifier">b</span></code>)</a>
</h5></div></div></div>
<a name="spirit.qi.reference.operator.expect.description"></a><h6>
<a name="id924046"></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 &gt;&gt; b)">Sequence</a>,
the expectation operator, <code class="computeroutput"><span class="identifier">a</span>
<span class="special">&gt;</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">&gt;</span> <span class="identifier">b</span> <span class="special">&gt;</span> <span class="special">...</span>
</pre>
<p>
However, while the plain <a class="link" href="sequence.html" title="Sequence (a &gt;&gt; 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">&gt;</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">&lt;</span><span class="identifier">Iter</span><span class="special">&gt;</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="id928987"></a>
<a class="link" href="expect.html#spirit.qi.reference.operator.expect.header">Header</a>
</h6>
<pre class="programlisting"><span class="comment">// forwards to &lt;boost/spirit/home/qi/operator/expect.hpp&gt;
</span><span class="preprocessor">#include</span> <span class="special">&lt;</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">&gt;</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="id929062"></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="id929164"></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">&lt;</span><span class="identifier">Iter</span><span class="special">&gt;</span></code> is thrown:
</p>
<pre class="programlisting"><span class="keyword">template</span> <span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Iter</span><span class="special">&gt;</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="id929317"></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">&gt;</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">&lt;</span><span class="identifier">Iter</span><span class="special">&gt;</span></code>
</p>
</td>
</tr></tbody>
</table></div>
<a name="spirit.qi.reference.operator.expect.attributes"></a><h6>
<a name="id929462"></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">&gt;</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">--&gt;</span> <span class="special">(</span><span class="identifier">a</span> <span class="special">&gt;</span> <span class="identifier">b</span><span class="special">):</span> <span class="identifier">tuple</span><span class="special">&lt;</span><span class="identifier">A</span><span class="special">,</span> <span class="identifier">B</span><span class="special">&gt;</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">--&gt;</span> <span class="special">(</span><span class="identifier">a</span> <span class="special">&gt;</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">--&gt;</span> <span class="special">(</span><span class="identifier">a</span> <span class="special">&gt;</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">--&gt;</span> <span class="special">(</span><span class="identifier">a</span> <span class="special">&gt;</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">--&gt;</span> <span class="special">(</span><span class="identifier">a</span> <span class="special">&gt;</span> <span class="identifier">b</span><span class="special">):</span> <span class="identifier">vector</span><span class="special">&lt;</span><span class="identifier">A</span><span class="special">&gt;</span>
<span class="identifier">a</span><span class="special">:</span> <span class="identifier">vector</span><span class="special">&lt;</span><span class="identifier">A</span><span class="special">&gt;,</span> <span class="identifier">b</span><span class="special">:</span> <span class="identifier">A</span> <span class="special">--&gt;</span> <span class="special">(</span><span class="identifier">a</span> <span class="special">&gt;</span> <span class="identifier">b</span><span class="special">):</span> <span class="identifier">vector</span><span class="special">&lt;</span><span class="identifier">A</span><span class="special">&gt;</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">&lt;</span><span class="identifier">A</span><span class="special">&gt;</span> <span class="special">--&gt;</span> <span class="special">(</span><span class="identifier">a</span> <span class="special">&gt;</span> <span class="identifier">b</span><span class="special">):</span> <span class="identifier">vector</span><span class="special">&lt;</span><span class="identifier">A</span><span class="special">&gt;</span>
<span class="identifier">a</span><span class="special">:</span> <span class="identifier">vector</span><span class="special">&lt;</span><span class="identifier">A</span><span class="special">&gt;,</span> <span class="identifier">b</span><span class="special">:</span> <span class="identifier">vector</span><span class="special">&lt;</span><span class="identifier">A</span><span class="special">&gt;</span> <span class="special">--&gt;</span> <span class="special">(</span><span class="identifier">a</span> <span class="special">&gt;</span> <span class="identifier">b</span><span class="special">):</span> <span class="identifier">vector</span><span class="special">&lt;</span><span class="identifier">A</span><span class="special">&gt;</span></pre>
<p>
</p>
</td>
</tr></tbody>
</table></div>
<a name="spirit.qi.reference.operator.expect.complexity"></a><h6>
<a name="id930146"></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="id930167"></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">&gt;</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">&lt;</span><span class="keyword">char</span> <span class="keyword">const</span><span class="special">*&gt;</span> <span class="keyword">const</span><span class="special">&amp;</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">&lt;&lt;</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">&lt;&lt;</span> <span class="string">"got: \""</span> <span class="special">&lt;&lt;</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">&lt;&lt;</span> <span class="char">'"'</span> <span class="special">&lt;&lt;</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 &#169; 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>
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