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<div class="titlepage"><div><div><h5 class="title">
<a name="spirit.qi.reference.parser_concepts.nonterminal"></a><a class="link" href="nonterminal.html" title="Nonterminal">Nonterminal</a>
</h5></div></div></div>
<a name="spirit.qi.reference.parser_concepts.nonterminal.description"></a><h6>
<a name="id886278"></a>
<a class="link" href="nonterminal.html#spirit.qi.reference.parser_concepts.nonterminal.description">Description</a>
</h6>
<p>
A Nonterminal is a symbol in a <a class="link" href="../../../abstracts/parsing_expression_grammar.html" title="Parsing Expression Grammar">Parsing
Expression Grammar</a> production that represents a grammar fragment.
Nonterminals may self reference to specify recursion. This is one of
the most important concepts and the reason behind the word "recursive"
in recursive descent parsing.
</p>
<a name="spirit.qi.reference.parser_concepts.nonterminal.refinement_of"></a><h6>
<a name="id886304"></a>
<a class="link" href="nonterminal.html#spirit.qi.reference.parser_concepts.nonterminal.refinement_of">Refinement
of</a>
</h6>
<div class="blockquote"><blockquote class="blockquote"><p>
<a class="link" href="parser.html" title="Parser"><code class="computeroutput"><span class="identifier">Parser</span></code></a>
</p></blockquote></div>
<a name="spirit.qi.reference.parser_concepts.nonterminal.signature"></a><h6>
<a name="id886339"></a>
<a class="link" href="nonterminal.html#spirit.qi.reference.parser_concepts.nonterminal.signature">Signature</a>
</h6>
<p>
Nonterminals can have both synthesized and inherited attributes. The
Nonterminal's <span class="emphasis"><em>Signature</em></span> specifies both the synthesized
and inherited attributes. The specification uses the function declarator
syntax:
</p>
<pre class="programlisting"><span class="identifier">RT</span><span class="special">(</span><span class="identifier">A0</span><span class="special">,</span> <span class="identifier">A1</span><span class="special">,</span> <span class="identifier">A2</span><span class="special">,</span> <span class="special">...,</span> <span class="identifier">AN</span><span class="special">)</span>
</pre>
<p>
where <code class="computeroutput"><span class="identifier">RT</span></code> is the Nonterminal's
synthesized attribute and <code class="computeroutput"><span class="identifier">A0</span></code>
... <code class="computeroutput"><span class="identifier">AN</span></code> are the Nonterminal's
inherited attributes.
</p>
<a name="spirit.qi.reference.parser_concepts.nonterminal.attributes"></a><h6>
<a name="id886440"></a>
<a class="link" href="nonterminal.html#spirit.qi.reference.parser_concepts.nonterminal.attributes">Attributes</a>
</h6>
<p>
The Nonterminal models a C++ function. The Nonterminal's synthesized
attribute is analogous to the function return value and its inherited
attributes are analogous to function arguments. The inherited attributes
(arguments) can be passed in just like any <a class="link" href="../basics.html#spirit.qi.reference.basics.lazy_argument">Lazy
Argument</a>, e.g.:
</p>
<pre class="programlisting"><span class="identifier">r</span><span class="special">(</span><span class="identifier">expr</span><span class="special">)</span> <span class="comment">// Evaluate expr at parse time and pass the result to the Nonterminal r
</span></pre>
<a name="spirit.qi.reference.parser_concepts.nonterminal._code__phrase_role__identifier___val__phrase___code_"></a><h6>
<a name="id886486"></a>
<a class="link" href="nonterminal.html#spirit.qi.reference.parser_concepts.nonterminal._code__phrase_role__identifier___val__phrase___code_"><code class="computeroutput"><span class="identifier">_val</span></code></a>
</h6>
<p>
The <code class="computeroutput"><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">_val</span></code> placeholder can be used in <a href="../../../../../../phoenix/doc/html/index.html" target="_top">Phoenix</a> semantic actions
anywhere in the Nonterminal's definition. This <a href="../../../../../../phoenix/doc/html/index.html" target="_top">Phoenix</a>
placeholder refers to the Nonterminal's (synthesized) attribute. The
<code class="computeroutput"><span class="identifier">_val</span></code> placeholder acts
like a mutable reference to the Nonterminal's attribute.
</p>
<a name="spirit.qi.reference.parser_concepts.nonterminal._code__phrase_role__identifier___r1__phrase___code_______code__phrase_role__identifier__r10__phrase___code_"></a><h6>
<a name="id886554"></a>
<a class="link" href="nonterminal.html#spirit.qi.reference.parser_concepts.nonterminal._code__phrase_role__identifier___r1__phrase___code_______code__phrase_role__identifier__r10__phrase___code_"><code class="computeroutput"><span class="identifier">_r1</span></code> ... <code class="computeroutput"><span class="identifier">r10</span></code></a>
</h6>
<p>
The <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">spirit</span><span class="special">::</span><span class="identifier">_r1</span></code> ... <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">spirit</span><span class="special">::</span><span class="identifier">r10</span></code>
placeholders can be used in <a href="../../../../../../phoenix/doc/html/index.html" target="_top">Phoenix</a>
semantic actions anywhere in the Nonterminal's definition. These <a href="../../../../../../phoenix/doc/html/index.html" target="_top">Phoenix</a> placeholders
refer to the Nonterminal's inherited attributes.
</p>
<a name="spirit.qi.reference.parser_concepts.nonterminal.locals"></a><h6>
<a name="id886635"></a>
<a class="link" href="nonterminal.html#spirit.qi.reference.parser_concepts.nonterminal.locals">Locals</a>
</h6>
<p>
Nonterminals can have local variables that will be created on the stack
at parse time. A locals descriptor added to the Nonterminal declaration
will give the Nonterminal local variables:
</p>
<pre class="programlisting"><span class="keyword">template</span> <span class="special"><</span><span class="keyword">typename</span> <span class="identifier">T0</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">T1</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">T2</span><span class="special">,</span> <span class="special">...,</span> <span class="keyword">typename</span> <span class="identifier">TN</span><span class="special">></span>
<span class="keyword">struct</span> <span class="identifier">locals</span><span class="special">;</span>
</pre>
<p>
where <code class="computeroutput"><span class="identifier">T0</span></code> ... <code class="computeroutput"><span class="identifier">TN</span></code> are the types of local variables
accessible in your <a href="../../../../../../phoenix/doc/html/index.html" target="_top">Phoenix</a>
semantic actions using the placeholders:
</p>
<div class="itemizedlist"><ul class="itemizedlist" type="disc">
<li class="listitem">
<code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">spirit</span><span class="special">::</span><span class="identifier">_a</span></code>
</li>
<li class="listitem">
<code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">spirit</span><span class="special">::</span><span class="identifier">_b</span></code>
</li>
<li class="listitem">
<code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">spirit</span><span class="special">::</span><span class="identifier">_c</span></code>
</li>
<li class="listitem">
<code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">spirit</span><span class="special">::</span><span class="identifier">_d</span></code>
</li>
<li class="listitem">
<code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">spirit</span><span class="special">::</span><span class="identifier">_e</span></code>
</li>
<li class="listitem">
<code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">spirit</span><span class="special">::</span><span class="identifier">_f</span></code>
</li>
<li class="listitem">
<code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">spirit</span><span class="special">::</span><span class="identifier">_g</span></code>
</li>
<li class="listitem">
<code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">spirit</span><span class="special">::</span><span class="identifier">_h</span></code>
</li>
<li class="listitem">
<code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">spirit</span><span class="special">::</span><span class="identifier">_i</span></code>
</li>
<li class="listitem">
<code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">spirit</span><span class="special">::</span><span class="identifier">_j</span></code>
</li>
</ul></div>
<p>
which correspond to the Nonterminal's local variables <code class="computeroutput"><span class="identifier">T0</span></code>
... <code class="computeroutput"><span class="identifier">T9</span></code>.
</p>
<div class="variablelist">
<p class="title"><b>Notation</b></p>
<dl>
<dt><span class="term"><code class="computeroutput"><span class="identifier">x</span></code></span></dt>
<dd><p>
A Nonterminal
</p></dd>
<dt><span class="term"><code class="computeroutput"><span class="identifier">X</span></code></span></dt>
<dd><p>
A Nonterminal type
</p></dd>
<dt><span class="term"><code class="computeroutput"><span class="identifier">arg1</span></code>, <code class="computeroutput"><span class="identifier">arg2</span></code>, ..., <code class="computeroutput"><span class="identifier">argN</span></code></span></dt>
<dd><p>
<a class="link" href="../basics.html#spirit.qi.reference.basics.lazy_argument">Lazy Arguments</a>
that evaluate to each of the Nonterminal's inherited attributes.
</p></dd>
</dl>
</div>
<a name="spirit.qi.reference.parser_concepts.nonterminal.valid_expressions"></a><h6>
<a name="id887151"></a>
<a class="link" href="nonterminal.html#spirit.qi.reference.parser_concepts.nonterminal.valid_expressions">Valid
Expressions</a>
</h6>
<p>
In addition to the requirements defined in <a class="link" href="parser.html" title="Parser"><code class="computeroutput"><span class="identifier">Parser</span></code></a>, for any Nonterminal
the following must be met:
</p>
<div class="informaltable"><table class="table">
<colgroup>
<col>
<col>
<col>
</colgroup>
<thead><tr>
<th>
<p>
Expression
</p>
</th>
<th>
<p>
Semantics
</p>
</th>
<th>
<p>
Return type
</p>
</th>
</tr></thead>
<tbody>
<tr>
<td>
<p>
<code class="computeroutput"><span class="identifier">x</span></code>
</p>
</td>
<td>
<p>
In a parser expression, invoke Nonterminal <code class="computeroutput"><span class="identifier">x</span></code>
</p>
</td>
<td>
<p>
<code class="computeroutput"><span class="identifier">X</span></code>
</p>
</td>
</tr>
<tr>
<td>
<p>
<code class="computeroutput"><span class="identifier">x</span><span class="special">(</span><span class="identifier">arg1</span><span class="special">,</span>
<span class="identifier">arg2</span><span class="special">,</span>
<span class="special">...,</span> <span class="identifier">argN</span><span class="special">)</span></code>
</p>
</td>
<td>
<p>
In a parser expression, invoke Nonterminal <code class="computeroutput"><span class="identifier">r</span></code>
passing in inherited attributes <code class="computeroutput"><span class="identifier">arg1</span></code>
... <code class="computeroutput"><span class="identifier">argN</span></code>
</p>
</td>
<td>
<p>
<code class="computeroutput"><span class="identifier">X</span></code>
</p>
</td>
</tr>
<tr>
<td>
<p>
<code class="computeroutput"><span class="identifier">x</span><span class="special">.</span><span class="identifier">name</span><span class="special">(</span><span class="identifier">name</span><span class="special">)</span></code>
</p>
</td>
<td>
<p>
Naming a Nonterminal.
</p>
</td>
<td>
<p>
<code class="computeroutput"><span class="keyword">void</span></code>
</p>
</td>
</tr>
<tr>
<td>
<p>
<code class="computeroutput"><span class="identifier">x</span><span class="special">.</span><span class="identifier">name</span><span class="special">()</span></code>
</p>
</td>
<td>
<p>
Getting the name of a Nonterminal.
</p>
</td>
<td>
<p>
<code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span></code>
</p>
</td>
</tr>
<tr>
<td>
<p>
debug(x)
</p>
</td>
<td>
<p>
Debug Nonterminal <code class="computeroutput"><span class="identifier">x</span></code>.
</p>
</td>
<td>
<p>
<code class="computeroutput"><span class="keyword">void</span></code>
</p>
</td>
</tr>
</tbody>
</table></div>
<a name="spirit.qi.reference.parser_concepts.nonterminal.type_expressions"></a><h6>
<a name="id888136"></a>
<a class="link" href="nonterminal.html#spirit.qi.reference.parser_concepts.nonterminal.type_expressions">Type
Expressions</a>
</h6>
<div class="informaltable"><table class="table">
<colgroup>
<col>
<col>
</colgroup>
<thead><tr>
<th>
<p>
Expression
</p>
</th>
<th>
<p>
Description
</p>
</th>
</tr></thead>
<tbody>
<tr>
<td>
<p>
<code class="computeroutput"><span class="identifier">X</span><span class="special">::</span><span class="identifier">sig_type</span></code>
</p>
</td>
<td>
<p>
The Signature of <code class="computeroutput"><span class="identifier">X</span></code>:
An <a href="../../../../../../../../libs/mpl/doc/refmanual/forward-sequence.html" target="_top">MPL
Forward Sequence</a>. The first element is the Nonterminal's
synthesized attribute type and the rest are the inherited attribute
types.
</p>
</td>
</tr>
<tr>
<td>
<p>
<code class="computeroutput"><span class="identifier">X</span><span class="special">::</span><span class="identifier">locals_type</span></code>
</p>
</td>
<td>
<p>
The local variables of <code class="computeroutput"><span class="identifier">X</span></code>:
An <a href="../../../../../../../../libs/mpl/doc/refmanual/forward-sequence.html" target="_top">MPL
Forward Sequence</a>.
</p>
</td>
</tr>
</tbody>
</table></div>
<a name="spirit.qi.reference.parser_concepts.nonterminal.models"></a><h6>
<a name="id888275"></a>
<a class="link" href="nonterminal.html#spirit.qi.reference.parser_concepts.nonterminal.models">Models</a>
</h6>
<div class="itemizedlist"><ul class="itemizedlist" type="disc">
<li class="listitem">
<a class="link" href="../nonterminal/rule.html" title="Rule">rule</a>
</li>
<li class="listitem">
<a class="link" href="../nonterminal/grammar.html" title="Grammar">grammar</a>
</li>
</ul></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 © 2001-2010 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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