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<font size="6" face="Verdana, Arial, Helvetica, sans-serif"><b>Semantic Actions</b></font>
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<p>Semantic actions have the form: <b>expression[action]</b></p>
<p>Ultimately, after having defined our grammar and having generated a corresponding
parser, we will need to produce some output and do some work besides syntax
analysis; unless, of course, what we want is merely to check for the conformance
of an input with our grammar, which is very seldom the case. Semantic actions
may be attached to any expression at any level within the parser hierarchy.
An action is a C/C++ function or function object that will be called if a match
is found in the particular context where it is attached. The action function
serves as a hook into the parser and may be used to, for example:</p>
<blockquote>
<p><img src="theme/bullet.gif" width="13" height="13"> Generate output from
the parser (ASTs, for example)<br>
<img src="theme/bullet.gif" width="13" height="13"> Report warnings or errors<br>
<img src="theme/bullet.gif" width="13" height="13"> Manage symbol tables</p>
</blockquote>
<h2>Generic Semantic Actions (Transduction Interface)</h2>
<p>A generic semantic action can be any free function or function object that
is compatible with the interface:</p>
<pre><code><font color="#000000"><span class=identifier></span><span class=keyword> void </span><span class=identifier>f</span><span class=special>(</span><span class=identifier>IteratorT </span><span class=identifier>first</span><span class=special>, </span><span class=identifier>IteratorT </span><span class=identifier>last</span><span class=special>);</span></font></code></pre>
<p>where <tt>IteratorT</tt> is the type of iterator used, <tt>first</tt> points
to the current input and <tt>last</tt> points to one after the end of the input
(identical to STL iterator ranges). A function object (functor) should have
a member <tt>operator()</tt> with the same signature as above:</p>
<pre><code><font color="#000000"><span class=special> </span><span class=keyword>struct </span><span class=identifier>my_functor
</span><span class=special>{
</span><span class=keyword>void </span><span class=keyword>operator</span><span class=special>()(</span><span class=identifier>IteratorT </span><span class=identifier>first</span><span class=special>, </span><span class=identifier>IteratorT </span><span class=identifier>last</span><span class=special>) </span><span class=keyword>const</span><span class=special>;
</span><span class=special>};</span></font></code></pre>
<p>Iterators pointing to the matching portion of the input are passed into the
function/functor.</p>
<p>In general, semantic actions accept the first-last iterator pair. This is the
transduction interface. The action functions or functors receive the unprocessed
data representing the matching production directly from the input. In many cases,
this is sufficient. Examples are source to source translation, pre-processing,
etc. </p>
<h3>Example:</h3>
<pre><code><font color="#000000"><span class=special> </span><span class=keyword>void
</span><span class=identifier>my_action</span><span class=special>(</span><span class=keyword>char const</span><span class=special>* </span><span class=identifier>first</span><span class=special>, </span><span class=keyword>char const</span><span class=special>* </span><span class=identifier>last</span><span class=special>)
{
</span><span class=identifier>std</span><span class=special>::</span><span class=identifier>string</span><span class=special> </span><span class="identifier">str</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>std</span><span class=special>::</span><span class=identifier>cout </span><span class=special><< </span><span class=identifier>str </span><span class=special><< </span><span class=identifier>std</span><span class=special>::</span><span class=identifier>endl</span><span class=special>;
}
</span><span class=identifier>rule</span><span class=special><> </span><span class=identifier>myrule </span><span class=special>= (</span><span class=identifier>a </span><span class=special>| </span><span class=identifier>b </span><span class=special>| *(</span><span class=identifier>c </span><span class=special>>> </span><span class=identifier>d</span><span class=special>))[&</span><span class=identifier>my_action</span><span class=special>];</span></font></code></pre>
<p>The function <tt>my_action</tt> will be called whenever the expression <tt>(a
| b | *(c >> d)</tt> matches a portion of the input stream while parsing.
Two iterators, <tt>first</tt> and <tt>last</tt>, are passed into the function.
These iterators point to the start and end, respectively, of the portion of
input stream where the match is found.</p>
<h3>Const-ness:</h3>
<p>With functors, take note that the <tt>operator()</tt> should be <tt>const</tt>.
This implies that functors are immutable. One may wish to have some member variables
that are modified when the action gets called. This is not a good idea. First
of all, functors are preferably lightweight. Functors are passed around a lot
and it would incur a lot of overhead if the functors are heavily laden. Second,
functors are passed by value. Thus, the actual functor object that finally attaches
to the parser, will surely not be the original instance supplied by the client.
What this means is that changes to a functor's state will not affect the original
functor that the client passed in since they are distinct copies. If a functor
needs to update some state variables, which is often the case, it is better
to use references to external data. The following example shows how this can
be done:</p>
<pre><code><font color="#000000"><span class=special> </span><span class=keyword>struct </span><span class=identifier>my_functor
</span><span class=special>{
</span><span class=identifier>my_functor</span><span class=special>(</span><span class=identifier>std</span><span class=special>::</span><span class=identifier>string</span><span class=special>& </span><span class=identifier>str_</span><span class=special>)
</span><span class=special>: </span><span class=identifier>str</span><span class=special>(</span><span class=identifier>str_</span><span class=special>) </span><span class=special>{}
</span><span class=keyword>void
</span><span class=keyword>operator</span><span class=special>()(</span><span class=identifier>IteratorT </span><span class=identifier>first</span><span class=special>, </span><span class=identifier>IteratorT </span><span class=identifier>last</span><span class=special>) </span><span class=keyword>const
</span><span class=special>{
</span><span class=identifier>str</span><span class=special>.</span><span class=identifier>assign</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=special>}
</span><span class=identifier>std</span><span class=special>::</span><span class=identifier>string</span><span class=special>& </span><span class=identifier>str</span><span class=special>;
</span><span class=special>};</span></font></code></pre>
<h3>Full Example:</h3>
<p>Here now is our calculator enhanced with semantic actions:</p>
<pre><code><font color="#000000"><span class=special> </span><span class=keyword>namespace
</span><span class=special>{
</span><span class=keyword>void </span><span class=identifier>do_int</span><span class=special>(</span><span class=keyword>char </span><span class=keyword>const</span><span class=special>* </span><span class=identifier>str</span><span class=special>, </span><span class=keyword>char </span><span class=keyword>const</span><span class=special>* </span><span class=identifier>end</span><span class=special>)
</span><span class=special>{
</span><span class=identifier>string </span><span class=identifier>s</span><span class=special>(</span><span class=identifier>str</span><span class=special>, </span><span class=identifier>end</span><span class=special>);
</span><span class=identifier>cout </span><span class=special><< </span><span class=string>"PUSH(" </span><span class=special><< </span><span class=identifier>s </span><span class=special><< </span><span class=literal>')' </span><span class=special><< </span><span class=identifier>endl</span><span class=special>;
</span><span class=special>}
</span><span class=keyword>void </span><span class=identifier>do_add</span><span class=special>(</span><span class=keyword>char </span><span class=keyword>const</span><span class=special>*, </span><span class=keyword>char </span><span class=keyword>const</span><span class=special>*) </span><span class=special>{ </span><span class=identifier>cout </span><span class=special><< </span><span class=string>"ADD\n"</span><span class=special>; </span><span class=special>}
</span><span class=keyword>void </span><span class=identifier>do_subt</span><span class=special>(</span><span class=keyword>char </span><span class=keyword>const</span><span class=special>*, </span><span class=keyword>char </span><span class=keyword>const</span><span class=special>*) </span><span class=special>{ </span><span class=identifier>cout </span><span class=special><< </span><span class=string>"SUBTRACT\n"</span><span class=special>; </span><span class=special>}
</span><span class=keyword>void </span><span class=identifier>do_mult</span><span class=special>(</span><span class=keyword>char </span><span class=keyword>const</span><span class=special>*, </span><span class=keyword>char </span><span class=keyword>const</span><span class=special>*) </span><span class=special>{ </span><span class=identifier>cout </span><span class=special><< </span><span class=string>"MULTIPLY\n"</span><span class=special>; </span><span class=special>}
</span><span class=keyword>void </span><span class=identifier>do_div</span><span class=special>(</span><span class=keyword>char </span><span class=keyword>const</span><span class=special>*, </span><span class=keyword>char </span><span class=keyword>const</span><span class=special>*) </span><span class=special>{ </span><span class=identifier>cout </span><span class=special><< </span><span class=string>"DIVIDE\n"</span><span class=special>; </span><span class=special>}
</span><span class=keyword>void </span><span class=identifier>do_neg</span><span class=special>(</span><span class=keyword>char </span><span class=keyword>const</span><span class=special>*, </span><span class=keyword>char </span><span class=keyword>const</span><span class=special>*) </span><span class=special>{ </span><span class=identifier>cout </span><span class=special><< </span><span class=string>"NEGATE\n"</span><span class=special>; </span><span class=special>}
</span><span class=special>}</span></font></code></pre>
<p>We augment our grammar with semantic actions:</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><</span><span class=identifier>calculator</span><span class=special>>
</span><span class=special>{
</span><span class=keyword>template </span><span class=special><</span><span class=keyword>typename </span><span class=identifier>ScannerT</span><span class=special>>
</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>expression
</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><span class=identifier>do_add</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><span class=identifier>do_subt</span><span class=special>]
</span><span class=special>)
</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=special>(</span><span class=literal>'*' </span><span class=special>>> </span><span class=identifier>factor</span><span class=special>)[&</span><span class=identifier>do_mult</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>do_div</span><span class=special>]
</span><span class=special>)
</span><span class=special>;
</span><span class=identifier>factor
</span><span class=special>= </span><span class=identifier>lexeme_d</span><span class=special>[(+</span><span class=identifier>digit_p</span><span class=special>)[&</span><span class=identifier>do_int</span><span class=special>]]
</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=special>(</span><span class=literal>'-' </span><span class=special>>> </span><span class=identifier>factor</span><span class=special>)[&</span><span class=identifier>do_neg</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=special>;
</span><span class=special>}
</span><span class=identifier>rule</span><span class=special><</span><span class=identifier>ScannerT</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=identifier>factor</span><span class=special>;
</span><span class=identifier>rule</span><span class=special><</span><span class=identifier>ScannerT</span><span class=special>> </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>Feeding in the expression <tt>(-1 + 2) * (3 + -4)</tt>, for example, to the
rule <tt>expression</tt> will produce the expected output:</p>
<pre><code><span class=special>-</span><span class=number>1
</span><span class=number>2
</span><span class=identifier>ADD
</span><span class=number>3
</span><span class=special>-</span><span class=number>4
</span><span class=identifier>ADD
</span><span class=identifier>MULT</span></code></pre>
<p>which, by the way, is the Reverse Polish Notation (RPN) of the given expression,
reminiscent of some primitive calculators and the language Forth.</p>
<p><img src="theme/lens.gif" width="15" height="16"> <a href="../example/fundamental/calc_plain.cpp">View
the complete source code here</a>. This is part of the Spirit distribution.
</p>
<h2><a name="specialized_actions"></a>Specialized Actions</h2>
<p>In general, semantic actions accept the first-last iterator pair. There are
situations though where we might want to pass data in its processed form. A
concrete example is the numeric parser. It is unwise to pass unprocessed data
to a semantic action attached to a numeric parser and just throw away what has
been parsed by the parser. We want to pass the actual parsed number.</p>
<p>The function and functor signature of a semantic action varies depending on
the parser where it is attached to. The following table lists the parsers that
accept unique signatures.</p>
<table width="80%" border="0" align="center">
<tr>
<td class="note_box"><img src="theme/note.gif" width="16" height="16"> Unless
explicitly stated in the documentation of a specific parser type, parsers
not included in the list by default expect the generic signature as explained
above.</td>
</tr>
</table>
<h3>Numeric Actions</h3>
<p><b>Applies to:</b></p>
<blockquote>
<p><img src="theme/bullet.gif" width="13" height="13"> uint_p<br>
<img src="theme/bullet.gif" width="13" height="13"> int_p<br>
<img src="theme/bullet.gif" width="13" height="13"> ureal_p<br>
<img src="theme/bullet.gif" width="13" height="13"> real_p</p>
</blockquote>
<p><b>Signature for functions:</b></p>
<pre><code><font color="#000000"><span class=identifier> </span><span class=keyword>void </span><span class=identifier>func</span><span class=special>(</span><span class=identifier>NumT </span><span class=identifier>val</span><span class=special>);</span></font></code></pre>
<p><b>Signature for functors:</b> </p>
<pre><code><font color="#000000"><span class=special> </span><span class=keyword>struct </span><span class=identifier>ftor
</span><span class=special>{
</span><span class=keyword>void </span><span class=keyword>operator</span><span class=special>()(</span><span class=identifier>NumT </span><span class=identifier>val</span><span class=special>) </span><span class=keyword>const</span><span class=special>;
</span><span class=special>};</span></font></code></pre>
<p>Where <tt>NumT</tt> is any primitive numeric type such as <tt>int</tt>, <tt>long</tt>,
<tt>float</tt>, <tt>double</tt>, etc., or a user defined numeric type such as
big_int. <tt>NumT</tt> is the same type used as template parameter to <tt>uint_p</tt>,
<tt>int_p</tt>, <tt>ureal_p</tt> or <tt>real_p</tt>. The parsed number is passed
into the function/functor.</p>
<h3>Character Actions</h3>
<p><b>Applies to:</b></p>
<blockquote>
<p><img src="theme/bullet.gif" width="13" height="13"> chlit, ch_p<br>
<img src="theme/bullet.gif" width="13" height="13"> range, range_p<br>
<img src="theme/bullet.gif" width="13" height="13"> anychar<br>
<img src="theme/bullet.gif" width="13" height="13"> alnum, alpha<br>
<img src="theme/bullet.gif" width="13" height="13"> cntrl, digit<br>
<img src="theme/bullet.gif" width="13" height="13"> graph, lower<br>
<img src="theme/bullet.gif" width="13" height="13"> print, punct<br>
<img src="theme/bullet.gif" width="13" height="13"> space, upper<br>
<img src="theme/bullet.gif" width="13" height="13"> xdigit</p>
</blockquote>
<p><b>Signature for functions:</b></p>
<pre><code><font color="#000000"><span class=identifier> </span><span class=keyword>void </span><span class=identifier>func</span><span class=special>(</span><span class=identifier>CharT </span><span class=identifier>ch</span><span class=special>);</span></font></code></pre>
<p><b>Signature for functors:</b></p>
<pre><code><font color="#000000"><span class=special> </span><span class=keyword>struct </span><span class=identifier>ftor
</span><span class=special>{
</span><span class=keyword>void </span><span class=keyword>operator</span><span class=special>()(</span><span class=identifier>CharT </span><span class=identifier>ch</span><span class=special>) </span><span class=keyword>const</span><span class=special>;
</span><span class=special>};</span></font></code></pre>
<p>Where <tt>CharT</tt> is the value_type of the iterator used in parsing. A <tt>char
const*</tt> iterator for example has a <tt>value_type</tt> of <tt>char</tt>.
The matching character is passed into the function/functor.</p>
<h2>Cascading Actions</h2>
<p>Actions can be cascaded. Cascaded actions also inherit the function/functor
interface of the original. For example:</p>
<pre><code><font color="#000000"><span class=special> </span><span class=identifier>uint_p</span><span class=special>[</span><span class=identifier>fa</span><span class=special>][</span><span class=identifier>fb</span><span class=special>][</span><span class=identifier>fc</span><span class=special>]</span></font></code></pre>
<p>Here, the functors <tt>fa</tt>, <tt>fb</tt> and <tt>fc</tt> all expect the
signature <tt>void operator()(unsigned n) const</tt>.</p>
<h2>Directives and Actions</h2>
<p>Directives inherit the the function/functor interface of the subject it is
enclosing. Example:</p>
<pre><code><font color="#000000"><span class=special> </span><span class=identifier>as_lower_d</span><span class=special>[</span><span class=identifier>ch_p</span><span class=special>(</span><span class=literal>'x'</span><span class=special>)][</span><span class=identifier>f</span><span class=special>]</span></font></code></pre>
<p>Here, the functor <tt>f</tt> expects the signature <tt>void operator()(char
ch) const</tt>, assuming that the iterator used is a <tt>char const*</tt>.</p>
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