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- <div class="titlepage"><div><div><h2 class="title" style="clear: both">
- <a name="spirit_x3.introduction"></a><a class="link" href="introduction.html" title="Introduction">Introduction</a>
- </h2></div></div></div>
- <p>
- Boost Spirit X3 is an object-oriented, recursive-descent parser for C++. It
- allows you to write grammars using a format similar to Extended Backus Naur
- Form (EBNF)<a href="#ftn.spirit_x3.introduction.f0" class="footnote" name="spirit_x3.introduction.f0"><sup class="footnote">[1]</sup></a> directly in C++. These inline grammar specifications can mix freely
- with other C++ code and, thanks to the generative power of C++ templates, are
- immediately executable. Conventional compiler-compilers or parser-generators
- have to perform an additional translation step from the source EBNF code to
- C or C++ code.
- </p>
- <p>
- Since the target input grammars are written entirely in C++ we do not need
- any separate tools to compile, preprocess or integrate those into the build
- process. <a href="http://boost-spirit.com" target="_top">Spirit</a> allows seamless
- integration of the parsing process with other C++ code. This often allows for
- simpler and more efficient code.
- </p>
- <p>
- The created parsers are fully attributed, which allows you to easily build
- and handle hierarchical data structures in memory. These data structures resemble
- the structure of the input data and can directly be used to generate arbitrarily-formatted
- output.
- </p>
- <p>
- A simple EBNF grammar snippet:
- </p>
- <pre class="programlisting"><span class="identifier">group</span> <span class="special">::=</span> <span class="char">'('</span> <span class="identifier">expression</span> <span class="char">')'</span>
- <span class="identifier">factor</span> <span class="special">::=</span> <span class="identifier">integer</span> <span class="special">|</span> <span class="identifier">group</span>
- <span class="identifier">term</span> <span class="special">::=</span> <span class="identifier">factor</span> <span class="special">((</span><span class="char">'*'</span> <span class="identifier">factor</span><span class="special">)</span> <span class="special">|</span> <span class="special">(</span><span class="char">'/'</span> <span class="identifier">factor</span><span class="special">))*</span>
- <span class="identifier">expression</span> <span class="special">::=</span> <span class="identifier">term</span> <span class="special">((</span><span class="char">'+'</span> <span class="identifier">term</span><span class="special">)</span> <span class="special">|</span> <span class="special">(</span><span class="char">'-'</span> <span class="identifier">term</span><span class="special">))*</span>
- </pre>
- <p>
- is approximated using facilities of Spirit's <span class="emphasis"><em>X3</em></span> as seen
- in this code snippet:
- </p>
- <pre class="programlisting"><span class="identifier">group</span> <span class="special">=</span> <span class="char">'('</span> <span class="special">>></span> <span class="identifier">expression</span> <span class="special">>></span> <span class="char">')'</span><span class="special">;</span>
- <span class="identifier">factor</span> <span class="special">=</span> <span class="identifier">integer</span> <span class="special">|</span> <span class="identifier">group</span><span class="special">;</span>
- <span class="identifier">term</span> <span class="special">=</span> <span class="identifier">factor</span> <span class="special">>></span> <span class="special">*((</span><span class="char">'*'</span> <span class="special">>></span> <span class="identifier">factor</span><span class="special">)</span> <span class="special">|</span> <span class="special">(</span><span class="char">'/'</span> <span class="special">>></span> <span class="identifier">factor</span><span class="special">));</span>
- <span class="identifier">expression</span> <span class="special">=</span> <span class="identifier">term</span> <span class="special">>></span> <span class="special">*((</span><span class="char">'+'</span> <span class="special">>></span> <span class="identifier">term</span><span class="special">)</span> <span class="special">|</span> <span class="special">(</span><span class="char">'-'</span> <span class="special">>></span> <span class="identifier">term</span><span class="special">));</span>
- </pre>
- <p>
- Through the magic of expression templates, this is perfectly valid and executable
- C++ code. The production rule <code class="computeroutput"><span class="identifier">expression</span></code>
- is, in fact, an object that has a member function <code class="computeroutput"><span class="identifier">parse</span></code>
- that does the work given a source code written in the grammar that we have
- just declared. Yes, it's a calculator. We shall simplify for now by skipping
- the type declarations and the definition of the rule <code class="computeroutput"><span class="identifier">integer</span></code>
- invoked by <code class="computeroutput"><span class="identifier">factor</span></code>. Now, the
- production rule <code class="computeroutput"><span class="identifier">expression</span></code>
- in our grammar specification, traditionally called the <code class="computeroutput"><span class="identifier">start</span></code>
- symbol, can recognize inputs such as:
- </p>
- <pre class="programlisting"><span class="number">12345</span>
- <span class="special">-</span><span class="number">12345</span>
- <span class="special">+</span><span class="number">12345</span>
- <span class="number">1</span> <span class="special">+</span> <span class="number">2</span>
- <span class="number">1</span> <span class="special">*</span> <span class="number">2</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="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="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">1</span> <span class="special">+</span> <span class="number">2</span><span class="special">)</span> <span class="special">*</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="special">(-</span><span class="number">1</span> <span class="special">+</span> <span class="number">2</span><span class="special">)</span> <span class="special">*</span> <span class="special">(</span><span class="number">3</span> <span class="special">+</span> <span class="special">-</span><span class="number">4</span><span class="special">)</span>
- <span class="number">1</span> <span class="special">+</span> <span class="special">((</span><span class="number">6</span> <span class="special">*</span> <span class="number">200</span><span class="special">)</span> <span class="special">-</span> <span class="number">20</span><span class="special">)</span> <span class="special">/</span> <span class="number">6</span>
- <span class="special">(</span><span class="number">1</span> <span class="special">+</span> <span class="special">(</span><span class="number">2</span> <span class="special">+</span> <span class="special">(</span><span class="number">3</span> <span class="special">+</span> <span class="special">(</span><span class="number">4</span> <span class="special">+</span> <span class="number">5</span><span class="special">))))</span>
- </pre>
- <p>
- Certainly we have modified the original EBNF syntax. This is done to conform
- to C++ syntax rules. Most notably we see the abundance of shift >> operators.
- Since there are no juxtaposition operators in C++, it is simply not possible
- to write something like:
- </p>
- <pre class="programlisting"><span class="identifier">a</span> <span class="identifier">b</span>
- </pre>
- <p>
- as seen in math syntax, for example, to mean multiplication or, in our case,
- as seen in EBNF syntax to mean sequencing (b should follow a). <span class="emphasis"><em>Spirit.X3</em></span>
- uses the shift <code class="computeroutput"><span class="special">>></span></code> operator
- instead for this purpose. We take the <code class="computeroutput"><span class="special">>></span></code>
- operator, with arrows pointing to the right, to mean "is followed by".
- Thus we write:
- </p>
- <pre class="programlisting"><span class="identifier">a</span> <span class="special">>></span> <span class="identifier">b</span>
- </pre>
- <p>
- The alternative operator <code class="computeroutput"><span class="special">|</span></code> and
- the parentheses <code class="computeroutput"><span class="special">()</span></code> remain as is.
- The assignment operator <code class="computeroutput"><span class="special">=</span></code> is used
- in place of EBNF's <code class="computeroutput"><span class="special">::=</span></code>. Last but
- not least, the Kleene star <code class="computeroutput"><span class="special">*</span></code>,
- which in this case is a postfix operator in EBNF becomes a prefix. Instead
- of:
- </p>
- <pre class="programlisting"><span class="identifier">a</span><span class="special">*</span> <span class="comment">//... in EBNF syntax,</span>
- </pre>
- <p>
- we write:
- </p>
- <pre class="programlisting"><span class="special">*</span><span class="identifier">a</span> <span class="comment">//... in Spirit.</span>
- </pre>
- <p>
- since there are no postfix stars, <code class="computeroutput"><span class="special">*</span></code>,
- in C/C++. Finally, we terminate each rule with the ubiquitous semi-colon,
- <code class="computeroutput"><span class="special">;</span></code>.
- </p>
- <div class="footnotes">
- <br><hr style="width:100; text-align:left;margin-left: 0">
- <div id="ftn.spirit_x3.introduction.f0" class="footnote"><p><a href="#spirit_x3.introduction.f0" class="para"><sup class="para">[1] </sup></a>
- <a href="http://www.cl.cam.ac.uk/%7Emgk25/iso-14977.pdf" target="_top">ISO-EBNF</a>
- </p></div>
- </div>
- </div>
- <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
- <td align="left"></td>
- <td align="right"><div class="copyright-footer">Copyright © 2001-2018 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>
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