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Introduction
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<font size="6" face=
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<p>
Spirit is an object-oriented recursive-descent parser generator framework
implemented using template meta-programming techniques. Expression
templates allow us to approximate the syntax of Extended Backus-Normal
Form (EBNF) completely in C++.
</p>
<p>
The Spirit framework enables a target grammar to be written exclusively
in C++. Inline EBNF grammar specifications can mix freely with other C++
code and, thanks to the generative power of C++ templates, are
immediately executable. In retrospect, 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>
A simple EBNF grammar snippet:
</p>
<pre><code><font color="#000000"> </font></code><code><font color="#000000"><span class="identifier">group </span> <span class="special">::=</span> <span class="literal">'('</span> <span class="identifier">expression</span> <span class="literal">')'
</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="literal">'*'</span> <span class=
"identifier">factor</span><span class="special">)</span> <span class=
"special">|</span> <span class="special">(</span><span class=
"literal">'/'</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="literal">'+'</span> <span class=
"identifier">term</span><span class="special">)</span> <span class=
"special">|</span> <span class="special">(</span><span class=
"literal">'-'</span> <span class="identifier">term</span><span class=
"special">))*</span></font></code></pre>
<p>
is approximated using Spirit's facilities as seen in this code snippet:
</p>
<pre><code><font color="#000000"> </font></code><code><font color="#000000"><span class=
"identifier">group </span> <span class=
"special">=</span> <span class="literal">'('</span> <span class=
"special">>></span> <span class=
"identifier">expression</span> <span class=
"special">>></span> <span class="literal">')'</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=
"literal">'*'</span> <span class="special">>></span> <span class=
"identifier">factor</span><span class="special">)</span> <span class=
"special">|</span> <span class="special">(</span><span class=
"literal">'/'</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=
"literal">'+'</span> <span class="special">>></span> <span class=
"identifier">term</span><span class="special">)</span> <span class=
"special">|</span> <span class="special">(</span><span class=
"literal">'-'</span> <span class="special">>></span> <span class=
"identifier">term</span><span class="special">));</span></font></code>
</pre>
<p>
Through the magic of expression templates, this is perfectly valid and
executable C++ code. The production rule <tt>expression</tt> is in fact
an object that has a member function parse 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 <tt>integer</tt> invoked by
<tt>factor</tt>. The production rule <tt>expression</tt> in our grammar
specification, traditionally called the start symbol, can recognize
inputs such as:
</p>
<pre><code><font color="#000000"> </font></code><span class="number">12345
</span><code><font color="#000000"> </font></code><span class="special">-</span><span class="number">12345
</span><code><font color="#000000"> </font></code><span class="special">+</span><span class="number">12345
</span><code><font color="#000000"> </font></code><span class="number">1</span> <span class=
"special">+</span> <span class="number">2
</span><code><font color="#000000"> </font></code><span class="number">1</span> <span class=
"special">*</span> <span class="number">2
</span><code><font color="#000000"> </font></code><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><code><font color="#000000"> </font></code><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><code><font color="#000000"> </font></code><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><code><font color="#000000"> </font></code><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><code><font color="#000000"> </font></code><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><code><font color="#000000"> </font></code><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><code><font color="#000000"> </font></code><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 done some modifications to the original EBNF syntax.
This is done to conform to C++ syntax rules. Most notably we see the
abundance of shift <tt>>></tt> operators. Since there are no
'empty' operators in C++, it is simply not possible to write something
like:
</p>
<pre><code><font color="#000000"> </font></code><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). The
framework uses the shift <tt class="operators">>></tt> operator
instead for this purpose. We take the <tt class="operators">>></tt>
operator, with arrows pointing to the right, to mean "is followed by".
Thus we write:
</p>
<pre><code><font color="#000000"> </font></code><span class=
"identifier">a</span> <span class="special">>></span> <span class=
"identifier">b</span>
</pre>
<p>
The alternative operator <tt class="operators">|</tt> and the parentheses
<tt class="operators">()</tt> remain as is. The assignment operator
<tt class="operators">=</tt> is used in place of EBNF's <tt class=
"operators">::=</tt>. Last but not least, the Kleene star <tt class=
"operators">*</tt> which used to be a postfix operator in EBNF becomes a
prefix. Instead of:
</p>
<pre><code><font color="#000000"> </font></code><span class="identifier">a</span><span class=
"special">*</span> <span class="comment">//... in EBNF syntax,</span>
</pre>
<p>
we write:
</p>
<pre><code><font color="#000000"> </font></code><span class="special">*</span><span class=
"identifier">a</span> <span class="comment">//... in Spirit.</span>
</pre>
<p>
since there are no postfix stars, "<tt class="operators">*</tt>", in
C/C++. Finally, we terminate each rule with the ubiquitous semi-colon,
"<tt>;</tt>".
</p>
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Copyright © 1998-2003 Joel de Guzman<br>
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<font size="2">Use, modification and distribution is subject to the
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