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<title>Qi Confix Parser Directive</title>
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<div class="section">
<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><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">>></span> <span class="identifier">subject</span> <span class="special">>></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"><</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">></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"><</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">>::</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"><</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">>::</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"><</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">>::</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">>></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">>></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><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 <boost/spirit/repository/home/qi/directive/confix.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_confix</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span>
</pre>
<h6>
<a name="spirit_repository.qi_components.directives.confix.h2"></a>
<span><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><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><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">--></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><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><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"><</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">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">></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><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"><</span><span class="keyword">typename</span> <span class="identifier">Iterator</span><span class="special">></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">&</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"><</span><span class="keyword">typename</span> <span class="identifier">Iterator</span><span class="special">></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">&</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><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"><</span><span class="identifier">b</span><span class="special">></span><span class="identifier">The</span> <span class="identifier">Body</span><span class="special"></</span><span class="identifier">b</span><span class="special">></span></code>"
is not very hard, either:
</p>
<p>
</p>
<pre class="programlisting"><span class="keyword">template</span> <span class="special"><</span><span class="keyword">typename</span> <span class="identifier">Iterator</span><span class="special">></span>
<span class="keyword">bool</span> <span class="identifier">parse_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">&</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">"<b>"</span><span class="special">,</span> <span class="string">"</b>"</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">'<'</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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