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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>
<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">&gt;&gt;</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">&gt;&gt;</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">&gt;&gt;</span> <span class="identifier">advance</span><span class="special">(</span><span class="identifier">_a</span><span class="special">)</span> <span class="special">&gt;&gt;</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 &lt;boost/spirit/repository/home/qi/primitive/advance.hpp&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</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">&gt;</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">--&gt;</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">&lt;</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">&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</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">&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</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">&gt;</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">&lt;</span><span class="keyword">typename</span> <span class="identifier">Iterator</span><span class="special">&gt;</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">&lt;</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">&lt;</span><span class="keyword">int</span><span class="special">&gt;</span> <span class="special">&gt;</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">&gt;&gt;</span> <span class="identifier">advance</span><span class="special">(</span><span class="identifier">_a</span><span class="special">)</span>
            <span class="special">&gt;&gt;</span> <span class="string">"boost"</span>
            <span class="special">&gt;&gt;</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">&gt;&gt;</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">&gt;&gt;</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">&lt;</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">&lt;</span><span class="keyword">int</span><span class="special">&gt;</span> <span class="special">&gt;</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">&lt;</span><span class="keyword">unsigned</span> <span class="keyword">char</span><span class="special">&gt;</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">&lt;</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">&gt;())</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">&lt;&lt;</span> <span class="string">"Parsing alt1 using random access iterator "</span> <span class="special">&lt;&lt;</span> <span class="identifier">result</span> <span class="special">&lt;&lt;</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">&lt;</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">list</span><span class="special">&lt;</span><span class="keyword">unsigned</span> <span class="keyword">char</span><span class="special">&gt;::</span><span class="identifier">const_iterator</span><span class="special">&gt;())</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">&lt;&lt;</span> <span class="string">"Parsing alt1 using bidirectional iterator "</span> <span class="special">&lt;&lt;</span> <span class="identifier">result</span> <span class="special">&lt;&lt;</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">&lt;</span><span class="keyword">unsigned</span> <span class="keyword">char</span><span class="special">&gt;</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">&lt;</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">&gt;())</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">&lt;&lt;</span> <span class="string">"Parsing alt2 using random access iterator "</span> <span class="special">&lt;&lt;</span> <span class="identifier">result</span> <span class="special">&lt;&lt;</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">&lt;</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">list</span><span class="special">&lt;</span><span class="keyword">unsigned</span> <span class="keyword">char</span><span class="special">&gt;::</span><span class="identifier">const_iterator</span><span class="special">&gt;())</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">&lt;&lt;</span> <span class="string">"Parsing alt2 using bidirectional iterator "</span> <span class="special">&lt;&lt;</span> <span class="identifier">result</span> <span class="special">&lt;&lt;</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>
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        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>)
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