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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> |
| <a name="spirit.qi.tutorials.mini_xml___error_handling.readable_names"></a><h6> |
| <a name="id820933"></a> |
| <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> |
| <a name="spirit.qi.tutorials.mini_xml___error_handling.on_error"></a><h6> |
| <a name="id821666"></a> |
| <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 occurred. |
| </p> |
| </td> |
| </tr> |
| <tr> |
| <td> |
| <p> |
| what |
| </p> |
| </td> |
| <td> |
| <p> |
| What failed: a string describing the failure. |
| </p> |
| </td> |
| </tr> |
| </tbody> |
| </table></div> |
| <a name="spirit.qi.tutorials.mini_xml___error_handling.expectation_points"></a><h6> |
| <a name="id822189"></a> |
| <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> |
| </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> |
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