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| <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> |
| <a name="spirit_repository.qi_components.primitive.advance.description"></a><h6> |
| <a name="spirit_repository.qi_components.primitive.advance.description-heading"></a> |
| <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> |
| <a name="spirit_repository.qi_components.primitive.advance.header"></a><h6> |
| <a name="spirit_repository.qi_components.primitive.advance.header-heading"></a> |
| <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> |
| <a name="spirit_repository.qi_components.primitive.advance.synopsis"></a><h6> |
| <a name="spirit_repository.qi_components.primitive.advance.synopsis-heading"></a> |
| <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> |
| <a name="spirit_repository.qi_components.primitive.advance.parameters"></a><h6> |
| <a name="spirit_repository.qi_components.primitive.advance.parameters-heading"></a> |
| <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> |
| <a name="spirit_repository.qi_components.primitive.advance.attribute"></a><h6> |
| <a name="spirit_repository.qi_components.primitive.advance.attribute-heading"></a> |
| <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> |
| <a name="spirit_repository.qi_components.primitive.advance.example"></a><h6> |
| <a name="spirit_repository.qi_components.primitive.advance.example-heading"></a> |
| <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> |
| <a name="spirit_repository.qi_components.primitive.advance.prerequisites"></a><h6> |
| <a name="spirit_repository.qi_components.primitive.advance.prerequisites-heading"></a> |
| <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> |
| <a name="spirit_repository.qi_components.primitive.advance.setting_up_the_grammar"></a><h6> |
| <a name="spirit_repository.qi_components.primitive.advance.setting_up_the_grammar-heading"></a> |
| <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> |
| <a name="spirit_repository.qi_components.primitive.advance.parsing_a_correctly_delimited_string_of_data"></a><h6> |
| <a name="spirit_repository.qi_components.primitive.advance.parsing_a_correctly_delimited_string_of_data-heading"></a> |
| <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> |
| <a name="spirit_repository.qi_components.primitive.advance.parsing_the_alternative_representation"></a><h6> |
| <a name="spirit_repository.qi_components.primitive.advance.parsing_the_alternative_representation-heading"></a> |
| <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> |
| <a name="spirit_repository.qi_components.primitive.advance.notes"></a><h6> |
| <a name="spirit_repository.qi_components.primitive.advance.notes-heading"></a> |
| <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> |
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| <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> |
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