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- </div>
- <div class="section">
- <div class="titlepage"><div><div><h4 class="title">
- <a name="spirit_repository.qi_components.primitive.advance"></a><a class="link" href="advance.html" title="Qi advance Parser">Qi
- advance Parser</a>
- </h4></div></div></div>
- <h6>
- <a name="spirit_repository.qi_components.primitive.advance.h0"></a>
- <span class="phrase"><a name="spirit_repository.qi_components.primitive.advance.description"></a></span><a class="link" href="advance.html#spirit_repository.qi_components.primitive.advance.description">Description</a>
- </h6>
- <p>
- The <span class="emphasis"><em>Spirit.Qi</em></span> <code class="computeroutput"><span class="identifier">advance</span></code>
- is a primitive parser component allowing the parser to skip (advance) through
- a specified number of iterations without performing unnecessary work:
- </p>
- <pre class="programlisting"><span class="identifier">advance</span><span class="special">(</span><span class="identifier">distance</span><span class="special">)</span>
- </pre>
- <p>
- The most obvious existing alternative to this, the <code class="computeroutput"><span class="identifier">repeat</span></code>
- directive, will cause the parser to advance one iterator at a time while
- usually performing operations at each step. In some cases that work is
- unnecessary, as in the case where large binary objects are being parsed.
- Take, for example, the following binary data:
- </p>
- <pre class="programlisting">00 00 00 01 77 fc b4 51 0a b3 b7 ... 1e 60 70 b6 00 00 01 00
- </pre>
- <p>
- If the first 4 bytes are a little-endian 32-bit integer describing the
- length of the subsequent data, but the data itself is not relevant to parsing,
- then the repeat directive would cause all of the subsequent 16 MB of data
- to be consumed one byte at a time while generating page faults or other
- superfluous I/O. If the value is large, as it is in this case, the parser
- becomes very slow.
- </p>
- <pre class="programlisting"><span class="identifier">little_dword</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="identifier">repeat</span><span class="special">(</span><span class="identifier">_a</span><span class="special">)[</span><span class="identifier">byte_</span><span class="special">]</span> <span class="special">>></span> <span class="identifier">little_dword</span><span class="special">...</span>
- </pre>
- <p>
- The <code class="computeroutput"><span class="identifier">advance</span></code> parser component
- solves this problem by performing as little work as possible to advance
- the parser's iterator, and will optimize for the case of random-access
- iterators by advancing directly to the desired relative iterator position.
- </p>
- <pre class="programlisting"><span class="identifier">little_dword</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="identifier">advance</span><span class="special">(</span><span class="identifier">_a</span><span class="special">)</span> <span class="special">>></span> <span class="identifier">little_dword</span><span class="special">...</span>
- </pre>
- <h6>
- <a name="spirit_repository.qi_components.primitive.advance.h1"></a>
- <span class="phrase"><a name="spirit_repository.qi_components.primitive.advance.header"></a></span><a class="link" href="advance.html#spirit_repository.qi_components.primitive.advance.header">Header</a>
- </h6>
- <pre class="programlisting"><span class="comment">// forwards to <boost/spirit/repository/home/qi/primitive/advance.hpp></span>
- <span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">spirit</span><span class="special">/</span><span class="identifier">repository</span><span class="special">/</span><span class="identifier">include</span><span class="special">/</span><span class="identifier">qi_advance</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span>
- </pre>
- <h6>
- <a name="spirit_repository.qi_components.primitive.advance.h2"></a>
- <span class="phrase"><a name="spirit_repository.qi_components.primitive.advance.synopsis"></a></span><a class="link" href="advance.html#spirit_repository.qi_components.primitive.advance.synopsis">Synopsis</a>
- </h6>
- <pre class="programlisting"><span class="identifier">advance</span><span class="special">(</span><span class="identifier">distance</span><span class="special">)</span>
- </pre>
- <h6>
- <a name="spirit_repository.qi_components.primitive.advance.h3"></a>
- <span class="phrase"><a name="spirit_repository.qi_components.primitive.advance.parameters"></a></span><a class="link" href="advance.html#spirit_repository.qi_components.primitive.advance.parameters">Parameters</a>
- </h6>
- <div class="informaltable"><table class="table">
- <colgroup>
- <col>
- <col>
- </colgroup>
- <thead><tr>
- <th>
- <p>
- Parameter
- </p>
- </th>
- <th>
- <p>
- Description
- </p>
- </th>
- </tr></thead>
- <tbody><tr>
- <td>
- <p>
- 'distance'
- </p>
- </td>
- <td>
- <p>
- The distance that the iterator shall be advanced
- </p>
- </td>
- </tr></tbody>
- </table></div>
- <h6>
- <a name="spirit_repository.qi_components.primitive.advance.h4"></a>
- <span class="phrase"><a name="spirit_repository.qi_components.primitive.advance.attribute"></a></span><a class="link" href="advance.html#spirit_repository.qi_components.primitive.advance.attribute">Attribute</a>
- </h6>
- <p>
- The <code class="computeroutput"><span class="identifier">advance</span></code> component exposes
- no attribute (the exposed attribute type is <code class="computeroutput"><span class="identifier">unused_type</span></code>):
- </p>
- <pre class="programlisting"><span class="identifier">advance</span> <span class="special">--></span> <span class="identifier">unused</span>
- </pre>
- <h6>
- <a name="spirit_repository.qi_components.primitive.advance.h5"></a>
- <span class="phrase"><a name="spirit_repository.qi_components.primitive.advance.example"></a></span><a class="link" href="advance.html#spirit_repository.qi_components.primitive.advance.example">Example</a>
- </h6>
- <p>
- The following example shows simple use cases of the <code class="computeroutput"><span class="identifier">advance</span></code>
- component. We will illustrate its usage by generating parsers for some
- binary data (for the full example code see <a href="../../../../../example/qi/advance.cpp" target="_top">advance.cpp</a>)
- </p>
- <h6>
- <a name="spirit_repository.qi_components.primitive.advance.h6"></a>
- <span class="phrase"><a name="spirit_repository.qi_components.primitive.advance.prerequisites"></a></span><a class="link" href="advance.html#spirit_repository.qi_components.primitive.advance.prerequisites">Prerequisites</a>
- </h6>
- <p>
- In addition to the main header file needed to include the core components
- implemented in <span class="emphasis"><em>Spirit.Qi</em></span> we add the header file needed
- for the new <code class="computeroutput"><span class="identifier">advance</span></code> component.
- </p>
- <p>
- </p>
- <pre class="programlisting"><span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">spirit</span><span class="special">/</span><span class="identifier">include</span><span class="special">/</span><span class="identifier">qi</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span>
- <span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">spirit</span><span class="special">/</span><span class="identifier">include</span><span class="special">/</span><span class="identifier">phoenix_operator</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span>
- <span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">spirit</span><span class="special">/</span><span class="identifier">repository</span><span class="special">/</span><span class="identifier">include</span><span class="special">/</span><span class="identifier">qi_advance</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span>
- </pre>
- <p>
- </p>
- <p>
- In order to make the examples below more readable we introduce or use the
- following namespaces.
- </p>
- <p>
- </p>
- <pre class="programlisting"><span class="keyword">namespace</span> <span class="identifier">qi</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">spirit</span><span class="special">::</span><span class="identifier">qi</span><span class="special">;</span>
- <span class="keyword">using</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">spirit</span><span class="special">::</span><span class="identifier">repository</span><span class="special">::</span><span class="identifier">qi</span><span class="special">::</span><span class="identifier">advance</span><span class="special">;</span>
- </pre>
- <p>
- </p>
- <h6>
- <a name="spirit_repository.qi_components.primitive.advance.h7"></a>
- <span class="phrase"><a name="spirit_repository.qi_components.primitive.advance.setting_up_the_grammar"></a></span><a class="link" href="advance.html#spirit_repository.qi_components.primitive.advance.setting_up_the_grammar">Setting
- up the Grammar</a>
- </h6>
- <p>
- This is a very simple grammar that recognizes several fields of a binary
- stream of data. There are two fields explicitly delimited by a field indicating
- the number of bytes that are spanned. They are separated by a literal string.
- </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">advance_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">qi</span><span class="special">::</span><span class="identifier">locals</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span> <span class="special">></span>
- <span class="special">{</span>
- <span class="identifier">advance_grammar</span><span class="special">()</span> <span class="special">:</span> <span class="identifier">advance_grammar</span><span class="special">::</span><span class="identifier">base_type</span><span class="special">(</span><span class="identifier">start</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">byte_</span><span class="special">;</span>
- <span class="keyword">using</span> <span class="identifier">qi</span><span class="special">::</span><span class="identifier">eoi</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="identifier">start</span>
- <span class="special">=</span> <span class="identifier">byte_</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="identifier">advance</span><span class="special">(</span><span class="identifier">_a</span><span class="special">)</span>
- <span class="special">>></span> <span class="string">"boost"</span>
- <span class="special">>></span> <span class="identifier">byte_</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">advance</span><span class="special">(</span><span class="identifier">_a</span><span class="special">)</span> <span class="special">|</span> <span class="string">"qi"</span><span class="special">)</span> <span class="comment">// note alternative when advance fails</span>
- <span class="special">>></span> <span class="identifier">eoi</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">qi</span><span class="special">::</span><span class="identifier">locals</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span> <span class="special">></span> <span class="identifier">start</span><span class="special">;</span>
- <span class="special">};</span>
- </pre>
- <p>
- </p>
- <p>
- Note that the second binary field may either contain the number of specified
- bytes, or the word "qi". If the <code class="computeroutput"><span class="identifier">advance</span></code>
- parser component fails to advance the specified number of bytes before
- reaching the end of input, it will fail and the parser will attempt to
- descend into alternatives.
- </p>
- <h6>
- <a name="spirit_repository.qi_components.primitive.advance.h8"></a>
- <span class="phrase"><a name="spirit_repository.qi_components.primitive.advance.parsing_a_correctly_delimited_string_of_data"></a></span><a class="link" href="advance.html#spirit_repository.qi_components.primitive.advance.parsing_a_correctly_delimited_string_of_data">Parsing
- a Correctly-delimited String of Data</a>
- </h6>
- <p>
- The data below is correctly delimited and will thus result in a valid parse.
- Note that both random-access and bidirectional iterators are used here.
- </p>
- <p>
- </p>
- <pre class="programlisting"><span class="keyword">unsigned</span> <span class="keyword">char</span> <span class="keyword">const</span> <span class="identifier">alt1</span><span class="special">[]</span> <span class="special">=</span>
- <span class="special">{</span>
- <span class="number">5</span><span class="special">,</span> <span class="comment">// number of bytes to advance</span>
- <span class="number">1</span><span class="special">,</span> <span class="number">2</span><span class="special">,</span> <span class="number">3</span><span class="special">,</span> <span class="number">4</span><span class="special">,</span> <span class="number">5</span><span class="special">,</span> <span class="comment">// data to advance through</span>
- <span class="char">'b'</span><span class="special">,</span> <span class="char">'o'</span><span class="special">,</span> <span class="char">'o'</span><span class="special">,</span> <span class="char">'s'</span><span class="special">,</span> <span class="char">'t'</span><span class="special">,</span> <span class="comment">// word to parse</span>
- <span class="number">2</span><span class="special">,</span> <span class="comment">// number of bytes to advance</span>
- <span class="number">11</span><span class="special">,</span> <span class="number">12</span> <span class="comment">// more data to advance through</span>
- <span class="comment">// eoi</span>
- <span class="special">};</span>
- <span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span> <span class="keyword">const</span> <span class="identifier">alt1_string</span><span class="special">(</span><span class="identifier">alt1</span><span class="special">,</span> <span class="identifier">alt1</span> <span class="special">+</span> <span class="keyword">sizeof</span> <span class="identifier">alt1</span><span class="special">);</span>
- <span class="identifier">std</span><span class="special">::</span><span class="identifier">list</span><span class="special"><</span><span class="keyword">unsigned</span> <span class="keyword">char</span><span class="special">></span> <span class="keyword">const</span> <span class="identifier">alt1_list</span><span class="special">(</span><span class="identifier">alt1</span><span class="special">,</span> <span class="identifier">alt1</span> <span class="special">+</span> <span class="keyword">sizeof</span> <span class="identifier">alt1</span><span class="special">);</span>
- <span class="identifier">result</span> <span class="special">=</span>
- <span class="identifier">qi</span><span class="special">::</span><span class="identifier">parse</span><span class="special">(</span><span class="identifier">alt1_string</span><span class="special">.</span><span class="identifier">begin</span><span class="special">(),</span> <span class="identifier">alt1_string</span><span class="special">.</span><span class="identifier">end</span><span class="special">()</span>
- <span class="special">,</span> <span class="identifier">client</span><span class="special">::</span><span class="identifier">advance_grammar</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">const_iterator</span><span class="special">>())</span>
- <span class="special">?</span> <span class="string">"succeeded"</span> <span class="special">:</span> <span class="string">"failed"</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="string">"Parsing alt1 using random access iterator "</span> <span class="special"><<</span> <span class="identifier">result</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="identifier">result</span> <span class="special">=</span>
- <span class="identifier">qi</span><span class="special">::</span><span class="identifier">parse</span><span class="special">(</span><span class="identifier">alt1_list</span><span class="special">.</span><span class="identifier">begin</span><span class="special">(),</span> <span class="identifier">alt1_list</span><span class="special">.</span><span class="identifier">end</span><span class="special">()</span>
- <span class="special">,</span> <span class="identifier">client</span><span class="special">::</span><span class="identifier">advance_grammar</span><span class="special"><</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">list</span><span class="special"><</span><span class="keyword">unsigned</span> <span class="keyword">char</span><span class="special">>::</span><span class="identifier">const_iterator</span><span class="special">>())</span>
- <span class="special">?</span> <span class="string">"succeeded"</span> <span class="special">:</span> <span class="string">"failed"</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="string">"Parsing alt1 using bidirectional iterator "</span> <span class="special"><<</span> <span class="identifier">result</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>
- </p>
- <h6>
- <a name="spirit_repository.qi_components.primitive.advance.h9"></a>
- <span class="phrase"><a name="spirit_repository.qi_components.primitive.advance.parsing_the_alternative_representation"></a></span><a class="link" href="advance.html#spirit_repository.qi_components.primitive.advance.parsing_the_alternative_representation">Parsing
- the Alternative Representation</a>
- </h6>
- <p>
- The data below is not correctly delimited, but will correctly parse because
- the alternative word "qi" is available.
- </p>
- <p>
- </p>
- <pre class="programlisting"><span class="keyword">unsigned</span> <span class="keyword">char</span> <span class="keyword">const</span> <span class="identifier">alt2</span><span class="special">[]</span> <span class="special">=</span>
- <span class="special">{</span>
- <span class="number">2</span><span class="special">,</span> <span class="comment">// number of bytes to advance</span>
- <span class="number">1</span><span class="special">,</span> <span class="number">2</span><span class="special">,</span> <span class="comment">// data to advance through</span>
- <span class="char">'b'</span><span class="special">,</span> <span class="char">'o'</span><span class="special">,</span> <span class="char">'o'</span><span class="special">,</span> <span class="char">'s'</span><span class="special">,</span> <span class="char">'t'</span><span class="special">,</span> <span class="comment">// word to parse</span>
- <span class="number">4</span><span class="special">,</span> <span class="comment">// number of bytes to advance</span>
- <span class="char">'q'</span><span class="special">,</span> <span class="char">'i'</span> <span class="comment">// alternative (advance won't work)</span>
- <span class="comment">// eoi</span>
- <span class="special">};</span>
- <span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span> <span class="keyword">const</span> <span class="identifier">alt2_string</span><span class="special">(</span><span class="identifier">alt2</span><span class="special">,</span> <span class="identifier">alt2</span> <span class="special">+</span> <span class="keyword">sizeof</span> <span class="identifier">alt2</span><span class="special">);</span>
- <span class="identifier">std</span><span class="special">::</span><span class="identifier">list</span><span class="special"><</span><span class="keyword">unsigned</span> <span class="keyword">char</span><span class="special">></span> <span class="keyword">const</span> <span class="identifier">alt2_list</span><span class="special">(</span><span class="identifier">alt2</span><span class="special">,</span> <span class="identifier">alt2</span> <span class="special">+</span> <span class="keyword">sizeof</span> <span class="identifier">alt2</span><span class="special">);</span>
- <span class="identifier">result</span> <span class="special">=</span>
- <span class="identifier">qi</span><span class="special">::</span><span class="identifier">parse</span><span class="special">(</span><span class="identifier">alt2_string</span><span class="special">.</span><span class="identifier">begin</span><span class="special">(),</span> <span class="identifier">alt2_string</span><span class="special">.</span><span class="identifier">end</span><span class="special">()</span>
- <span class="special">,</span> <span class="identifier">client</span><span class="special">::</span><span class="identifier">advance_grammar</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">const_iterator</span><span class="special">>())</span>
- <span class="special">?</span> <span class="string">"succeeded"</span> <span class="special">:</span> <span class="string">"failed"</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="string">"Parsing alt2 using random access iterator "</span> <span class="special"><<</span> <span class="identifier">result</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="identifier">result</span> <span class="special">=</span>
- <span class="identifier">qi</span><span class="special">::</span><span class="identifier">parse</span><span class="special">(</span><span class="identifier">alt2_list</span><span class="special">.</span><span class="identifier">begin</span><span class="special">(),</span> <span class="identifier">alt2_list</span><span class="special">.</span><span class="identifier">end</span><span class="special">()</span>
- <span class="special">,</span> <span class="identifier">client</span><span class="special">::</span><span class="identifier">advance_grammar</span><span class="special"><</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">list</span><span class="special"><</span><span class="keyword">unsigned</span> <span class="keyword">char</span><span class="special">>::</span><span class="identifier">const_iterator</span><span class="special">>())</span>
- <span class="special">?</span> <span class="string">"succeeded"</span> <span class="special">:</span> <span class="string">"failed"</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="string">"Parsing alt2 using bidirectional iterator "</span> <span class="special"><<</span> <span class="identifier">result</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>
- </p>
- <h6>
- <a name="spirit_repository.qi_components.primitive.advance.h10"></a>
- <span class="phrase"><a name="spirit_repository.qi_components.primitive.advance.notes"></a></span><a class="link" href="advance.html#spirit_repository.qi_components.primitive.advance.notes">Notes</a>
- </h6>
- <p>
- The <code class="computeroutput"><span class="identifier">advance</span></code> parser component
- will fail unconditionally on negative values. It will never attempt to
- advance the iterator in the reverse direction.
- </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 © 2001-2011 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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