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<html>
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<title>Karma subrules</title>
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<div class="section">
<div class="titlepage"><div><div><h4 class="title">
<a name="spirit_repository.karma_components.nonterminal.subrule"></a><a class="link" href="subrule.html" title="Karma subrules">Karma
subrules</a>
</h4></div></div></div>
<h6>
<a name="spirit_repository.karma_components.nonterminal.subrule.h0"></a>
<span><a name="spirit_repository.karma_components.nonterminal.subrule.description"></a></span><a class="link" href="subrule.html#spirit_repository.karma_components.nonterminal.subrule.description">Description</a>
</h6>
<p>
The <span class="emphasis"><em>Spirit.Karma</em></span> <code class="computeroutput"><span class="identifier">subrule</span></code>
is a component allowing to create a named generator, and to refer to it
by name -- much like rules and grammars. It is in fact a fully static version
of the rule.
</p>
<p>
The strength of subrules is performance. Replacing some rules with subrules
can make a generator slightly faster (see <a class="link" href="subrule.html#spirit_repository.karma_components.nonterminal.subrule.performance">Performance</a>
below for measurements). The reason is that subrules allow aggressive inlining
by the C++ compiler, whereas the implementation of rules is based on a
virtual function call which, depending on the compiler, can have some run-time
overhead and stop inlining.
</p>
<p>
The weaknesses of subrules are:
</p>
<div class="itemizedlist"><ul class="itemizedlist" type="disc">
<li class="listitem">
subrules can only be defined and used within the same generator expression.
A subrule cannot be defined at one location, and then used in another
location.
</li>
<li class="listitem">
subrules put a massive strain on the C++ compiler. They increase compile
times and memory usage during compilation, and also increase the risk
of hitting compiler limits and/or bugs.
</li>
</ul></div>
<p>
</p>
<pre class="programlisting"><span class="identifier">entry</span> <span class="special">%=</span> <span class="special">(</span>
<span class="identifier">ast_node</span> <span class="special">%=</span> <span class="identifier">int_</span> <span class="special">|</span> <span class="identifier">binary_node</span> <span class="special">|</span> <span class="identifier">unary_node</span>
<span class="special">,</span> <span class="identifier">binary_node</span> <span class="special">%=</span> <span class="char">'('</span> <span class="special"><<</span> <span class="identifier">ast_node</span> <span class="special"><<</span> <span class="identifier">char_</span> <span class="special"><<</span> <span class="identifier">ast_node</span> <span class="special"><<</span> <span class="char">')'</span>
<span class="special">,</span> <span class="identifier">unary_node</span> <span class="special">%=</span> <span class="char">'('</span> <span class="special"><<</span> <span class="identifier">char_</span> <span class="special"><<</span> <span class="identifier">ast_node</span> <span class="special"><<</span> <span class="char">')'</span>
<span class="special">);</span>
</pre>
<p>
</p>
<p>
The example above can be found here: <a href="../../../../../example/karma/calc2_ast_dump_sr.cpp" target="_top">../../example/karma/calc2_ast_dump_sr.cpp</a>
</p>
<p>
As shown in this code snippet (an extract from the calc2_ast_dump_sr example),
subrules can be freely mixed with rules and grammars. Here, a group of
3 subrules (<code class="computeroutput"><span class="identifier">ast_node</span></code>,
<code class="computeroutput"><span class="identifier">binary_node</span></code>, <code class="computeroutput"><span class="identifier">unary_node</span></code>) is assigned to a rule (named
<code class="computeroutput"><span class="identifier">entry</span></code>). This means that
parts of a generator can use subrules (typically the innermost, most performance-critical
parts), whereas the rest can use rules and grammars.
</p>
<h6>
<a name="spirit_repository.karma_components.nonterminal.subrule.h1"></a>
<span><a name="spirit_repository.karma_components.nonterminal.subrule.header"></a></span><a class="link" href="subrule.html#spirit_repository.karma_components.nonterminal.subrule.header">Header</a>
</h6>
<pre class="programlisting"><span class="comment">// forwards to <boost/spirit/repository/home/karma/nonterminal/subrule.hpp></span>
<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">spirit</span><span class="special">/</span><span class="identifier">repository</span><span class="special">/</span><span class="identifier">include</span><span class="special">/</span><span class="identifier">karma_subrule</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span>
</pre>
<h6>
<a name="spirit_repository.karma_components.nonterminal.subrule.h2"></a>
<span><a name="spirit_repository.karma_components.nonterminal.subrule.synopsis__declaration_"></a></span><a class="link" href="subrule.html#spirit_repository.karma_components.nonterminal.subrule.synopsis__declaration_">Synopsis
(declaration)</a>
</h6>
<pre class="programlisting"><span class="identifier">subrule</span><span class="special"><</span><span class="identifier">ID</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="identifier">sr</span><span class="special">(</span><span class="identifier">name</span><span class="special">);</span>
</pre>
<h6>
<a name="spirit_repository.karma_components.nonterminal.subrule.h3"></a>
<span><a name="spirit_repository.karma_components.nonterminal.subrule.parameters__declaration_"></a></span><a class="link" href="subrule.html#spirit_repository.karma_components.nonterminal.subrule.parameters__declaration_">Parameters
(declaration)</a>
</h6>
<div class="informaltable"><table class="table">
<colgroup>
<col>
<col>
</colgroup>
<thead><tr>
<th>
<p>
Parameter
</p>
</th>
<th>
<p>
Description
</p>
</th>
</tr></thead>
<tbody>
<tr>
<td>
<p>
<code class="computeroutput"><span class="identifier">ID</span></code>
</p>
</td>
<td>
<p>
Required numeric argument. Gives the subrule a unique 'identification
tag'.
</p>
</td>
</tr>
<tr>
<td>
<p>
<code class="computeroutput"><span class="identifier">A1</span></code>, <code class="computeroutput"><span class="identifier">A2</span></code>
</p>
</td>
<td>
<p>
Optional types, can be specified in any order. Can be one of
1. signature, 2. locals (see rules reference for more information
on those parameters).
</p>
<p>
Note that the delimiter type need not be specified in the parameters,
unlike with grammars and rules. Subrules will automatically use
the delimiter type which is in effect when they are invoked.
</p>
</td>
</tr>
<tr>
<td>
<p>
<code class="computeroutput"><span class="identifier">name</span></code>
</p>
</td>
<td>
<p>
Optional string. Gives the subrule a name, useful for debugging
and error handling.
</p>
</td>
</tr>
</tbody>
</table></div>
<h6>
<a name="spirit_repository.karma_components.nonterminal.subrule.h4"></a>
<span><a name="spirit_repository.karma_components.nonterminal.subrule.synopsis__usage_"></a></span><a class="link" href="subrule.html#spirit_repository.karma_components.nonterminal.subrule.synopsis__usage_">Synopsis
(usage)</a>
</h6>
<p>
Subrules are defined and used within groups, typically (and by convention)
enclosed inside parentheses.
</p>
<pre class="programlisting"><span class="comment">// Group containing N subrules</span>
<span class="special">(</span>
<span class="identifier">sr1</span> <span class="special">=</span> <span class="identifier">expr1</span>
<span class="special">,</span> <span class="identifier">sr2</span> <span class="special">=</span> <span class="identifier">expr2</span>
<span class="special">,</span> <span class="special">...</span> <span class="comment">// Any number of subrules</span>
<span class="special">}</span>
</pre>
<p>
The IDs of all subrules defined within the same group must be different.
It is an error to define several subrules with the same ID (or to define
the same subrule multiple times) in the same group.
</p>
<pre class="programlisting"><span class="comment">// Auto-subrules and inherited attributes</span>
<span class="special">(</span>
<span class="identifier">srA</span> <span class="special">%=</span> <span class="identifier">exprA</span> <span class="special"><<</span> <span class="identifier">srB</span> <span class="special"><<</span> <span class="identifier">srC</span><span class="special">(</span><span class="identifier">c1</span><span class="special">,</span> <span class="identifier">c2</span><span class="special">,</span> <span class="special">...)</span> <span class="comment">// Arguments to subrule srC</span>
<span class="special">,</span> <span class="identifier">srB</span> <span class="special">%=</span> <span class="identifier">exprB</span>
<span class="special">,</span> <span class="identifier">srC</span> <span class="special">=</span> <span class="identifier">exprC</span>
<span class="special">,</span> <span class="special">...</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="comment">// Arguments to group, i.e. to start subrule srA</span>
</pre>
<h6>
<a name="spirit_repository.karma_components.nonterminal.subrule.h5"></a>
<span><a name="spirit_repository.karma_components.nonterminal.subrule.parameters__usage_"></a></span><a class="link" href="subrule.html#spirit_repository.karma_components.nonterminal.subrule.parameters__usage_">Parameters
(usage)</a>
</h6>
<div class="informaltable"><table class="table">
<colgroup>
<col>
<col>
</colgroup>
<thead><tr>
<th>
<p>
Parameter
</p>
</th>
<th>
<p>
Description
</p>
</th>
</tr></thead>
<tbody>
<tr>
<td>
<p>
<code class="computeroutput"><span class="identifier">sr1</span></code>, <code class="computeroutput"><span class="identifier">sr2</span></code>
</p>
</td>
<td>
<p>
Subrules with different IDs.
</p>
</td>
</tr>
<tr>
<td>
<p>
<code class="computeroutput"><span class="identifier">expr1</span></code>, <code class="computeroutput"><span class="identifier">expr2</span></code>
</p>
</td>
<td>
<p>
Generator expressions. Can include <code class="computeroutput"><span class="identifier">sr1</span></code>
and <code class="computeroutput"><span class="identifier">sr2</span></code>, as well
as any other valid generator expressions.
</p>
</td>
</tr>
<tr>
<td>
<p>
<code class="computeroutput"><span class="identifier">srA</span></code>
</p>
</td>
<td>
<p>
Subrule with a synthesized attribute and inherited attributes.
</p>
</td>
</tr>
<tr>
<td>
<p>
<code class="computeroutput"><span class="identifier">srB</span></code>
</p>
</td>
<td>
<p>
Subrule with a synthesized attribute.
</p>
</td>
</tr>
<tr>
<td>
<p>
<code class="computeroutput"><span class="identifier">srC</span></code>
</p>
</td>
<td>
<p>
Subrule with inherited attributes.
</p>
</td>
</tr>
<tr>
<td>
<p>
<code class="computeroutput"><span class="identifier">exprA</span></code>, <code class="computeroutput"><span class="identifier">exprB</span></code>, <code class="computeroutput"><span class="identifier">exprC</span></code>
</p>
</td>
<td>
<p>
Generator expressions.
</p>
</td>
</tr>
<tr>
<td>
<p>
<code class="computeroutput"><span class="identifier">a1</span></code>, <code class="computeroutput"><span class="identifier">a2</span></code>
</p>
</td>
<td>
<p>
Arguments passed to the subrule group. They are passed as inherited
attributes to the group's start subrule, <code class="computeroutput"><span class="identifier">srA</span></code>.
</p>
</td>
</tr>
<tr>
<td>
<p>
<code class="computeroutput"><span class="identifier">c1</span></code>, <code class="computeroutput"><span class="identifier">c2</span></code>
</p>
</td>
<td>
<p>
Arguments passed as inherited attributes to subrule <code class="computeroutput"><span class="identifier">srC</span></code>.
</p>
</td>
</tr>
</tbody>
</table></div>
<h6>
<a name="spirit_repository.karma_components.nonterminal.subrule.h6"></a>
<span><a name="spirit_repository.karma_components.nonterminal.subrule.groups"></a></span><a class="link" href="subrule.html#spirit_repository.karma_components.nonterminal.subrule.groups">Groups</a>
</h6>
<p>
A subrule group (a set of subrule definitions) is a generator, which can
be used anywhere in a generator expression (in assignments to rules, as
well as directly in arguments to functions such as <code class="computeroutput"><span class="identifier">generate</span></code>).
In a group, generation proceeds from the start subrule, which is the first
(topmost) subrule defined in that group. In the two groups in the synopsis
above, <code class="computeroutput"><span class="identifier">sr1</span></code> and <code class="computeroutput"><span class="identifier">srA</span></code> are the start subrules respectively
-- for example when the first subrule group is called forth, the <code class="computeroutput"><span class="identifier">sr1</span></code> subrule is called.
</p>
<p>
A subrule can only be used in a group which defines it. Groups can be viewed
as scopes: a definition of a subrule is limited to its enclosing group.
</p>
<pre class="programlisting"><span class="identifier">rule</span><span class="special"><</span><span class="identifier">outiter_type</span><span class="special">></span> <span class="identifier">r1</span><span class="special">,</span> <span class="identifier">r2</span><span class="special">,</span> <span class="identifier">r3</span><span class="special">;</span>
<span class="identifier">subrule</span><span class="special"><</span><span class="number">1</span><span class="special">></span> <span class="identifier">sr1</span><span class="special">;</span>
<span class="identifier">subrule</span><span class="special"><</span><span class="number">2</span><span class="special">></span> <span class="identifier">sr2</span><span class="special">;</span>
<span class="identifier">r1</span> <span class="special">=</span>
<span class="special">(</span> <span class="identifier">sr1</span> <span class="special">=</span> <span class="char">'a'</span> <span class="special"><<</span> <span class="identifier">space</span> <span class="special">)</span> <span class="comment">// First group in r1.</span>
<span class="special"><<</span> <span class="special">(</span> <span class="identifier">sr2</span> <span class="special">=</span> <span class="special">+</span><span class="identifier">sr1</span> <span class="special">)</span> <span class="comment">// Second group in r1.</span>
<span class="comment">// ^^^</span>
<span class="comment">// DOES NOT COMPILE: sr1 is not defined in this</span>
<span class="comment">// second group, it cannot be used here (its</span>
<span class="comment">// previous definition is out of scope).</span>
<span class="special">;</span>
<span class="identifier">r2</span> <span class="special">=</span>
<span class="special">(</span> <span class="identifier">sr1</span> <span class="special">=</span> <span class="char">'a'</span> <span class="special"><<</span> <span class="identifier">space</span> <span class="special">)</span> <span class="comment">// Only group in r2.</span>
<span class="special"><<</span> <span class="identifier">sr1</span>
<span class="comment">// ^^^</span>
<span class="comment">// DOES NOT COMPILE: not in a subrule group,</span>
<span class="comment">// sr1 cannot be used here (here too, its</span>
<span class="comment">// previous definition is out of scope).</span>
<span class="special">;</span>
<span class="identifier">r3</span> <span class="special">=</span>
<span class="special">(</span> <span class="identifier">sr1</span> <span class="special">=</span> <span class="identifier">space</span> <span class="special"><<</span> <span class="char">'x'</span> <span class="special">)</span> <span class="comment">// Another group. The same subrule `sr1`</span>
<span class="comment">// can have another, independent</span>
<span class="comment">// definition in this group.</span>
<span class="special">;</span>
</pre>
<h6>
<a name="spirit_repository.karma_components.nonterminal.subrule.h7"></a>
<span><a name="spirit_repository.karma_components.nonterminal.subrule.attributes"></a></span><a class="link" href="subrule.html#spirit_repository.karma_components.nonterminal.subrule.attributes">Attributes</a>
</h6>
<p>
A subrule has the same behavior as a rule with respect to attributes. In
particular:
</p>
<div class="itemizedlist"><ul class="itemizedlist" type="disc">
<li class="listitem">
the type of its synthesized attribute is the one specified in the subrule's
signature, if any. Otherwise it is <code class="computeroutput"><span class="identifier">unused_type</span></code>.
</li>
<li class="listitem">
the types of its inherited attributes are the ones specified in the
subrule's signature, if any. Otherwise the subrule has no inherited
attributes.
</li>
<li class="listitem">
an auto-subrule can be defined by assigning it with the <code class="computeroutput"><span class="special">%=</span></code> syntax. In this case, the subrule's
synthesized attribute is automatically propagated to the RHS generator's
attribute.
</li>
<li class="listitem">
the Phoenix placeholders <code class="computeroutput"><span class="identifier">_val</span></code>,
<code class="computeroutput"><span class="identifier">_r1</span></code>, <code class="computeroutput"><span class="identifier">_r2</span></code>, ... are available to refer to
the subrule's synthesized and inherited attributes, if present.
</li>
</ul></div>
<h6>
<a name="spirit_repository.karma_components.nonterminal.subrule.h8"></a>
<span><a name="spirit_repository.karma_components.nonterminal.subrule.locals"></a></span><a class="link" href="subrule.html#spirit_repository.karma_components.nonterminal.subrule.locals">Locals</a>
</h6>
<p>
A subrule has the same behavior as a rule with respect to locals. In particular,
the Phoenix placeholders <code class="computeroutput"><span class="identifier">_a</span></code>,
<code class="computeroutput"><span class="identifier">_b</span></code>, ... are available to
refer to the subrule's locals, if present.
</p>
<h6>
<a name="spirit_repository.karma_components.nonterminal.subrule.h9"></a>
<span><a name="spirit_repository.karma_components.nonterminal.subrule.example"></a></span><a class="link" href="subrule.html#spirit_repository.karma_components.nonterminal.subrule.example">Example</a>
</h6>
<p>
Some includes:
</p>
<p>
</p>
<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">spirit</span><span class="special">/</span><span class="identifier">include</span><span class="special">/</span><span class="identifier">karma</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span>
<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">spirit</span><span class="special">/</span><span class="identifier">repository</span><span class="special">/</span><span class="identifier">include</span><span class="special">/</span><span class="identifier">karma_subrule</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span>
<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">spirit</span><span class="special">/</span><span class="identifier">include</span><span class="special">/</span><span class="identifier">phoenix_core</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span>
<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">spirit</span><span class="special">/</span><span class="identifier">include</span><span class="special">/</span><span class="identifier">phoenix_operator</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span>
<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">spirit</span><span class="special">/</span><span class="identifier">include</span><span class="special">/</span><span class="identifier">phoenix_fusion</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span>
</pre>
<p>
</p>
<p>
Some using declarations:
</p>
<p>
</p>
<pre class="programlisting"><span class="keyword">using</span> <span class="keyword">namespace</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">spirit</span><span class="special">;</span>
<span class="keyword">using</span> <span class="keyword">namespace</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">spirit</span><span class="special">::</span><span class="identifier">ascii</span><span class="special">;</span>
<span class="keyword">namespace</span> <span class="identifier">repo</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">spirit</span><span class="special">::</span><span class="identifier">repository</span><span class="special">;</span>
</pre>
<p>
</p>
<p>
A grammar containing only one rule, defined with a group of 2 subrules:
</p>
<p>
</p>
<pre class="programlisting"><span class="keyword">template</span> <span class="special"><</span><span class="keyword">typename</span> <span class="identifier">OutputIterator</span><span class="special">></span>
<span class="keyword">struct</span> <span class="identifier">mini_xml_generator</span>
<span class="special">:</span> <span class="identifier">karma</span><span class="special">::</span><span class="identifier">grammar</span><span class="special"><</span><span class="identifier">OutputIterator</span><span class="special">,</span> <span class="identifier">mini_xml</span><span class="special">()></span>
<span class="special">{</span>
<span class="identifier">mini_xml_generator</span><span class="special">()</span> <span class="special">:</span> <span class="identifier">mini_xml_generator</span><span class="special">::</span><span class="identifier">base_type</span><span class="special">(</span><span class="identifier">entry</span><span class="special">)</span>
<span class="special">{</span>
<span class="identifier">entry</span> <span class="special">%=</span> <span class="special">(</span>
<span class="identifier">xml</span> <span class="special">=</span>
<span class="char">'<'</span> <span class="special"><<</span> <span class="identifier">string</span><span class="special">[</span><span class="identifier">_1</span> <span class="special">=</span> <span class="identifier">at_c</span><span class="special"><</span><span class="number">0</span><span class="special">>(</span><span class="identifier">_val</span><span class="special">)]</span> <span class="special"><<</span> <span class="char">'>'</span>
<span class="special"><<</span> <span class="special">(*</span><span class="identifier">node</span><span class="special">)[</span><span class="identifier">_1</span> <span class="special">=</span> <span class="identifier">at_c</span><span class="special"><</span><span class="number">1</span><span class="special">>(</span><span class="identifier">_val</span><span class="special">)]</span>
<span class="special"><<</span> <span class="string">"</"</span> <span class="special"><<</span> <span class="identifier">string</span><span class="special">[</span><span class="identifier">_1</span> <span class="special">=</span> <span class="identifier">at_c</span><span class="special"><</span><span class="number">0</span><span class="special">>(</span><span class="identifier">_val</span><span class="special">)]</span> <span class="special"><<</span> <span class="char">'>'</span>
<span class="special">,</span> <span class="identifier">node</span> <span class="special">%=</span> <span class="identifier">string</span> <span class="special">|</span> <span class="identifier">xml</span>
<span class="special">);</span>
<span class="special">}</span>
<span class="identifier">karma</span><span class="special">::</span><span class="identifier">rule</span><span class="special"><</span><span class="identifier">OutputIterator</span><span class="special">,</span> <span class="identifier">mini_xml</span><span class="special">()></span> <span class="identifier">entry</span><span class="special">;</span>
<span class="identifier">repo</span><span class="special">::</span><span class="identifier">karma</span><span class="special">::</span><span class="identifier">subrule</span><span class="special"><</span><span class="number">0</span><span class="special">,</span> <span class="identifier">mini_xml</span><span class="special">()></span> <span class="identifier">xml</span><span class="special">;</span>
<span class="identifier">repo</span><span class="special">::</span><span class="identifier">karma</span><span class="special">::</span><span class="identifier">subrule</span><span class="special"><</span><span class="number">1</span><span class="special">,</span> <span class="identifier">mini_xml_node</span><span class="special">()></span> <span class="identifier">node</span><span class="special">;</span>
<span class="special">};</span>
</pre>
<p>
</p>
<p>
The definitions of the <code class="computeroutput"><span class="identifier">mini_xml</span></code>
and <code class="computeroutput"><span class="identifier">mini_xml_node</span></code> data
structures are not shown here. The full example above can be found here:
<a href="../../../../../example/karma/mini_xml_karma_sr.cpp" target="_top">../../example/karma/mini_xml_karma_sr.cpp</a>
</p>
<h6>
<a name="spirit_repository.karma_components.nonterminal.subrule.h10"></a>
<span><a name="spirit_repository.karma_components.nonterminal.subrule.performance"></a></span><a class="link" href="subrule.html#spirit_repository.karma_components.nonterminal.subrule.performance">Performance</a>
</h6>
<p>
For comparison of run-time and compile-time performance when using subrules,
please see the <a class="link" href="../../qi_components/nonterminal/subrule.html#spirit_repository.qi_components.nonterminal.subrule.performance">Performance</a>
section of <span class="emphasis"><em>Spirit.Qi</em></span> subrules (the implementation
of <span class="emphasis"><em>Spirit.Karma</em></span> and <span class="emphasis"><em>Spirit.Qi</em></span>
subrules is very similar, so performance is very similar too).
</p>
<h6>
<a name="spirit_repository.karma_components.nonterminal.subrule.h11"></a>
<span><a name="spirit_repository.karma_components.nonterminal.subrule.notes"></a></span><a class="link" href="subrule.html#spirit_repository.karma_components.nonterminal.subrule.notes">Notes</a>
</h6>
<p>
Subrules push the C++ compiler hard. A group of subrules is a single C++
expression. Current C++ compilers cannot handle very complex expressions
very well. One restricting factor is the typical compiler's limit on template
recursion depth. Some, but not all, compilers allow this limit to be configured.
</p>
<p>
g++'s maximum can be set using a compiler flag: <code class="computeroutput"><span class="special">-</span><span class="identifier">ftemplate</span><span class="special">-</span><span class="identifier">depth</span></code>. Set this appropriately if you
use relatively complex subrules.
</p>
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<td align="right"><div class="copyright-footer">Copyright © 2001-2011 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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