<html> <head> <meta http-equiv="Content-Type" content="text/html; charset=US-ASCII"> <title>Mini XML - Error Handling</title> <link rel="stylesheet" href="../../../../../../../doc/html/boostbook.css" type="text/css"> <meta name="generator" content="DocBook XSL Stylesheets V1.75.0"> <link rel="home" href="../../../index.html" title="Spirit 2.2"> <link rel="up" href="../tutorials.html" title="Tutorials"> <link rel="prev" href="mini_xml___asts_.html" title="Mini XML - ASTs!"> <link rel="next" href="../quick_reference.html" title="Quick Reference"> </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="../../../../../../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="mini_xml___asts_.html"><img src="../../../../../../../doc/html/images/prev.png" alt="Prev"></a><a accesskey="u" href="../tutorials.html"><img src="../../../../../../../doc/html/images/up.png" alt="Up"></a><a accesskey="h" href="../../../index.html"><img src="../../../../../../../doc/html/images/home.png" alt="Home"></a><a accesskey="n" href="../quick_reference.html"><img src="../../../../../../../doc/html/images/next.png" alt="Next"></a> </div> <div class="section" title="Mini XML - Error Handling"> <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> <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> </p> <p> What's new? </p> <a name="spirit.qi.tutorials.mini_xml___error_handling.readable_names"></a><h6> <a name="id643850"></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="id644030"></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 we attach the handler to. 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 decribing the failure. </p> </td> </tr> </tbody> </table></div> <a name="spirit.qi.tutorials.mini_xml___error_handling.expectation_points"></a><h6> <a name="id644538"></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 forward 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> <hr> <div class="spirit-nav"> <a accesskey="p" href="mini_xml___asts_.html"><img src="../../../../../../../doc/html/images/prev.png" alt="Prev"></a><a accesskey="u" href="../tutorials.html"><img src="../../../../../../../doc/html/images/up.png" alt="Up"></a><a accesskey="h" href="../../../index.html"><img src="../../../../../../../doc/html/images/home.png" alt="Home"></a><a accesskey="n" href="../quick_reference.html"><img src="../../../../../../../doc/html/images/next.png" alt="Next"></a> </div> </body> </html>