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<title>Qi Confix Parser Directive</title>
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<div class="titlepage"><div><div><h4 class="title">
<a name="spirit_repository.qi_components.directives.confix"></a><a class="link" href="confix.html" title="Qi Confix Parser Directive">Qi
        Confix Parser Directive</a>
</h4></div></div></div>
<h6>
<a name="spirit_repository.qi_components.directives.confix.h0"></a>
          <span class="phrase"><a name="spirit_repository.qi_components.directives.confix.description"></a></span><a class="link" href="confix.html#spirit_repository.qi_components.directives.confix.description">Description</a>
        </h6>
<p>
          The <span class="emphasis"><em>Spirit.Qi</em></span> <code class="computeroutput"><span class="identifier">confix</span></code>
          directive is a unary parser component allowing to embed a parser (the subject)
          inside an opening (the prefix) and a closing (the suffix):
        </p>
<pre class="programlisting"><span class="identifier">confix</span><span class="special">(</span><span class="identifier">prefix</span><span class="special">,</span> <span class="identifier">suffix</span><span class="special">)[</span><span class="identifier">subject</span><span class="special">]</span>
</pre>
<p>
          This results in a parser that is equivalent to the sequence
        </p>
<pre class="programlisting"><span class="identifier">omit</span><span class="special">[</span><span class="identifier">prefix</span><span class="special">]</span> <span class="special">&gt;&gt;</span> <span class="identifier">subject</span> <span class="special">&gt;&gt;</span> <span class="identifier">omit</span><span class="special">[</span><span class="identifier">suffix</span><span class="special">]</span>
</pre>
<p>
          A simple example is a parser for non-nested comments which can now be written
          as:
        </p>
<pre class="programlisting"><span class="identifier">confix</span><span class="special">(</span><span class="string">"/*"</span><span class="special">,</span> <span class="string">"*/"</span><span class="special">)[*(</span><span class="identifier">char_</span> <span class="special">-</span> <span class="string">"*/"</span><span class="special">)]</span>  <span class="comment">// C style comment</span>
<span class="identifier">confix</span><span class="special">(</span><span class="string">"//"</span><span class="special">,</span> <span class="identifier">eol</span><span class="special">)[*(</span><span class="identifier">char_</span> <span class="special">-</span> <span class="identifier">eol</span><span class="special">)]</span>    <span class="comment">// C++ style comment</span>
</pre>
<p>
          Using the <code class="computeroutput"><span class="identifier">confix</span></code> directive
          instead of the explicit sequence has the advantage of being able to encapsulate
          the prefix and the suffix into a separate construct. The following code
          snippet illustrates the idea:
        </p>
<pre class="programlisting"><span class="keyword">namespace</span> <span class="identifier">spirit</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="keyword">namespace</span> <span class="identifier">repo</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="comment">// Define a metafunction allowing to compute the type</span>
<span class="comment">// of the confix() construct</span>
<span class="keyword">template</span> <span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Prefix</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">Suffix</span> <span class="special">=</span> <span class="identifier">Prefix</span><span class="special">&gt;</span>
<span class="keyword">struct</span> <span class="identifier">confix_spec</span>
<span class="special">{</span>
    <span class="keyword">typedef</span> <span class="keyword">typename</span> <span class="identifier">spirit</span><span class="special">::</span><span class="identifier">result_of</span><span class="special">::</span><span class="identifier">terminal</span><span class="special">&lt;</span>
        <span class="identifier">repo</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">confix</span><span class="special">(</span><span class="identifier">Prefix</span><span class="special">,</span> <span class="identifier">Suffix</span><span class="special">)</span>
    <span class="special">&gt;::</span><span class="identifier">type</span> <span class="identifier">type</span><span class="special">;</span>
<span class="special">};</span>

<span class="identifier">confix_spec</span><span class="special">&lt;</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">&gt;::</span><span class="identifier">type</span> <span class="keyword">const</span> <span class="identifier">c_comment</span> <span class="special">=</span> <span class="identifier">repo</span><span class="special">::</span><span class="identifier">confix</span><span class="special">(</span><span class="string">"/*"</span><span class="special">,</span> <span class="string">"*/"</span><span class="special">);</span>
<span class="identifier">confix_spec</span><span class="special">&lt;</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">&gt;::</span><span class="identifier">type</span> <span class="keyword">const</span> <span class="identifier">cpp_comment</span> <span class="special">=</span> <span class="identifier">repo</span><span class="special">::</span><span class="identifier">confix</span><span class="special">(</span><span class="string">"//"</span><span class="special">,</span> <span class="string">"\n"</span><span class="special">);</span>
</pre>
<p>
          Now, the comment parsers can be written as
        </p>
<pre class="programlisting"><span class="identifier">c_comment</span><span class="special">[*(</span><span class="identifier">char_</span> <span class="special">-</span> <span class="string">"*/"</span><span class="special">)]</span>    <span class="comment">// C style comment</span>
<span class="identifier">cpp_comment</span><span class="special">[*(</span><span class="identifier">char_</span> <span class="special">-</span> <span class="identifier">eol</span><span class="special">)]</span>   <span class="comment">// C++ style comment</span>
</pre>
<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>
            While the <code class="computeroutput"><span class="identifier">confix_p</span><span class="special">(</span><span class="identifier">prefix</span><span class="special">,</span> <span class="identifier">subject</span><span class="special">,</span>
            <span class="identifier">suffix</span><span class="special">)</span></code>
            parser in <span class="emphasis"><em>Spirit.Classic</em></span> was equivalent to the sequence
            <code class="computeroutput"><span class="identifier">prefix</span> <span class="special">&gt;&gt;</span>
            <span class="special">*(</span><span class="identifier">subject</span>
            <span class="special">-</span> <span class="identifier">suffix</span><span class="special">)</span> <span class="special">&gt;&gt;</span> <span class="identifier">suffix</span><span class="special">,</span> <span class="identifier">the</span> <span class="emphasis"><em>Spirit.Qi</em></span> </code>confix`
            directive will not perform this refactoring any more. This simplifies
            the code and makes things more explicit.
          </p></td></tr>
</table></div>
<h6>
<a name="spirit_repository.qi_components.directives.confix.h1"></a>
          <span class="phrase"><a name="spirit_repository.qi_components.directives.confix.header"></a></span><a class="link" href="confix.html#spirit_repository.qi_components.directives.confix.header">Header</a>
        </h6>
<pre class="programlisting"><span class="comment">// forwards to &lt;boost/spirit/repository/home/qi/directive/confix.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_confix</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
</pre>
<h6>
<a name="spirit_repository.qi_components.directives.confix.h2"></a>
          <span class="phrase"><a name="spirit_repository.qi_components.directives.confix.synopsis"></a></span><a class="link" href="confix.html#spirit_repository.qi_components.directives.confix.synopsis">Synopsis</a>
        </h6>
<pre class="programlisting"><span class="identifier">confix</span><span class="special">(</span><span class="identifier">prefix</span><span class="special">,</span> <span class="identifier">suffix</span><span class="special">)[</span><span class="identifier">subject</span><span class="special">]</span>
</pre>
<h6>
<a name="spirit_repository.qi_components.directives.confix.h3"></a>
          <span class="phrase"><a name="spirit_repository.qi_components.directives.confix.parameters"></a></span><a class="link" href="confix.html#spirit_repository.qi_components.directives.confix.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>
                    <code class="computeroutput"><span class="identifier">prefix</span></code>
                  </p>
                </td>
<td>
                  <p>
                    The parser for the opening (the prefix).
                  </p>
                </td>
</tr>
<tr>
<td>
                  <p>
                    <code class="computeroutput"><span class="identifier">suffix</span></code>
                  </p>
                </td>
<td>
                  <p>
                    The parser for the ending (the suffix).
                  </p>
                </td>
</tr>
<tr>
<td>
                  <p>
                    <code class="computeroutput"><span class="identifier">subject</span></code>
                  </p>
                </td>
<td>
                  <p>
                    The parser for the input sequence between the <code class="computeroutput"><span class="identifier">prefix</span></code>
                    and <code class="computeroutput"><span class="identifier">suffix</span></code> parts.
                  </p>
                </td>
</tr>
</tbody>
</table></div>
<p>
          All three parameters can be arbitrarily complex parsers themselves.
        </p>
<h6>
<a name="spirit_repository.qi_components.directives.confix.h4"></a>
          <span class="phrase"><a name="spirit_repository.qi_components.directives.confix.attribute"></a></span><a class="link" href="confix.html#spirit_repository.qi_components.directives.confix.attribute">Attribute</a>
        </h6>
<p>
          The <code class="computeroutput"><span class="identifier">confix</span></code> directive exposes
          the attribute type of its subject as its own attribute type. If the <code class="computeroutput"><span class="identifier">subject</span></code> does not expose any attribute
          (the type is <code class="computeroutput"><span class="identifier">unused_type</span></code>),
          then the <code class="computeroutput"><span class="identifier">confix</span></code> does not
          expose any attribute either.
        </p>
<pre class="programlisting"><span class="identifier">a</span><span class="special">:</span> <span class="identifier">A</span><span class="special">,</span> <span class="identifier">p</span><span class="special">:</span> <span class="identifier">P</span><span class="special">,</span> <span class="identifier">s</span><span class="special">:</span> <span class="identifier">S</span><span class="special">:</span> <span class="special">--&gt;</span> <span class="identifier">confix</span><span class="special">(</span><span class="identifier">p</span><span class="special">,</span> <span class="identifier">s</span><span class="special">)[</span><span class="identifier">a</span><span class="special">]:</span> <span class="identifier">A</span>
</pre>
<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>
            This notation is used all over the Spirit documentation and reads as:
            Given, <code class="computeroutput"><span class="identifier">a</span></code>, <code class="computeroutput"><span class="identifier">p</span></code>, and <code class="computeroutput"><span class="identifier">s</span></code>
            are parsers, and <code class="computeroutput"><span class="identifier">A</span></code>,
            <code class="computeroutput"><span class="identifier">P</span></code>, and <code class="computeroutput"><span class="identifier">S</span></code> are the types of their attributes,
            then the type of the attribute exposed by <code class="computeroutput"><span class="identifier">confix</span><span class="special">(</span><span class="identifier">p</span><span class="special">,</span> <span class="identifier">s</span><span class="special">)[</span><span class="identifier">a</span><span class="special">]</span></code> will be <code class="computeroutput"><span class="identifier">A</span></code>.
          </p></td></tr>
</table></div>
<h6>
<a name="spirit_repository.qi_components.directives.confix.h5"></a>
          <span class="phrase"><a name="spirit_repository.qi_components.directives.confix.example"></a></span><a class="link" href="confix.html#spirit_repository.qi_components.directives.confix.example">Example</a>
        </h6>
<p>
          The following example shows simple use cases of the <code class="computeroutput"><span class="identifier">confix</span></code>
          directive. We will illustrate its usage by generating parsers for different
          comment styles and for some simple tagged data (for the full example code
          see <a href="../../../../../example/qi/confix.cpp" target="_top">confix.cpp</a>)
        </p>
<h6>
<a name="spirit_repository.qi_components.directives.confix.h6"></a>
          <span class="phrase"><a name="spirit_repository.qi_components.directives.confix.prerequisites"></a></span><a class="link" href="confix.html#spirit_repository.qi_components.directives.confix.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">confix</span></code> directive.
        </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">repository</span><span class="special">/</span><span class="identifier">include</span><span class="special">/</span><span class="identifier">qi_confix</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 import a number of
          elements into the current namespace:
        </p>
<p>
</p>
<pre class="programlisting"><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">eol</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">lexeme</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">ascii</span><span class="special">::</span><span class="identifier">alnum</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">ascii</span><span class="special">::</span><span class="identifier">char_</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">ascii</span><span class="special">::</span><span class="identifier">space</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">qi</span><span class="special">::</span><span class="identifier">parse</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">qi</span><span class="special">::</span><span class="identifier">phrase_parse</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">confix</span><span class="special">;</span>
</pre>
<p>
        </p>
<h6>
<a name="spirit_repository.qi_components.directives.confix.h7"></a>
          <span class="phrase"><a name="spirit_repository.qi_components.directives.confix.parsing_different_comment_styles"></a></span><a class="link" href="confix.html#spirit_repository.qi_components.directives.confix.parsing_different_comment_styles">Parsing
          Different Comment Styles</a>
        </h6>
<p>
          We will show how to parse different comment styles. First we will parse
          a C++ comment:
        </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">bool</span> <span class="identifier">parse_cpp_comment</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="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">&amp;</span> <span class="identifier">attr</span><span class="special">)</span>
<span class="special">{</span>
    <span class="keyword">bool</span> <span class="identifier">r</span> <span class="special">=</span> <span class="identifier">parse</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="identifier">confix</span><span class="special">(</span><span class="string">"//"</span><span class="special">,</span> <span class="identifier">eol</span><span class="special">)[*(</span><span class="identifier">char_</span> <span class="special">-</span> <span class="identifier">eol</span><span class="special">)],</span>  <span class="comment">// grammar</span>
        <span class="identifier">attr</span><span class="special">);</span>                              <span class="comment">// attribute</span>

    <span class="keyword">if</span> <span class="special">(!</span><span class="identifier">r</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>
<p>
          This function will obviously parse input such as "<code class="computeroutput"><span class="comment">//
          This is a comment \n </span></code>". Similarily parsing a 'C'-style
          comment proves to be straightforward:
        </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">bool</span> <span class="identifier">parse_c_comment</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="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">&amp;</span> <span class="identifier">attr</span><span class="special">)</span>
<span class="special">{</span>
    <span class="keyword">bool</span> <span class="identifier">r</span> <span class="special">=</span> <span class="identifier">parse</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="identifier">confix</span><span class="special">(</span><span class="string">"/*"</span><span class="special">,</span> <span class="string">"*/"</span><span class="special">)[*(</span><span class="identifier">char_</span> <span class="special">-</span> <span class="string">"*/"</span><span class="special">)],</span>    <span class="comment">// grammar</span>
        <span class="identifier">attr</span><span class="special">);</span>                                  <span class="comment">// attribute</span>

    <span class="keyword">if</span> <span class="special">(!</span><span class="identifier">r</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>
<p>
          which again will be able to parse e.g. "<code class="computeroutput"><span class="comment">/*
          This is a comment */</span> </code>".
        </p>
<h6>
<a name="spirit_repository.qi_components.directives.confix.h8"></a>
          <span class="phrase"><a name="spirit_repository.qi_components.directives.confix.parsing_tagged_data"></a></span><a class="link" href="confix.html#spirit_repository.qi_components.directives.confix.parsing_tagged_data">Parsing
          Tagged Data</a>
        </h6>
<p>
          Generating a parser that extracts the body from the HTML snippet "<code class="computeroutput"><span class="special">&lt;</span><span class="identifier">b</span><span class="special">&gt;</span><span class="identifier">The</span> <span class="identifier">Body</span><span class="special">&lt;/</span><span class="identifier">b</span><span class="special">&gt;</span></code>"
          is not very hard, either:
        </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">bool</span> <span class="identifier">parse_tagged</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="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">&amp;</span> <span class="identifier">attr</span><span class="special">)</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> <span class="identifier">last</span><span class="special">,</span>
        <span class="identifier">confix</span><span class="special">(</span><span class="string">"&lt;b&gt;"</span><span class="special">,</span> <span class="string">"&lt;/b&gt;"</span><span class="special">)[</span><span class="identifier">lexeme</span><span class="special">[*(</span><span class="identifier">char_</span> <span class="special">-</span> <span class="char">'&lt;'</span><span class="special">)]],</span>  <span class="comment">// grammar</span>
        <span class="identifier">space</span><span class="special">,</span>                                          <span class="comment">// skip</span>
        <span class="identifier">attr</span><span class="special">);</span>                                          <span class="comment">// attribute</span>

    <span class="keyword">if</span> <span class="special">(!</span><span class="identifier">r</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>
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        Distributed under the Boost Software License, Version 1.0. (See accompanying
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