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<html>
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<title>Mini XML - Error Handling</title>
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<div class="section">
<div class="titlepage"><div><div><h4 class="title">
<a name="spirit.qi.tutorials.mini_xml___error_handling"></a><a class="link" href="mini_xml___error_handling.html" title="Mini XML - Error Handling">Mini
XML - Error Handling</a>
</h4></div></div></div>
<p>
A parser will not be complete without error handling. Spirit2 provides
some facilities to make it easy to adapt a grammar for error handling.
We'll wrap up the Qi tutorial with another version of the mini xml parser,
this time, with error handling.
</p>
<p>
The full cpp file for this example can be found here: <a href="../../../../../example/qi/mini_xml3.cpp" target="_top">../../example/qi/mini_xml3.cpp</a>
</p>
<p>
Here's the grammar:
</p>
<p>
</p>
<pre class="programlisting"><span class="keyword">template</span> <span class="special"><</span><span class="keyword">typename</span> <span class="identifier">Iterator</span><span class="special">></span>
<span class="keyword">struct</span> <span class="identifier">mini_xml_grammar</span>
<span class="special">:</span> <span class="identifier">qi</span><span class="special">::</span><span class="identifier">grammar</span><span class="special"><</span><span class="identifier">Iterator</span><span class="special">,</span> <span class="identifier">mini_xml</span><span class="special">(),</span> <span class="identifier">qi</span><span class="special">::</span><span class="identifier">locals</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">ascii</span><span class="special">::</span><span class="identifier">space_type</span><span class="special">></span>
<span class="special">{</span>
<span class="identifier">mini_xml_grammar</span><span class="special">()</span>
<span class="special">:</span> <span class="identifier">mini_xml_grammar</span><span class="special">::</span><span class="identifier">base_type</span><span class="special">(</span><span class="identifier">xml</span><span class="special">,</span> <span class="string">"xml"</span><span class="special">)</span>
<span class="special">{</span>
<span class="keyword">using</span> <span class="identifier">qi</span><span class="special">::</span><span class="identifier">lit</span><span class="special">;</span>
<span class="keyword">using</span> <span class="identifier">qi</span><span class="special">::</span><span class="identifier">lexeme</span><span class="special">;</span>
<span class="keyword">using</span> <span class="identifier">qi</span><span class="special">::</span><span class="identifier">on_error</span><span class="special">;</span>
<span class="keyword">using</span> <span class="identifier">qi</span><span class="special">::</span><span class="identifier">fail</span><span class="special">;</span>
<span class="keyword">using</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">ascii</span><span class="special">::</span><span class="identifier">string</span><span class="special">;</span>
<span class="keyword">using</span> <span class="keyword">namespace</span> <span class="identifier">qi</span><span class="special">::</span><span class="identifier">labels</span><span class="special">;</span>
<span class="keyword">using</span> <span class="identifier">phoenix</span><span class="special">::</span><span class="identifier">construct</span><span class="special">;</span>
<span class="keyword">using</span> <span class="identifier">phoenix</span><span class="special">::</span><span class="identifier">val</span><span class="special">;</span>
<span class="identifier">text</span> <span class="special">%=</span> <span class="identifier">lexeme</span><span class="special">[+(</span><span class="identifier">char_</span> <span class="special">-</span> <span class="char">'<'</span><span class="special">)];</span>
<span class="identifier">node</span> <span class="special">%=</span> <span class="identifier">xml</span> <span class="special">|</span> <span class="identifier">text</span><span class="special">;</span>
<span class="identifier">start_tag</span> <span class="special">%=</span>
<span class="char">'<'</span>
<span class="special">>></span> <span class="special">!</span><span class="identifier">lit</span><span class="special">(</span><span class="char">'/'</span><span class="special">)</span>
<span class="special">></span> <span class="identifier">lexeme</span><span class="special">[+(</span><span class="identifier">char_</span> <span class="special">-</span> <span class="char">'>'</span><span class="special">)]</span>
<span class="special">></span> <span class="char">'>'</span>
<span class="special">;</span>
<span class="identifier">end_tag</span> <span class="special">=</span>
<span class="string">"</"</span>
<span class="special">></span> <span class="identifier">string</span><span class="special">(</span><span class="identifier">_r1</span><span class="special">)</span>
<span class="special">></span> <span class="char">'>'</span>
<span class="special">;</span>
<span class="identifier">xml</span> <span class="special">%=</span>
<span class="identifier">start_tag</span><span class="special">[</span><span class="identifier">_a</span> <span class="special">=</span> <span class="identifier">_1</span><span class="special">]</span>
<span class="special">></span> <span class="special">*</span><span class="identifier">node</span>
<span class="special">></span> <span class="identifier">end_tag</span><span class="special">(</span><span class="identifier">_a</span><span class="special">)</span>
<span class="special">;</span>
<span class="identifier">xml</span><span class="special">.</span><span class="identifier">name</span><span class="special">(</span><span class="string">"xml"</span><span class="special">);</span>
<span class="identifier">node</span><span class="special">.</span><span class="identifier">name</span><span class="special">(</span><span class="string">"node"</span><span class="special">);</span>
<span class="identifier">text</span><span class="special">.</span><span class="identifier">name</span><span class="special">(</span><span class="string">"text"</span><span class="special">);</span>
<span class="identifier">start_tag</span><span class="special">.</span><span class="identifier">name</span><span class="special">(</span><span class="string">"start_tag"</span><span class="special">);</span>
<span class="identifier">end_tag</span><span class="special">.</span><span class="identifier">name</span><span class="special">(</span><span class="string">"end_tag"</span><span class="special">);</span>
<span class="identifier">on_error</span><span class="special"><</span><span class="identifier">fail</span><span class="special">></span>
<span class="special">(</span>
<span class="identifier">xml</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="identifier">val</span><span class="special">(</span><span class="string">"Error! Expecting "</span><span class="special">)</span>
<span class="special"><<</span> <span class="identifier">_4</span> <span class="comment">// what failed?</span>
<span class="special"><<</span> <span class="identifier">val</span><span class="special">(</span><span class="string">" here: \""</span><span class="special">)</span>
<span class="special"><<</span> <span class="identifier">construct</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">_3</span><span class="special">,</span> <span class="identifier">_2</span><span class="special">)</span> <span class="comment">// iterators to error-pos, end</span>
<span class="special"><<</span> <span class="identifier">val</span><span class="special">(</span><span class="string">"\""</span><span class="special">)</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>
<span class="identifier">qi</span><span class="special">::</span><span class="identifier">rule</span><span class="special"><</span><span class="identifier">Iterator</span><span class="special">,</span> <span class="identifier">mini_xml</span><span class="special">(),</span> <span class="identifier">qi</span><span class="special">::</span><span class="identifier">locals</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">ascii</span><span class="special">::</span><span class="identifier">space_type</span><span class="special">></span> <span class="identifier">xml</span><span class="special">;</span>
<span class="identifier">qi</span><span class="special">::</span><span class="identifier">rule</span><span class="special"><</span><span class="identifier">Iterator</span><span class="special">,</span> <span class="identifier">mini_xml_node</span><span class="special">(),</span> <span class="identifier">ascii</span><span class="special">::</span><span class="identifier">space_type</span><span class="special">></span> <span class="identifier">node</span><span class="special">;</span>
<span class="identifier">qi</span><span class="special">::</span><span class="identifier">rule</span><span class="special"><</span><span class="identifier">Iterator</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">ascii</span><span class="special">::</span><span class="identifier">space_type</span><span class="special">></span> <span class="identifier">text</span><span class="special">;</span>
<span class="identifier">qi</span><span class="special">::</span><span class="identifier">rule</span><span class="special"><</span><span class="identifier">Iterator</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">ascii</span><span class="special">::</span><span class="identifier">space_type</span><span class="special">></span> <span class="identifier">start_tag</span><span class="special">;</span>
<span class="identifier">qi</span><span class="special">::</span><span class="identifier">rule</span><span class="special"><</span><span class="identifier">Iterator</span><span class="special">,</span> <span class="keyword">void</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">ascii</span><span class="special">::</span><span class="identifier">space_type</span><span class="special">></span> <span class="identifier">end_tag</span><span class="special">;</span>
<span class="special">};</span>
</pre>
<p>
</p>
<p>
What's new?
</p>
<h6>
<a name="spirit.qi.tutorials.mini_xml___error_handling.h0"></a>
<span><a name="spirit.qi.tutorials.mini_xml___error_handling.readable_names"></a></span><a class="link" href="mini_xml___error_handling.html#spirit.qi.tutorials.mini_xml___error_handling.readable_names">Readable
Names</a>
</h6>
<p>
First, when we call the base class, we give the grammar a name:
</p>
<pre class="programlisting"><span class="special">:</span> <span class="identifier">mini_xml_grammar</span><span class="special">::</span><span class="identifier">base_type</span><span class="special">(</span><span class="identifier">xml</span><span class="special">,</span> <span class="string">"xml"</span><span class="special">)</span>
</pre>
<p>
Then, we name all our rules:
</p>
<pre class="programlisting"><span class="identifier">xml</span><span class="special">.</span><span class="identifier">name</span><span class="special">(</span><span class="string">"xml"</span><span class="special">);</span>
<span class="identifier">node</span><span class="special">.</span><span class="identifier">name</span><span class="special">(</span><span class="string">"node"</span><span class="special">);</span>
<span class="identifier">text</span><span class="special">.</span><span class="identifier">name</span><span class="special">(</span><span class="string">"text"</span><span class="special">);</span>
<span class="identifier">start_tag</span><span class="special">.</span><span class="identifier">name</span><span class="special">(</span><span class="string">"start_tag"</span><span class="special">);</span>
<span class="identifier">end_tag</span><span class="special">.</span><span class="identifier">name</span><span class="special">(</span><span class="string">"end_tag"</span><span class="special">);</span>
</pre>
<h6>
<a name="spirit.qi.tutorials.mini_xml___error_handling.h1"></a>
<span><a name="spirit.qi.tutorials.mini_xml___error_handling.on_error"></a></span><a class="link" href="mini_xml___error_handling.html#spirit.qi.tutorials.mini_xml___error_handling.on_error">On Error</a>
</h6>
<p>
<code class="computeroutput"><span class="identifier">on_error</span></code> declares our error
handler:
</p>
<pre class="programlisting"><span class="identifier">on_error</span><span class="special"><</span><span class="identifier">Action</span><span class="special">>(</span><span class="identifier">rule</span><span class="special">,</span> <span class="identifier">handler</span><span class="special">)</span>
</pre>
<p>
This will specify what we will do when we get an error. We will print out
an error message using phoenix:
</p>
<pre class="programlisting"><span class="identifier">on_error</span><span class="special"><</span><span class="identifier">fail</span><span class="special">></span>
<span class="special">(</span>
<span class="identifier">xml</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="identifier">val</span><span class="special">(</span><span class="string">"Error! Expecting "</span><span class="special">)</span>
<span class="special"><<</span> <span class="identifier">_4</span> <span class="comment">// what failed?</span>
<span class="special"><<</span> <span class="identifier">val</span><span class="special">(</span><span class="string">" here: \""</span><span class="special">)</span>
<span class="special"><<</span> <span class="identifier">construct</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">_3</span><span class="special">,</span> <span class="identifier">_2</span><span class="special">)</span> <span class="comment">// iterators to error-pos, end</span>
<span class="special"><<</span> <span class="identifier">val</span><span class="special">(</span><span class="string">"\""</span><span class="special">)</span>
<span class="special"><<</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span>
<span class="special">);</span>
</pre>
<p>
we choose to <code class="computeroutput"><span class="identifier">fail</span></code> in our
example for the <code class="computeroutput"><span class="identifier">Action</span></code>:
Quit and fail. Return a no_match (false). It can be one of:
</p>
<div class="informaltable"><table class="table">
<colgroup>
<col>
<col>
</colgroup>
<thead><tr>
<th>
<p>
<code class="computeroutput"><span class="identifier">Action</span></code>
</p>
</th>
<th>
<p>
Description
</p>
</th>
</tr></thead>
<tbody>
<tr>
<td>
<p>
fail
</p>
</td>
<td>
<p>
Quit and fail. Return a no_match.
</p>
</td>
</tr>
<tr>
<td>
<p>
retry
</p>
</td>
<td>
<p>
Attempt error recovery, possibly moving the iterator position.
</p>
</td>
</tr>
<tr>
<td>
<p>
accept
</p>
</td>
<td>
<p>
Force success, moving the iterator position appropriately.
</p>
</td>
</tr>
<tr>
<td>
<p>
rethrow
</p>
</td>
<td>
<p>
Rethrows the error.
</p>
</td>
</tr>
</tbody>
</table></div>
<p>
<code class="computeroutput"><span class="identifier">rule</span></code> is the rule to which
the handler is attached. In our case, we are attaching to the <code class="computeroutput"><span class="identifier">xml</span></code> rule.
</p>
<p>
<code class="computeroutput"><span class="identifier">handler</span></code> is the actual error
handling function. It expects 4 arguments:
</p>
<div class="informaltable"><table class="table">
<colgroup>
<col>
<col>
</colgroup>
<thead><tr>
<th>
<p>
Arg
</p>
</th>
<th>
<p>
Description
</p>
</th>
</tr></thead>
<tbody>
<tr>
<td>
<p>
first
</p>
</td>
<td>
<p>
The position of the iterator when the rule with the handler was
entered.
</p>
</td>
</tr>
<tr>
<td>
<p>
last
</p>
</td>
<td>
<p>
The end of input.
</p>
</td>
</tr>
<tr>
<td>
<p>
error-pos
</p>
</td>
<td>
<p>
The actual position of the iterator where the error occured.
</p>
</td>
</tr>
<tr>
<td>
<p>
what
</p>
</td>
<td>
<p>
What failed: a string describing the failure.
</p>
</td>
</tr>
</tbody>
</table></div>
<h6>
<a name="spirit.qi.tutorials.mini_xml___error_handling.h2"></a>
<span><a name="spirit.qi.tutorials.mini_xml___error_handling.expectation_points"></a></span><a class="link" href="mini_xml___error_handling.html#spirit.qi.tutorials.mini_xml___error_handling.expectation_points">Expectation
Points</a>
</h6>
<p>
You might not have noticed it, but some of our expressions changed from
using the <code class="computeroutput"><span class="special">>></span></code> to <code class="computeroutput"><span class="special">></span></code>. Look, for example:
</p>
<pre class="programlisting"><span class="identifier">end_tag</span> <span class="special">=</span>
<span class="string">"</"</span>
<span class="special">></span> <span class="identifier">lit</span><span class="special">(</span><span class="identifier">_r1</span><span class="special">)</span>
<span class="special">></span> <span class="char">'>'</span>
<span class="special">;</span>
</pre>
<p>
What is it? It's the <span class="emphasis"><em>expectation</em></span> operator. You will
have some "deterministic points" in the grammar. Those are the
places where backtracking <span class="bold"><strong>cannot</strong></span> occur.
For our example above, when you get a <code class="computeroutput"><span class="string">"</"</span></code>,
you definitely must see a valid end-tag label next. It should be the one
you got from the start-tag. After that, you definitely must have a <code class="computeroutput"><span class="char">'>'</span></code> next. Otherwise, there is no point in
proceeding and trying other branches, regardless where they are. The input
is definitely erroneous. When this happens, an expectation_failure exception
is thrown. Somewhere outward, the error handler will catch the exception.
</p>
<p>
Try building the parser: <a href="../../../../../example/qi/mini_xml3.cpp" target="_top">../../example/qi/mini_xml3.cpp</a>.
You can find some examples in: <a href="../../../../../example/qi/mini_xml_samples" target="_top">../../example/qi/mini_xml_samples</a>
for testing purposes. "4.toyxml" has an error in it:
</p>
<pre class="programlisting"><span class="special"><</span><span class="identifier">foo</span><span class="special">><</span><span class="identifier">bar</span><span class="special">></</span><span class="identifier">foo</span><span class="special">></</span><span class="identifier">bar</span><span class="special">></span>
</pre>
<p>
Running the example with this gives you:
</p>
<pre class="programlisting"><span class="identifier">Error</span><span class="special">!</span> <span class="identifier">Expecting</span> <span class="string">"bar"</span> <span class="identifier">here</span><span class="special">:</span> <span class="string">"foo></bar>"</span>
<span class="identifier">Error</span><span class="special">!</span> <span class="identifier">Expecting</span> <span class="identifier">end_tag</span> <span class="identifier">here</span><span class="special">:</span> <span class="string">"<bar></foo></bar>"</span>
<span class="special">-------------------------</span>
<span class="identifier">Parsing</span> <span class="identifier">failed</span>
<span class="special">-------------------------</span>
</pre>
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Distributed under the Boost Software License, Version 1.0. (See accompanying
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