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| <div class="section"> |
| <div class="titlepage"><div><div><h4 class="title"> |
| <a name="spirit.qi.tutorials.warming_up"></a><a class="link" href="warming_up.html" title="Warming up">Warming up</a> |
| </h4></div></div></div> |
| <p> |
| We'll start by showing examples of parser expressions to give you a feel |
| on how to build parsers from the simplest parser, building up as we go. |
| When comparing EBNF to <a href="http://boost-spirit.com" target="_top">Spirit</a>, |
| the expressions may seem awkward at first. <a href="http://boost-spirit.com" target="_top">Spirit</a> |
| heavily uses operator overloading to accomplish its magic. |
| </p> |
| <a name="spirit.qi.tutorials.warming_up.trivial_example__1_parsing_a_number"></a><h6> |
| <a name="id798191"></a> |
| <a class="link" href="warming_up.html#spirit.qi.tutorials.warming_up.trivial_example__1_parsing_a_number">Trivial |
| Example #1 Parsing a number</a> |
| </h6> |
| <p> |
| Create a parser that will parse a floating-point number. |
| </p> |
| <pre class="programlisting"><span class="identifier">double_</span> |
| </pre> |
| <p> |
| (You've got to admit, that's trivial!) The above code actually generates |
| a Spirit floating point parser (a built-in parser). Spirit has many pre-defined |
| parsers and consistent naming conventions help you keep from going insane! |
| </p> |
| <a name="spirit.qi.tutorials.warming_up.trivial_example__2_parsing_two_numbers"></a><h6> |
| <a name="id798220"></a> |
| <a class="link" href="warming_up.html#spirit.qi.tutorials.warming_up.trivial_example__2_parsing_two_numbers">Trivial |
| Example #2 Parsing two numbers</a> |
| </h6> |
| <p> |
| Create a parser that will accept a line consisting of two floating-point |
| numbers. |
| </p> |
| <pre class="programlisting"><span class="identifier">double_</span> <span class="special">>></span> <span class="identifier">double_</span> |
| </pre> |
| <p> |
| Here you see the familiar floating-point numeric parser <code class="computeroutput"><span class="identifier">double_</span></code> |
| used twice, once for each number. What's that <code class="computeroutput"><span class="special">>></span></code> |
| operator doing in there? Well, they had to be separated by something, and |
| this was chosen as the "followed by" sequence operator. The above |
| program creates a parser from two simpler parsers, glueing them together |
| with the sequence operator. The result is a parser that is a composition |
| of smaller parsers. Whitespace between numbers can implicitly be consumed |
| depending on how the parser is invoked (see below). |
| </p> |
| <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> |
| When we combine parsers, we end up with a "bigger" parser, |
| but it's still a parser. Parsers can get bigger and bigger, nesting more |
| and more, but whenever you glue two parsers together, you end up with |
| one bigger parser. This is an important concept. |
| </p></td></tr> |
| </table></div> |
| <a name="spirit.qi.tutorials.warming_up.trivial_example__3_parsing_zero_or_more_numbers"></a><h6> |
| <a name="id798281"></a> |
| <a class="link" href="warming_up.html#spirit.qi.tutorials.warming_up.trivial_example__3_parsing_zero_or_more_numbers">Trivial |
| Example #3 Parsing zero or more numbers</a> |
| </h6> |
| <p> |
| Create a parser that will accept zero or more floating-point numbers. |
| </p> |
| <pre class="programlisting"><span class="special">*</span><span class="identifier">double_</span> |
| </pre> |
| <p> |
| This is like a regular-expression Kleene Star, though the syntax might |
| look a bit odd for a C++ programmer not used to seeing the <code class="computeroutput"><span class="special">*</span></code> operator overloaded like this. Actually, |
| if you know regular expressions it may look odd too since the star is before |
| the expression it modifies. C'est la vie. Blame it on the fact that we |
| must work with the syntax rules of C++. |
| </p> |
| <p> |
| Any expression that evaluates to a parser may be used with the Kleene Star. |
| Keep in mind that C++ operator precedence rules may require you to put |
| expressions in parentheses for complex expressions. The Kleene Star is |
| also known as a Kleene Closure, but we call it the Star in most places. |
| </p> |
| <a name="spirit.qi.tutorials.warming_up.trivial_example__4_parsing_a_comma_delimited_list_of_numbers"></a><h6> |
| <a name="id798326"></a> |
| <a class="link" href="warming_up.html#spirit.qi.tutorials.warming_up.trivial_example__4_parsing_a_comma_delimited_list_of_numbers">Trivial |
| Example #4 Parsing a comma-delimited list of numbers</a> |
| </h6> |
| <p> |
| This example will create a parser that accepts a comma-delimited list of |
| numbers. |
| </p> |
| <pre class="programlisting"><span class="identifier">double_</span> <span class="special">>></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="identifier">double_</span><span class="special">)</span> |
| </pre> |
| <p> |
| Notice <code class="computeroutput"><span class="identifier">char_</span><span class="special">(</span><span class="char">','</span><span class="special">)</span></code>. It is |
| a literal character parser that can recognize the comma <code class="computeroutput"><span class="char">','</span></code>. |
| In this case, the Kleene Star is modifying a more complex parser, namely, |
| the one generated by the expression: |
| </p> |
| <pre class="programlisting"><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="identifier">double_</span><span class="special">)</span> |
| </pre> |
| <p> |
| Note that this is a case where the parentheses are necessary. The Kleene |
| star encloses the complete expression above. |
| </p> |
| <a name="spirit.qi.tutorials.warming_up.let_s_parse_"></a><h6> |
| <a name="id798457"></a> |
| <a class="link" href="warming_up.html#spirit.qi.tutorials.warming_up.let_s_parse_">Let's Parse!</a> |
| </h6> |
| <p> |
| We're done with defining the parser. So the next step is now invoking this |
| parser to do its work. There are a couple of ways to do this. For now, |
| we will use the <code class="computeroutput"><span class="identifier">phrase_parse</span></code> |
| function. One overload of this function accepts four arguments: |
| </p> |
| <div class="orderedlist"><ol class="orderedlist" type="1"> |
| <li class="listitem"> |
| An iterator pointing to the start of the input |
| </li> |
| <li class="listitem"> |
| An iterator pointing to one past the end of the input |
| </li> |
| <li class="listitem"> |
| The parser object |
| </li> |
| <li class="listitem"> |
| Another parser called the skip parser |
| </li> |
| </ol></div> |
| <p> |
| In our example, we wish to skip spaces and tabs. Another parser named |
| <code class="computeroutput"><span class="identifier">space</span></code> is included in Spirit's |
| repertoire of predefined parsers. It is a very simple parser that simply |
| recognizes whitespace. We will use <code class="computeroutput"><span class="identifier">space</span></code> |
| as our skip parser. The skip parser is the one responsible for skipping |
| characters in between parser elements such as the <code class="computeroutput"><span class="identifier">double_</span></code> |
| and <code class="computeroutput"><span class="identifier">char_</span></code>. |
| </p> |
| <p> |
| Ok, so now let's parse! |
| </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">bool</span> <span class="identifier">parse_numbers</span><span class="special">(</span><span class="identifier">Iterator</span> <span class="identifier">first</span><span class="special">,</span> <span class="identifier">Iterator</span> <span class="identifier">last</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">double_</span><span class="special">;</span> |
| <span class="keyword">using</span> <span class="identifier">qi</span><span class="special">::</span><span class="identifier">phrase_parse</span><span class="special">;</span> |
| <span class="keyword">using</span> <span class="identifier">ascii</span><span class="special">::</span><span class="identifier">space</span><span class="special">;</span> |
| |
| <span class="keyword">bool</span> <span class="identifier">r</span> <span class="special">=</span> <span class="identifier">phrase_parse</span><span class="special">(</span> |
| <span class="identifier">first</span><span class="special">,</span> <a class="co" name="spirit0co" href="warming_up.html#spirit0"><img src="../../../../../../../doc/src/images/callouts/1.png" alt="1" border="0"></a> |
| <span class="identifier">last</span><span class="special">,</span> <a class="co" name="spirit1co" href="warming_up.html#spirit1"><img src="../../../../../../../doc/src/images/callouts/2.png" alt="2" border="0"></a> |
| <span class="identifier">double_</span> <span class="special">>></span> <span class="special">*(</span><span class="char">','</span> <span class="special">>></span> <span class="identifier">double_</span><span class="special">),</span> <a class="co" name="spirit2co" href="warming_up.html#spirit2"><img src="../../../../../../../doc/src/images/callouts/3.png" alt="3" border="0"></a> |
| <span class="identifier">space</span> <a class="co" name="spirit3co" href="warming_up.html#spirit3"><img src="../../../../../../../doc/src/images/callouts/4.png" alt="4" border="0"></a> |
| <span class="special">);</span> |
| <span class="keyword">if</span> <span class="special">(</span><span class="identifier">first</span> <span class="special">!=</span> <span class="identifier">last</span><span class="special">)</span> <span class="comment">// fail if we did not get a full match |
| </span> <span class="keyword">return</span> <span class="keyword">false</span><span class="special">;</span> |
| <span class="keyword">return</span> <span class="identifier">r</span><span class="special">;</span> |
| <span class="special">}</span> |
| </pre> |
| <p> |
| </p> |
| <div class="calloutlist"><table border="0" summary="Callout list"> |
| <tr> |
| <td width="5%" valign="top" align="left"><p><a name="spirit0"></a><a href="#spirit0co"><img src="../../../../../../../doc/src/images/callouts/1.png" alt="1" border="0"></a> </p></td> |
| <td valign="top" align="left"><p> |
| start iterator |
| </p></td> |
| </tr> |
| <tr> |
| <td width="5%" valign="top" align="left"><p><a name="spirit1"></a><a href="#spirit1co"><img src="../../../../../../../doc/src/images/callouts/2.png" alt="2" border="0"></a> </p></td> |
| <td valign="top" align="left"><p> |
| end iterator |
| </p></td> |
| </tr> |
| <tr> |
| <td width="5%" valign="top" align="left"><p><a name="spirit2"></a><a href="#spirit2co"><img src="../../../../../../../doc/src/images/callouts/3.png" alt="3" border="0"></a> </p></td> |
| <td valign="top" align="left"><p> |
| the parser |
| </p></td> |
| </tr> |
| <tr> |
| <td width="5%" valign="top" align="left"><p><a name="spirit3"></a><a href="#spirit3co"><img src="../../../../../../../doc/src/images/callouts/4.png" alt="4" border="0"></a> </p></td> |
| <td valign="top" align="left"><p> |
| the skip-parser |
| </p></td> |
| </tr> |
| </table></div> |
| <p> |
| The parse function returns <code class="computeroutput"><span class="keyword">true</span></code> |
| or <code class="computeroutput"><span class="keyword">false</span></code> depending on the |
| result of the parse. The first iterator is passed by reference. On a successful |
| parse, this iterator is repositioned to the rightmost position consumed |
| by the parser. If this becomes equal to <code class="computeroutput"><span class="identifier">last</span></code>, |
| then we have a full match. If not, then we have a partial match. A partial |
| match happens when the parser is only able to parse a portion of the input. |
| </p> |
| <p> |
| Note that we inlined the parser directly in the call to parse. Upon calling |
| parse, the expression evaluates into a temporary, unnamed parser which |
| is passed into the parse() function, used, and then destroyed. |
| </p> |
| <p> |
| Here, we opted to make the parser generic by making it a template, parameterized |
| by the iterator type. By doing so, it can take in data coming from any |
| STL conforming sequence as long as the iterators conform to a forward iterator. |
| </p> |
| <p> |
| You can find the full cpp file here: <a href="../../../../../example/qi/num_list1.cpp" target="_top">../../example/qi/num_list1.cpp</a> |
| </p> |
| <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> |
| <code class="computeroutput"><span class="keyword">char</span></code> and <code class="computeroutput"><span class="keyword">wchar_t</span></code> |
| operands |
| </p> |
| <p> |
| The careful reader may notice that the parser expression has <code class="computeroutput"><span class="char">','</span></code> instead of <code class="computeroutput"><span class="identifier">char_</span><span class="special">(</span><span class="char">','</span><span class="special">)</span></code> |
| as the previous examples did. This is ok due to C++ syntax rules of conversion. |
| There are <code class="computeroutput"><span class="special">>></span></code> operators |
| that are overloaded to accept a <code class="computeroutput"><span class="keyword">char</span></code> |
| or <code class="computeroutput"><span class="keyword">wchar_t</span></code> argument on its |
| left or right (but not both). An operator may be overloaded if at least |
| one of its parameters is a user-defined type. In this case, the <code class="computeroutput"><span class="identifier">double_</span></code> is the 2nd argument to <code class="computeroutput"><span class="keyword">operator</span><span class="special">>></span></code>, |
| and so the proper overload of <code class="computeroutput"><span class="special">>></span></code> |
| is used, converting <code class="computeroutput"><span class="char">','</span></code> into |
| a character literal parser. |
| </p> |
| <p> |
| The problem with omitting the <code class="computeroutput"><span class="identifier">char_</span></code> |
| should be obvious: <code class="computeroutput"><span class="char">'a'</span> <span class="special">>></span> |
| <span class="char">'b'</span></code> is not a spirit parser, it is |
| a numeric expression, right-shifting the ASCII (or another encoding) |
| value of <code class="computeroutput"><span class="char">'a'</span></code> by the ASCII value |
| of <code class="computeroutput"><span class="char">'b'</span></code>. However, both <code class="computeroutput"><span class="identifier">char_</span><span class="special">(</span><span class="char">'a'</span><span class="special">)</span> <span class="special">>></span> |
| <span class="char">'b'</span></code> and <code class="computeroutput"><span class="char">'a'</span> |
| <span class="special">>></span> <span class="identifier">char_</span><span class="special">(</span><span class="char">'b'</span><span class="special">)</span></code> |
| are Spirit sequence parsers for the letter <code class="computeroutput"><span class="char">'a'</span></code> |
| followed by <code class="computeroutput"><span class="char">'b'</span></code>. You'll get |
| used to it, sooner or later. |
| </p> |
| </td></tr> |
| </table></div> |
| <p> |
| Finally, take note that we test for a full match (i.e. the parser fully |
| parsed the input) by checking if the first iterator, after parsing, is |
| equal to the end iterator. You may strike out this part if partial matches |
| are to be allowed. |
| </p> |
| </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> |
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