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Joel de Guzman 994d4e48cc moving stuff to classic spirit 
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<title>The Grammar</title>
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<font size="6" face="Verdana, Arial, Helvetica, sans-serif"><b>The Grammar</b></font>
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<p>The <b>grammar</b> encapsulates a set of rules. The <tt>grammar</tt> class
is a protocol base class. It is essentially an interface contract. The <tt>grammar</tt>
is a template class that is parameterized by its derived class, <tt>DerivedT</tt>,
and its context, <tt>ContextT</tt>. The template parameter ContextT defaults
to <tt>parser_context</tt>, a predefined context. </p>
<p>You need not be concerned at all with the ContextT template parameter unless
you wish to tweak the low level behavior of the grammar. Detailed information
on the ContextT template parameter is provided <a href="indepth_the_parser_context.html">elsewhere</a>.
The <tt>grammar</tt> relies on the template parameter DerivedT, a grammar subclass
to define the actual rules.</p>
<p>Presented below is the public API. There may actually be more template parameters
after <tt>ContextT</tt>. Everything after the <tt>ContextT</tt> parameter should
not be of concern to the client and are strictly for internal use only.</p>
<pre><code><font color="#000000"><span class=identifier> </span><span class=keyword>template</span><span class=special>&lt;
</span><span class=keyword>typename </span><span class=identifier>DerivedT</span><span class=special>,
</span><span class=keyword>typename </span><span class=identifier>ContextT </span><span class=special>= </span><span class=identifier>parser_context</span><span class=special>&lt;</span><span class=special>&gt; &gt;
</span><span class=keyword>struct </span><span class=identifier>grammar</span><span class=special>;</span></font></code></pre>
<h2>Grammar definition</h2>
<p>A concrete sub-class inheriting from <tt>grammar</tt> is expected to have a
nested template class (or struct) named <tt>definition</tt>:</p>
<blockquote>
<p><img src="theme/bullet.gif" width="13" height="13"> It is a nested template
class with a typename <tt>ScannerT</tt> parameter.<br>
<img src="theme/bullet.gif" width="13" height="13"> Its constructor defines
the grammar rules.<br>
<img src="theme/bullet.gif" width="13" height="13"> Its constructor is passed
in a reference to the actual grammar <tt>self</tt>.<br>
<img src="theme/bullet.gif" width="13" height="13"> It has a member function
named <tt>start</tt> that returns a reference to the start <tt>rule</tt>.</p>
</blockquote>
<h2>Grammar skeleton</h2>
<pre><code><font color="#000000"><span class=special> </span><span class=keyword>struct </span><span class=identifier>my_grammar </span><span class=special>: </span><span class=keyword>public </span><span class=identifier>grammar</span><span class=special>&lt;</span><span class=identifier>my_grammar</span><span class=special>&gt;
</span><span class=special>{
</span><span class=keyword>template </span><span class=special>&lt;</span><span class=keyword>typename </span><span class=identifier>ScannerT</span><span class=special>&gt;
</span><span class=keyword>struct </span><span class=identifier>definition
</span><span class=special>{
</span><span class=identifier>rule</span><span class=special>&lt;</span><span class=identifier>ScannerT</span><span class=special>&gt; </span><span class=identifier>r</span><span class=special>;
</span><span class=identifier>definition</span><span class=special>(</span><span class=identifier>my_grammar </span><span class=keyword>const</span><span class=special>& </span><span class=identifier>self</span><span class=special>) </span><span class=special>{ </span><span class=identifier>r </span><span class=special>= </span><span class=comment>/*..define here..*/</span><span class=special>; </span><span class=special>}
</span><span class=identifier>rule</span><span class=special>&lt;</span><span class=identifier>ScannerT</span><span class=special>&gt; </span><span class=keyword>const</span><span class=special>& </span><span class=identifier>start</span><span class=special>() </span><span class=keyword>const </span><span class=special>{ </span><span class=keyword>return </span><span class=identifier>r</span><span class=special>; </span><span class=special>}
</span><span class=special>};
</span><span class=special>};</span></font></code></pre>
<p>Decoupling the scanner type from the rules that form a grammar allows the grammar
to be used in different contexts possibly using different scanners. We do not
care what scanner we are dealing with. The user-defined <tt>my_grammar</tt>
can be used with <b>any</b> type of scanner. Unlike the rule, the grammar is
not tied to a specific scanner type. See <a href="faq.html#scanner_business">&quot;Scanner
Business&quot;</a> to see why this is important and to gain further understanding
on this scanner-rule coupling problem.</p>
<h2>Instantiating and using my_grammar</h2>
<p>Our grammar above may be instantiated and put into action:</p>
<pre><code><font color="#000000"><span class=special> </span><span class=identifier>my_grammar </span><span class=identifier>g</span><span class=special>;
</span><span class=keyword>if </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>g</span><span class=special>, </span><span class=identifier>space_p</span><span class=special>).</span><span class=identifier>full</span><span class=special>)
</span><span class=identifier>cout </span><span class=special>&lt;&lt; </span><span class=string>"parsing succeeded\n"</span><span class=special>;
</span><span class=keyword>else
</span><span class=identifier>cout </span><span class=special>&lt;&lt; </span><span class=string>"parsing failed\n"</span><span class=special>;</span></font></code></pre>
<p><tt>my_grammar</tt> <b>IS-A </b>parser and can be used anywhere a parser is
expected, even referenced by another rule:</p>
<pre><code><font color="#000000"><span class=special> </span><span class=identifier>rule</span><span class=special>&lt;&gt; </span><span class=identifier>r </span><span class=special>= </span><span class=identifier>g </span><span class=special>&gt;&gt; </span><span class=identifier>str_p</span><span class=special>(</span><span class=string>"cool huh?"</span><span class=special>);</span></font></code></pre>
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<td class="note_box"><img src="theme/alert.gif" width="16" height="16"> <b>Referencing
grammars<br>
</b><br>
Like the rule, the grammar is also held by reference when it is placed in
the right hand side of an EBNF expression. It is the responsibility of the
client to ensure that the referenced grammar stays in scope and does not
get destructed while it is being referenced. </td>
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</table>
<h2><a name="full_grammar"></a>Full Grammar Example</h2>
<p>Recalling our original calculator example, here it is now rewritten using a
grammar:</p>
<pre><code><font color="#000000"><span class=special> </span><span class=keyword>struct </span><span class=identifier>calculator </span><span class=special>: </span><span class=keyword>public </span><span class=identifier>grammar</span><span class=special>&lt;</span><span class=identifier>calculator</span><span class=special>&gt;
</span><span class=special>{
</span><span class=keyword>template </span><span class=special>&lt;</span><span class=keyword>typename </span><span class=identifier>ScannerT</span><span class=special>&gt;
</span><span class=keyword>struct </span><span class=identifier>definition
</span><span class=special>{
</span><span class=identifier>definition</span><span class=special>(</span><span class=identifier>calculator </span><span class=keyword>const</span><span class=special>& </span><span class=identifier>self</span><span class=special>)
</span><span class=special>{
</span><span class=identifier>group </span><span class=special>= </span><span class=literal>'(' </span><span class=special>&gt;&gt; </span><span class=identifier>expression </span><span class=special>&gt;&gt; </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>&gt;&gt; </span><span class=special>*((</span><span class=literal>'*' </span><span class=special>&gt;&gt; </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>&gt;&gt; </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>&gt;&gt; </span><span class=special>*((</span><span class=literal>'+' </span><span class=special>&gt;&gt; </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>&gt;&gt; </span><span class=identifier>term</span><span class=special>));
</span><span class=special>}
</span><span class=identifier>rule</span><span class=special>&lt;</span><span class=identifier>ScannerT</span><span class=special>&gt; </span><span class=identifier>expression</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=identifier>group</span><span class=special>;
</span><span class=identifier>rule</span><span class=special>&lt;</span><span class=identifier>ScannerT</span><span class=special>&gt; </span><span class=keyword>const</span><span class=special>&
</span><span class=identifier>start</span><span class=special>() </span><span class=keyword>const </span><span class=special>{ </span><span class=keyword>return </span><span class=identifier>expression</span><span class=special>; </span><span class=special>}
</span><span class=special>};
</span><span class=special>};</span></font></code></pre>
<p><img src="theme/lens.gif" width="15" height="16"> A fully working example with
<a href="semantic_actions.html">semantic actions</a> can be <a href="../example/fundamental/calc_plain.cpp">viewed
here</a>. This is part of the Spirit distribution. </p>
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<td class="note_box"><img src="theme/lens.gif" width="15" height="16"> <b>self</b><br>
<br>
You might notice that the definition of the grammar has a constructor that
accepts a const reference to the outer grammar. In the example above, notice
that <tt>calculator::definition</tt> takes in a <tt>calculator const&amp;
self</tt>. While this is unused in the example above, in many cases, this
is very useful. The self argument is the definition's window to the outside
world. For example, the calculator class might have a reference to some
state information that the definition can update while parsing proceeds
through <a href="semantic_actions.html">semantic actions</a>. </td>
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<h2>Grammar Capsules</h2>
<p>As a grammar becomes complicated, it is a good idea to group parts into logical
modules. For instance, when writing a language, it might be wise to put expressions
and statements into separate grammar capsules. The grammar takes advantage of
the encapsulation properties of C++ classes. The declarative nature of classes
makes it a perfect fit for the definition of grammars. Since the grammar is
nothing more than a class declaration, we can conveniently publish it in header
files. The idea is that once written and fully tested, a grammar can be reused
in many contexts. We now have the notion of grammar libraries.</p>
<h2><a name="multithreading"></a>Reentrancy and multithreading</h2>
<p>An instance of a grammar may be used in different places multiple times without
any problem. The implementation is tuned to allow this at the expense of some
overhead. However, we can save considerable cycles and bytes if we are certain
that a grammar will only have a single instance. If this is desired, simply
define <tt>BOOST_SPIRIT_SINGLE_GRAMMAR_INSTANCE</tt> before including any spirit
header files.</p>
<pre><font face="Courier New, Courier, mono"><code><span class="preprocessor"> #define</span></code></font><span class="preprocessor"><code><font face="Courier New, Courier, mono"> </font><tt>BOOST_SPIRIT_SINGLE_GRAMMAR_INSTANCE</tt></code></span></pre>
<p> On the other hand, if a grammar is intended to be used in multithreaded code,
we should then define <tt>BOOST_SPIRIT_THREADSAFE</tt> before including any
spirit header files. In this case it will also be required to link against <a href="http://www.boost.org/libs/thread/doc/index.html">Boost.Threads</a></p>
<pre><font face="Courier New, Courier, mono"><span class="preprocessor"> #define</span></font> <span class="preprocessor"><tt>BOOST_SPIRIT_THREADSAFE</tt></span></pre>
<h2>Using more than one grammar start rule </h2>
<p>Sometimes it is desirable to have more than one visible entry point to a grammar
(apart from the start rule). To allow additional start points, Spirit provides
a helper template <tt>grammar_def</tt>, which may be used as a base class for
the <tt>definition</tt> subclass of your <tt>grammar</tt>. Here's an example:</p>
<pre><code> <span class="comment">// this header has to be explicitly included</span>
<span class="preprocessor">#include</span> <span class="string">&lt;boost/spirit/utility/grammar_def.hpp&gt;</span>
</span><span class=keyword>struct </span><span class=identifier>calculator2 </span><span class=special>: </span><span class=keyword>public </span><span class=identifier>grammar</span><span class=special>&lt;</span><span class=identifier>calculator2</span><span class=special>&gt;
{
</span> <span class="keyword">enum</span>
{
expression = 0,
term = 1,
factor = 2,
};
<span class=special> </span><span class=keyword>template </span><span class=special>&lt;</span><span class=keyword>typename </span><span class=identifier>ScannerT</span><span class=special>&gt;
</span><span class=keyword>struct </span><span class=identifier>definition
</span><span class="special">:</span> <span class="keyword">public</span><span class=identifier> grammar_def</span><span class="special">&lt;</span><span class=identifier>rule</span><span class=special>&lt;</span><span class=identifier>ScannerT</span><span class=special>&gt;,</span> same<span class="special">,</span> same<span class="special">&gt;</span>
<span class=special>{</span>
<span class=identifier>definition</span><span class=special>(</span><span class=identifier>calculator2 </span><span class=keyword>const</span><span class=special>& </span><span class=identifier>self</span><span class=special>)
{
</span><span class=identifier>group </span><span class=special>= </span><span class=literal>'(' </span><span class=special>&gt;&gt; </span><span class=identifier>expression </span><span class=special>&gt;&gt; </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>&gt;&gt; *((</span><span class=literal>'*' </span><span class=special>&gt;&gt; </span><span class=identifier>factor</span><span class=special>) | (</span><span class=literal>'/' </span><span class=special>&gt;&gt; </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>&gt;&gt; *((</span><span class=literal>'+' </span><span class=special>&gt;&gt; </span><span class=identifier>term</span><span class=special>) | (</span><span class=literal>'-' </span><span class=special>&gt;&gt; </span><span class=identifier>term</span><span class=special>));</span>
<span class="keyword">this</span><span class="special">-&gt;</span>start_parsers<span class="special">(</span>expression<span class="special">,</span> term<span class="special">,</span> factor<span class="special">);</span>
<span class="special">}</span>
<span class=identifier>rule</span><span class=special>&lt;</span><span class=identifier>ScannerT</span><span class=special>&gt; </span><span class=identifier>expression</span><span class=special>, </span><span class=identifier>term</span><span class=special>, </span><span class=identifier>factor, group</span><span class=special>;
</span><span class=special> };
};</span></font></code></pre>
<p>The <tt>grammar_def</tt> template has to be instantiated with the types of
all the rules you wish to make visible from outside the <tt>grammar</tt>:</p>
<pre><code><span class=identifier> </span><span class=identifier>grammar_def</span><span class="special">&lt;</span><span class=identifier>rule</span><span class=special>&lt;</span><span class=identifier>ScannerT</span><span class=special>&gt;,</span> same<span class="special">,</span> same<span class="special">&gt;</span></code> </pre>
<p>The shorthand notation <tt>same</tt> is used to indicate that the same type
be used as specified by the previous template parameter (e.g. <code><tt>rule&lt;ScannerT&gt;</tt></code>).
Obviously, <tt>same</tt> may not be used as the first template parameter. </p>
<table width="80%" border="0" align="center">
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<td class="note_box"> <img src="theme/bulb.gif" width="13" height="18"> <strong>grammar_def
start types</strong><br>
<br>
It may not be obvious, but it is interesting to note that aside from rule&lt;&gt;s,
any parser type may be specified (e.g. chlit&lt;&gt;, strlit&lt;&gt;, int_parser&lt;&gt;,
etc.).</td>
</tr>
</table>
<p>Using the grammar_def class, there is no need to provide a <tt>start()</tt>member
function anymore. Instead, you'll have to insert a call to the <tt>this-&gt;start_parsers()</tt>
(which is a member function of the <tt>grammar_def</tt> template) to define
the start symbols for your <tt>grammar</tt>. <img src="theme/note.gif" width="16" height="16">
Note that the number and the sequence of the rules used as the parameters to
the <tt>start_parsers()</tt> function should match the types specified in the
<tt>grammar_def</tt> template:</p>
<pre><code> <span class="keyword">this</span><span class="special">-&gt;</span>start_parsers<span class="special">(</span>expression<span class="special">,</span> term<span class="special">,</span> factor<span class="special">);</span></code></pre>
<p> The grammar entry point may be specified using the following syntax:</p>
<pre><code><font color="#000000"><span class=identifier> g</span><span class="special">.</span><span class=identifier>use_parser</span><span class="special">&lt;</span><span class=identifier>N</span><span class=special>&gt;() </span><span class="comment">// Where g is your grammar and N is the Nth entry.</span></font></code></pre>
<p>This sample shows how to use the <tt>term</tt> rule from the <tt>calculator2</tt>
grammar above:</p>
<pre><code><font color="#000000"><span class=identifier> calculator2 g</span><span class=special>;
</span><span class=keyword>if </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>g</span><span class="special">.</span><span class=identifier>use_parser</span><span class="special">&lt;</span><span class=identifier>calculator2::term</span><span class=special>&gt;(),</span><span class=identifier>
space_p</span><span class=special>
).</span><span class=identifier>full</span><span class=special>)
{
</span><span class=identifier>cout </span><span class=special>&lt;&lt; </span><span class=string>"parsing succeeded\n"</span><span class=special>;
}
</span><span class=keyword>else</span> <span class="special">{</span>
<span class=identifier>cout </span><span class=special>&lt;&lt; </span><span class=string>"parsing failed\n"</span><span class=special>;
}</span></font></code></pre>
<p>The template parameter for the <tt>use_parser&lt;&gt;</tt> template type should
be the zero based index into the list of rules specified in the <tt>start_parsers()</tt>
function call. </p>
<table width="80%" border="0" align="center">
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<td class="note_box"><img src="theme/note.gif" width="16" height="16"> <tt><strong>use_parser&lt;0&gt;</strong></tt><br>
<br>
Note, that using <span class="literal">0</span> (zero) as the template parameter
to <tt>use_parser</tt> is equivalent to using the start rule, exported by
conventional means through the <tt>start()</tt> function, as shown in the
first <tt><a href="grammar.html#full_grammar">calculator</a></tt> sample
above. So this notation may be used even for grammars exporting one rule
through its <tt>start()</tt> function only. On the other hand, calling a
<tt>grammar</tt> without the <tt>use_parser</tt> notation will execute the
rule specified as the first parameter to the <tt>start_parsers()</tt> function.
</td>
</tr>
</table>
<p>The maximum number of usable start rules is limited by the preprocessor constant:</p>
<pre> <span class="identifier">BOOST_SPIRIT_GRAMMAR_STARTRULE_TYPE_LIMIT</span> <span class="comment">// defaults to 3</span></pre>
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<br>
<hr size="1">
<p class="copyright">Copyright &copy; 1998-2003 Joel de Guzman<br>
Copyright &copy; 2003-2004 Hartmut Kaiser <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> </p>
<p>&nbsp;</p>
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