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| Introduction |
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| "Verdana, Arial, Helvetica, sans-serif"><b>Introduction</b></font> |
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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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| <p class="copyright"> |
| Copyright © 1998-2003 Joel de Guzman<br> |
| <br> |
| <font size="2">Use, modification and distribution is subject to the |
| Boost Software License, Version 1.0. (See accompanying file |
| LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)</font> |
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