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      <font size="6" face="Verdana, Arial, Helvetica, sans-serif"><b>Subrules</b></font>
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<p>Spirit is implemented using expression templates. This is a very powerful technique.
  Along with its power comes some complications. We almost take for granted that
  when we write <tt>i | j &gt;&gt; k</tt> where <tt>i</tt>, <tt>j</tt> and <tt>k</tt>
  are all integers the result is still an integer. Yet, with expression templates,
  the same expression <tt>i | j &gt;&gt; k</tt> where <tt>i</tt>, <tt>j</tt> and
  <tt>k</tt> are of type <tt>T</tt>, the result is a complex composite type [see
  <a href="basic_concepts.html">Basic Concepts</a>]. Spirit expressions, which
  are combinations of primitives and composites yield an infinite set of new types.
  One problem is that C++ offers no easy facility to deduce the type of an arbitrarily
  complex expression that yields a complex type. Thus, while it is easy to write:</p>
<pre><code><font color="#000000"><span class=identifier>    </span><span class=keyword>int </span><span class=identifier>r </span><span class=special>= </span><span class=identifier>i </span><span class=special>| </span><span class=identifier>j </span><span class=special>&gt;&gt; </span><span class=identifier>k</span><span class=special>; </span><span class=comment>// where i, j, and k are ints</span></font></code></pre>
<p>Expression templates yield an endless supply of types. Without the <a href="rule.html">rule</a>, 
  there is no easy way to do this in C++ if <tt>i</tt>, <tt>j</tt> and <tt>k</tt> 
  are Spirit parsers:</p>
<pre><code><font color="#000000"><span class=comment>    </span><span class=special>&lt;</span><span class=identifier>what_type???</span><span class=special>&gt; </span><span class=identifier>r </span><span class=special>= </span><span class=identifier>i </span><span class=special>| </span><span class=identifier>j </span><span class=special>&gt;&gt; </span><span class=identifier>k</span><span class=special>; </span><span class=comment>// where i, j, and k are Spirit parsers</span></font></code></pre>
<p>If <tt>i</tt>, <tt>j</tt> and <tt>k</tt> are all <tt>chlit&lt;&gt;</tt> objects, 
  the type that we want is:</p>
<pre><code><font color="#000000"><span class=comment>    </span><span class=keyword>typedef
        </span><span class=identifier>alternative</span><span class=special>&lt;
            </span><span class=identifier>chlit</span><span class=special>&lt;&gt;</span><span class=comment>      //  i
          </span><span class=special>,</span> <span class=identifier>sequence</span><span class=special>&lt;
                </span><span class=identifier>chlit</span><span class=special>&lt;&gt;  </span><span class=comment>//  j
          </span><span class=special>    ,</span><span class=comment> </span><span class=identifier>chlit</span><span class=special>&lt;&gt;  </span><span class=comment>//  k
            </span><span class=special>&gt;
        &gt;
    </span><span class=identifier>rule_t</span><span class=special>;

    </span><span class=identifier>rule_t r </span><span class=special>= </span><span class=identifier>i </span><span class=special>| </span><span class=identifier>j </span><span class=special>&gt;&gt; </span><span class=identifier>k</span><span class=special>; </span><span class=comment>// where i, j, and k are chlit&lt;&gt; objects</span></font></code></pre>
<p>We deliberately formatted the type declaration nicely to make it understandable. 
  Try that with a more complex expression. While it can be done, explicitly spelling 
  out the type of a Spirit expression template is tedious and error prone. The 
  right hand side (rhs) has to mirror the type of the left hand side (lhs). (<img src="theme/lens.gif" width="15" height="16"> 
  Yet, if you still wish to do it, see this <a href="techniques.html#no_rules">link</a> 
  for a technique). </p>
<table width="80%" border="0" align="center">
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    <td class="note_box"><p><img src="theme/lens.gif" width="15" height="16"><b> 
        typeof and auto</b> <br>
        <br>
        Some compilers already support the <tt>typeof</tt> keyword. This can be 
        used to free us from having to explicitly type the type (pun intentional). 
        Using the <tt>typeof</tt>, we can rewrite the Spirit expression above 
        as:<br>
        <br>
        <span class="keyword"><code>typeof</code><code></code></span><code><span class=special>(</span><span class=identifier>i 
        </span><span class=special>| </span><span class=identifier>j </span><span class=special>&gt;&gt; 
        </span><span class=identifier>k</span><span class=special>) </span><span class=identifier>r 
        </span><span class=special>= </span><span class=identifier>i </span><span class=special>| 
        </span><span class=identifier>j </span><span class=special>&gt;&gt; </span><span class=identifier>k</span><span class=special>;</span></code><br>
        <br>
        While this is better than having to explicitly declare a complex type, 
        it is redundant, error prone and still an eye sore. The expression is 
        typed twice. The only way to simplify this is to introduce a macro (See 
        this <a href="techniques.html#typeof">link</a> for more information).<br>
        <br>
        <a href="http://www.boost-consulting.com">David Abrahams</a> proposed 
        in comp.std.c++ to reuse the <tt>auto</tt> keyword for type deduced variables. 
        This has been extensibly discussed in <a href="http://www.boost.org">boost.org</a>. Example: 
        <br>
        <br>
        <span class=keyword><code>auto </code></span><code><span class=identifier>r 
        </span><span class=special>= </span><span class=identifier>i </span><span class=special>| 
        </span><span class=identifier>j </span><span class=special>&gt;&gt; </span><span class=identifier>k</span><span class=special>;</span></code><br>
        <br>
        Once such a C++ extension is accepted into the standard, this would be 
        a neat solution and a nice fit for our purpose. It's not a complete solution 
        though since there are still situations where we do not know the rhs beforehand; 
        for instance when pre-declaring cyclic dependent rules.</p>
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<p>Fortunately, rules come to the rescue. Rules can capture the type of the expression
  assigned to it. Thus:</p>
<pre><code><font color="#000000">    <span class=identifier>rule</span><span class=special>&lt;&gt; </span><span class=identifier>r </span><span class=special>= </span><span class=identifier>i </span><span class=special>| </span><span class=identifier>j </span><span class=special>&gt;&gt; </span><span class=identifier>k</span><span class=special>;  </span><span class=comment>// where i, j, and k are chlit&lt;&gt; objects</span></font></code></pre>
<p>It might not be apparent but behind the scenes, plain rules are actually implemented
  using a pointer to a runtime polymorphic abstract class that holds the dynamic
  type of the parser assigned to it. When a Spirit expression is assigned to a
  rule, its type is encapsulated in a concrete subclass of the abstract class.
  A virtual parse function delegates the parsing to the encapsulated object.</p>
<p>Rules have drawbacks though:</p>
<p><img src="theme/bullet.gif" width="12" height="12"> It is coupled to a specific 
  scanner type. The rule is tied to a specific scanner [see <a href="faq.html#scanner_business">The 
  Scanner Business</a>].<br>
  <img src="theme/bullet.gif" width="12" height="12"> The rule's parse member 
function has a virtual function call overhead that cannot be inlined.</p>
<h2>Static rules: subrules</h2>
<p>The subrule is a fully static version of the rule. The subrule does not have
  the drawbacks listed above. </p>
<p><img src="theme/bullet.gif" width="12" height="12"> The subrule is not tied 
  to a specific scanner so just about any scanner type may be used<br>
  <img src="theme/bullet.gif" width="12" height="12"> The subrule also allows 
  aggressive inlining since there are no virtual function calls</p>
<pre><code><font color="#000000"><span class=identifier>    </span><span class=keyword>template</span><span class=special>&lt;</span><span class=keyword>int </span></font><span class="identifier">ID</span><font color="#000000"><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;&gt;</span> <span class=special>&gt;
    </span><span class=keyword>class </span><span class=identifier>subrule</span><span class=special>;</span></font></code></pre>
<p>The first template parameter gives the subrule an identification tag. Like 
  the <a href="rule.html">rule</a>, there is a ContextT template parameter that 
  defaults to <code><tt>parser_context</tt></code>. You need not be concerned 
  at all with the <tt>ContextT</tt> template parameter unless you wish to tweak 
  the low level behavior of the subrule. Detailed information on the <tt>ContextT</tt> 
  template parameter is provided <a href="indepth_the_parser_context.html">elsewhere</a>. 
</p>
<p>Presented above 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>
<p>Apart from a few minor differences, the subrule follows the usage and syntax 
  of the rule closely. Here's the calculator grammar using subrules:</p>
<pre><code><font color="#000000"><span class=comment>    </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>first </span><span class=special>=
                </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=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>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>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=special>}

            </span><span class=identifier>subrule</span><span class=special>&lt;</span><span class=number>0</span><span class=special>&gt;  </span><span class=identifier>expression</span><span class=special>;
            </span><span class=identifier>subrule</span><span class=special>&lt;</span><span class=number>1</span><span class=special>&gt;  </span><span class=identifier>term</span><span class=special>;
            </span><span class=identifier>subrule</span><span class=special>&lt;</span><span class=number>2</span><span class=special>&gt;  </span><span class=identifier>factor</span><span class=special>;
            </span><span class=identifier>subrule</span><span class=special>&lt;</span><span class=number>3</span><span class=special>&gt;  </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=identifier>first</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>first</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/subrule_calc.cpp">viewed 
  here</a>. This is part of the Spirit distribution. </p>
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    <td width="199"><img src="theme/subrule1.png" width="234" height="224"></td>
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<p>The subrule as an efficient version of the rule. Compiler optimizations such
  as aggressive inlining help reduce the code size and increase performance significantly.
</p>
<p>The subrule is not a panacea however. Subrules push the C++ compiler hard to
  its knees. For example, current compilers have a limit on recursion depth that
  may not be exceeded. Don't even think about writing a full pascal grammar using
  subrules alone. A grammar using subrules is a single C++ expression. Current
  C++ compilers cannot handle very complex expressions very well. Finally, a plain
  rule is still needed to act as place holder for subrules.</p>
<p>The code above is a good example of the recommended way to use subrules. Notice 
  the hierarchy. We have a grammar that encapsulates the whole calculator. The 
  start rule is a plain rule that holds the set of subrules. The subrules in turn 
  defines the actual details of the grammar.</p>
<table width="80%" border="0" align="center">
  <tr> 
    <td class="note_box"><img src="theme/lens.gif" width="15" height="16"><b> 
      Template instantiation depth</b> <br> <br>
      Spirit pushes the C++ compiler hard. Current C++ compilers cannot handle 
      very complex heavily nested 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.<br>
      <br>
      g++'s maximum can be set using a compiler flag: -ftemplate-depth. Set this 
      appropriately if you have a relatively complex grammar.<br>
      <br>
      Microsoft Visual C++ can take greater than 1000 for both template class 
      and function instantiation depths. However, the linker chokes with deep 
      template function instantiation unless inline recursion depth is set using 
      these pragmas:<br>
      <br>
      <span class="preprocessor">#pragma</span> inline_depth<span class="special">(</span>255<span class="special">)</span><br>
      <span class="preprocessor">#pragma</span> inline_recursion<span class="special">(</span>on<span class="special">)<br>
      <br>
      </span>Perhaps this limitations no longer applies to more current versions 
      of these compilers. Be sure to check your compiler documentation.</td>
  </tr>
</table>
<p>This setup gives a good balance. The subrules do all the work. Each grammar 
  will have only one rule: <tt>first</tt>. The rule is used just to hold the subrules 
  and make them visible to the grammar. </p>
<h3>The subrule definition</h3>
<p>Like the rule, the expression after assignment operator <tt>=</tt> defines
  the subrule:</p>
<pre>    <span class=identifier>identifier </span><span class=special>= </span><span class=identifier>expression</span></pre>
<p>Unlike rules, subrules may be defined only once. Redefining a subrule is illegal 
  and will result to a compile time assertion.</p>
<h3>Separators [ , ]</h3>
<p>While rules are terminated by the semicollon <tt>';'</tt>. Subrules are not 
  terminated but are separated by the comma: <tt>','</tt>. Like Pascal statements, 
  the last subrule in a group may not have a trailing comma.</p>
<pre><span class=identifier>    </span><span class=identifier>a </span><span class=special>= </span><span class=identifier>ch_p</span><span class=special>(</span><span class=literal>'a'</span><span class=special>),
    </span><span class=identifier>b </span><span class=special>= </span><span class=identifier>ch_p</span><span class=special>(</span><span class=literal>'b'</span><span class=special>),
    </span><span class=identifier>c </span><span class=special>= </span><span class=identifier>ch_p</span><span class=special>(</span><span class=literal>'c'</span><span class=special>), </span><span class=comment>// BAD, trailing comma</span><code><font color="#000000"><font color="#800000"><i></i></font></font></code><code><font color="#000000"><font color="#800000"><i></i></font></font><i></i></code></pre>
<p>
<pre><code><span class=comment>    </span><span class=identifier>a </span><span class=special>= </span><span class=identifier>ch_p</span><span class=special>(</span><span class=literal>'a'</span><span class=special>),
    </span><span class=identifier>b </span><span class=special>= </span><span class=identifier>ch_p</span><span class=special>(</span><span class=literal>'b'</span><span class=special>),
    </span><span class=identifier>c </span><span class=special>= </span><span class=identifier>ch_p</span><span class=special>(</span><span class=literal>'c'</span><span class=special>)  </span><span class=comment>// OK</span></code></pre>
<h3> The start subrule</h3>
<p>Unlike rules, parsing proceeds from the start subrule. The first (topmost) 
  subrule in a group of subrules is called the <b>start subrule</b>. In our example 
  above, <tt>expression</tt> is the start subrule. When a group of subrules is 
  called forth, the start subrule <tt>expression</tt> is called first.</p>
<h3>IDs</h3>
<p>Each subrule has a corresponding ID; an integral constant that uniquely specifies 
  the subrule. Our example above has four subrules. They are declared as:</p>
<pre><code><span class=comment>    </span><span class=identifier>subrule</span><span class=special>&lt;</span><span class=number>0</span><span class=special>&gt;  </span><span class=identifier>expression</span><span class=special>;
    </span><span class=identifier>subrule</span><span class=special>&lt;</span><span class=number>1</span><span class=special>&gt;  </span><span class=identifier>term</span><span class=special>;
    </span><span class=identifier>subrule</span><span class=special>&lt;</span><span class=number>2</span><span class=special>&gt;  </span><span class=identifier>factor</span><span class=special>;
    </span><span class=identifier>subrule</span><span class=special>&lt;</span><span class=number>3</span><span class=special>&gt;  </span><span class=identifier>group</span><span class=special>;</span></code></pre>
<h3> Aliases</h3>
<p>It is possible to have subrules with similar IDs. A subrule with a similar
  ID to will be an alias of the other. Both subrules may be used interchangeably.</p>
<pre><code><span class=special>    </span><span class=identifier>subrule</span><span class=special>&lt;</span><span class=number>0</span><span class=special>&gt;  </span><span class=identifier>a</span><span class=special>;
    </span><span class=identifier>subrule</span><span class=special>&lt;</span><span class=number>0</span><span class=special>&gt;  </span><span class=identifier>alias</span><span class=special>;  </span><span class=comment>// alias of a</span></code></pre>
<h3>Groups: scope and nesting</h3>
<p>The scope of a subrule and its definition is the enclosing group, typically 
  (and by convention) enclosed inside the parentheses. IDs outside a scope are 
  not directly visible. Inner subrule groups can be nested by enclosing each sub-group 
  inside another set of parentheses. Each group is unique and acts independently. 
  Consequently, while it may not be advisable to do so, a subrule in a group may 
  share the same ID as a subrule in another group since both groups are independent 
  of each other.</p>
<pre><code><span class=comment>    </span><span class=identifier>subrule</span><span class=special>&lt;</span><span class=number>0</span><span class=special>&gt; </span><span class=identifier>a</span><span class=special>;
    </span><span class=identifier>subrule</span><span class=special>&lt;</span><span class=number>1</span><span class=special>&gt; </span><span class=identifier>b</span><span class=special>;
    </span><span class=identifier>subrule</span><span class=special>&lt;</span><span class=number>0</span><span class=special>&gt; </span><span class=identifier>c</span><span class=special>;
    </span><span class=identifier>subrule</span><span class=special>&lt;</span><span class=number>1</span><span class=special>&gt; </span><span class=identifier>d</span><span class=special>;

    </span><span class=special>(                       </span><span class=comment>// outer subrule group, scope of a and b
        </span><span class=identifier>a </span><span class=special>= </span><span class=identifier>ch_p</span><span class=special>(</span><span class=literal>'a'</span><span class=special>),
        </span><span class=identifier>b </span><span class=special>=
        </span><span class=special>(                   </span><span class=comment>// inner subrule group, scope of b and c
            </span><span class=identifier>c </span><span class=special>= </span><span class=identifier>ch_p</span><span class=special>(</span><span class=literal>'c'</span><span class=special>),
            </span><span class=identifier>d </span><span class=special>= </span><span class=identifier>ch_p</span><span class=special>(</span><span class=literal>'d'</span><span class=special>)
        </span><span class=special>)
    </span><span class=special>)</span></code></pre>
<p>Subrule IDs need to be unique only within a group. A grammar is an implicit 
  group. Furthermore, even subrules in a grammar may have the same IDs without 
  clashing if they are inside a group. Subrules may be explicitly grouped using 
  the parentheses. Parenthesized groups have unique scopes. In the code above, 
  the outer subrule group defines the subrules <tt>a</tt> and <tt>b</tt> while 
  the inner subrule group defines the subrules <tt>c</tt> and <tt>d</tt>. Notice 
  that the definition of <tt>b</tt> is the inner subrule.</p>
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<p class="copyright">Copyright &copy; 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></p>
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