| <html> |
| <head> |
| <title>Error Handling</title> |
| <meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1"> |
| <link rel="stylesheet" href="theme/style.css" type="text/css"> |
| </head> |
| |
| <body> |
| <table width="100%" border="0" background="theme/bkd2.gif" cellspacing="2"> |
| <tr> |
| <td width="10"> |
| </td> |
| <td width="85%"> <font size="6" face="Verdana, Arial, Helvetica, sans-serif"><b>Error |
| Handling </b></font></td> |
| <td width="112"><a href="http://spirit.sf.net"><img src="theme/spirit.gif" width="112" height="48" align="right" border="0"></a></td> |
| </tr> |
| </table> |
| <br> |
| <table border="0"> |
| <tr> |
| <td width="10"></td> |
| <td width="30"><a href="../index.html"><img src="theme/u_arr.gif" border="0"></a></td> |
| <td width="30"><a href="debugging.html"><img src="theme/l_arr.gif" border="0"></a></td> |
| <td width="30"><a href="quickref.html"><img src="theme/r_arr.gif" border="0"></a></td> |
| </tr> |
| </table> |
| <p>C++'s exception handling mechanism is a perfect match for error handling in |
| the framework. Imagine a complete parser as a maze. At each branch, the input |
| dictates where we will turn. Given an erroneous input, we may reach a dead end. |
| If we ever reach one, it would be a waste of time to backtrack from where we |
| came from. Instead, we supply guards in strategic points. Beyond a certain point, |
| we put put parser assertions in places where one is not allowed to go. </p> |
| <p>The assertions are like springs that catapult us back to the guard. If we ever |
| reach a brick wall given a specific input pattern, everything unwinds quickly |
| and we are thrown right back to the guard. This can be a very effective optimization |
| when used wisely. Right back at the guard, we have a chance to correct the situation, |
| if possible. The following illustration depicts the scenario.</p> |
| <table border="0" align="center"> |
| <tr> |
| <td><img src="theme/error_handling.png" width="313" height="238"></td> |
| </tr> |
| </table> |
| <a name="the_parser_exception"></a> |
| <h2>Parser Errors</h2> |
| <p> The <tt>parser_error</tt> class is the generic parser exception class used |
| by Spirit. This is the base class for all parser exceptions.</p> |
| <pre> <code><span class=keyword>template </span><span class=special><</span><span class=keyword>typename </span><span class=identifier>ErrorDescrT</span><span class=special>, </span><span class=keyword>typename </span><span class=identifier>IteratorT </span><span class=special>= </span><span class=keyword>char </span><span class=keyword>const</span><span class=special>*> |
| </span><span class=keyword>class </span><span class=identifier>parser_error </span><span class=special> |
| { |
| </span><span class=keyword>public</span><span class=special>: |
| </span><span class=identifier>parser_error</span><span class=special>(</span><span class=identifier>IteratorT </span><span class=identifier>where</span><span class=special>, </span><span class=identifier>ErrorDescrT </span><span class=identifier>descriptor</span><span class=special>); |
| </span><span class=identifier>IteratorT </span><span class=identifier>where</span><span class=special>; |
| </span><span class=identifier>ErrorDescrT</span><span class=identifier> descriptor</span><span class=special>; |
| </span><span class=special>}; |
| </span></code></pre> |
| <p> The exception holds the iterator position where the error was encountered |
| in its <tt>where</tt> member variable. In addition to the iterator, <tt>parser_error</tt> |
| also holds information regarding the error (error descriptor) in its <tt>descriptor |
| </tt> member variable.</p> |
| <p> Semantic actions are free to throw parser exceptions when necessary. A utility |
| function <tt>throw_</tt> may be called. This function creates and throws a <tt>parser_error</tt> |
| given an iterator and an error descriptor:</p> |
| <pre> |
| <code><span class=keyword>template </span><span class=special><</span><span class=keyword>typename </span><span class=identifier>ErrorDescrT</span><span class=special>, </span><span class=keyword>typename </span><span class=identifier>IteratorT</span><span class=special>> |
| </span><span class=keyword>void </span><span class=identifier>throw_</span><span class=special>(</span><span class=identifier>IteratorT where</span><span class=special>, </span><span class=identifier>ErrorDescrT descriptor</span><span class=special>); |
| </span></code></pre> |
| <a name="the_parser_assertion"></a> |
| <h2>Parser Assertions</h2> |
| <p> Assertions may be put in places where we don't have any other option other |
| than expect parsing to succeed. If parsing fails, a specific type of exception |
| is thrown.</p> |
| <p> Before declaring the grammar, we declare some assertion objects. <tt>assertion</tt> |
| is a template class parameterized by the type of error that will be thrown once |
| the assertion fails. The following assertions are parameterized by a user defined |
| Error enumeration.</p> |
| <a name="examples"></a> |
| <h3>Examples</h3> |
| <pre> |
| <code><span class=keyword>enum </span><span class=identifier>Errors |
| </span><span class=special>{ |
| </span><span class=identifier>program_expected</span><span class=special>, |
| </span><span class=identifier>begin_expected</span><span class=special>, |
| </span><span class=identifier>end_expected |
| </span><span class=special>}; |
| |
| </span><span class=identifier>assertion</span><span class=special><</span><span class=identifier>Errors</span><span class=special>> </span><span class=identifier>expect_program</span><span class=special>(</span><span class=identifier>program_expected</span><span class=special>); |
| </span><span class=identifier>assertion</span><span class=special><</span><span class=identifier>Errors</span><span class=special>> </span><span class=identifier>expect_begin</span><span class=special>(</span><span class=identifier>begin_expected</span><span class=special>); |
| </span><span class=identifier>assertion</span><span class=special><</span><span class=identifier>Errors</span><span class=special>> </span><span class=identifier>expect_end</span><span class=special>(</span><span class=identifier>end_expected</span><span class=special>); |
| </span></code></pre> |
| <p> The example above uses enums to hold the information regarding the error, |
| we are free to use other types such as integers and strings. For example, <tt>assertion<string></tt> |
| accepts a string as its info. It is advisable to use light-weight objects though, |
| after all, error descriptors are usually static. Enums are convenient for error |
| handlers to detect and easily catch since C++ treats enums as unique types.</p> |
| <table width="80%" border="0" align="center"> |
| <tr> |
| <td class="note_box"> <b><img src="theme/lens.gif" width="15" height="16"> |
| The assertive_parser</b><br> |
| <br> |
| Actually, the expression <tt>expect_end(str_p("end"))</tt>creates |
| an assertive_parser object. An assertive_parser is a parser that throws |
| an exception in response to a parsing failure. The assertive_parser throws |
| a parser_error exception rather than returning an unsuccessful match to |
| signal that the parser failed to match the input. During parsing, parsers |
| are given an iterator of type <tt>IteratorT</tt>. This is combined with |
| the error descriptor type <tt>ErrorDescrT</tt> of the assertion (in this |
| case enum <tt>Errors</tt>). Both are used to create a <tt>parser_error<Errors, |
| IteratorT></tt> which is then thrown to signal the exception. </td> |
| </tr> |
| </table> |
| <p> The predeclared <tt>expect_end</tt> assertion object may now be used in the |
| grammar as wrappers around parsers. For example:</p> |
| <pre> |
| <code><span class=identifier>expect_end</span><span class=special>(</span><span class=identifier>str_p</span><span class=special>(</span><span class=string>"end"</span><span class=special>)) |
| </span></code></pre> |
| <p> This will throw an exception if it fails to see "end" from the input.</p> |
| <a name="the_guard"></a> |
| <h2>The Guard</h2> |
| <p> The <tt>guard</tt> is used to catch a specific type of <tt>parser_error</tt>. |
| guards are typically predeclared just like assertions. Extending our previous |
| example:</p> |
| <pre> |
| <code><span class=identifier>guard</span><span class=special><</span><span class=identifier>Errors</span><span class=special>> </span><span class=identifier>my_guard</span><span class=special>; |
| </span></code></pre> |
| <p> <tt>Errors</tt>, in this example is the error descriptor type we want to detect. |
| This is the same enum as above. <tt>my_guard</tt> may now be used in a grammar |
| declaration:</p> |
| <pre> <code><span class=identifier>my_guard</span><span class=special>(</span><span class=identifier>p</span><span class=special>)[</span><span class=identifier>error_handler</span><span class=special>]</span></code></pre> |
| <p> where <tt>p</tt> is an expression that evaluates to a parser. Somewhere inside |
| <tt>p</tt>, a parser may throw a parser exception. <tt>error_handler</tt> is |
| the error handler which may be a function or functor compatible with the interface:</p> |
| <pre> <code>error_status<span class=special><</span>T<span class=special>></span><span class=identifier> |
| f</span><span class=special>(</span>ScannerT const& scan, ErrorT error<span class=special>); |
| </span></code></pre> |
| <p> Where scan points to the scanner state prior to parsing and error is the error |
| that arose. The handler is allowed to move the scanner position as it sees fit, |
| possibly in an attempt to perform error correction. The handler must then return |
| an <tt>error_status<T></tt> object. </p> |
| <table width="80%" border="0" align="center"> |
| <tr> |
| <td class="note_box"> <b><img src="theme/lens.gif" width="15" height="16"> |
| The fallback_parser </b><br> |
| <br> |
| The expression <tt>my_guard(expr, error_handler)</tt>creates a fallback_parser |
| object. The fallback_parser handles parser_error exceptions of a specific |
| type. Since <tt>my_guard</tt> is declared as <tt>guard<Errors></tt>, |
| the fallback_parser catches <tt>Errors</tt> specific parser errors: <tt>parser_error<Errors, |
| IteratorT></tt>. The class sets up a try block. When an exception is |
| caught, the catch block then calls the error_handler. </td> |
| </tr> |
| </table> |
| <h2>error_status<T></h2> |
| <pre> |
| <code><span class=keyword>template </span><span class=special><</span><span class=keyword>typename </span><span class=identifier>T </span><span class=special>= </span><span class=identifier>nil_t</span><span class=special>> |
| </span><span class=keyword>struct </span><span class=identifier>error_status |
| </span><span class=special>{ |
| </span><span class=keyword>enum </span><span class=identifier>result_t </span><span class=special>{ </span><span class=identifier>fail</span><span class=special>, </span><span class=identifier>retry</span><span class=special>, </span><span class=identifier>accept</span><span class=special>, </span><span class=identifier>rethrow </span><span class=special>}; |
| |
| </span><span class=identifier>error_status</span><span class=special>(</span><span class=identifier> |
| result_t result </span><span class=special>= </span><span class=identifier>fail</span><span class=special>, |
| </span><span class=keyword>int </span><span class=identifier>length </span><span class=special>= -</span><span class=number>1</span><span class=special>, |
| </span><span class=identifier>T </span><span class=keyword>const</span><span class=special>& </span><span class=identifier>value </span><span class=special>= </span><span class=identifier>T</span><span class=special>()); |
| </span> |
| <span class=identifier>result_t result</span><span class=special>; |
| </span><span class=keyword>int </span><span class=identifier>length</span><span class=special>; |
| </span><span class=identifier>T value</span><span class=special>; |
| };</span></code></pre> |
| <p>Where <tt>T</tt> is an attribute type compatible with the match attribute of |
| the <tt>fallback_parser</tt>'s subject (defaults to <tt>nil_t</tt>). The class |
| <tt>error_status</tt> reports the result of an error handler. This result can |
| be one of: </p> |
| <table width="90%" border="0" align="center"> |
| <tr> |
| <td class="table_title" colspan="8"> error_status result </td> |
| </tr> |
| <tr> |
| <tr> |
| <td class="table_cells"><b>fail</b></td> |
| <td class="table_cells">quit and fail. Return a <tt>no_match</tt></td> |
| </tr> |
| <td class="table_cells"><b>retry</b></td> |
| <td class="table_cells">attempt error recovery, possibly moving the scanner</td> |
| </tr> |
| <td class="table_cells"><b>accept</b></td> |
| <td class="table_cells">force success returning a matching length, moving the |
| scanner appropriately and returning an attribute value</td> |
| </tr> |
| <td class="table_cells"><b>rethrow</b></td> |
| <td class="table_cells">rethrows the error</td> |
| </tr> |
| </table> |
| <p><img src="theme/lens.gif" width="15" height="16"> See <a href="../example/fundamental/error_handling.cpp">error_handling.cpp</a> for a compilable example. This is part of the Spirit distribution.</p> |
| <table width="80%" border="0" align="center"> |
| </table> |
| <table border="0"> |
| <tr> |
| <td width="10"></td> |
| <td width="30"><a href="../index.html"><img src="theme/u_arr.gif" border="0"></a></td> |
| <td width="30"><a href="debugging.html"><img src="theme/l_arr.gif" border="0"></a></td> |
| <td width="30"><a href="quickref.html"><img src="theme/r_arr.gif" border="0"></a></td> |
| </tr> |
| </table> |
| <br> |
| <hr size="1"> |
| <p class="copyright">Copyright © 1998-2003 Joel de Guzman<br> |
| <br> |
| <font size="2">Use, modification and distribution is subject to the Boost Software |
| License, Version 1.0. (See accompanying file LICENSE_1_0.txt or copy at |
| http://www.boost.org/LICENSE_1_0.txt)</font></p> |
| <p class="copyright"> </p> |
| </body> |
| </html> |
