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<div class="titlepage"><div><div><h4 class="title">
<a name="spirit.lex.abstracts.lexer_semantic_actions"></a><a class="link" href="lexer_semantic_actions.html" title="Lexer Semantic Actions"> Lexer
Semantic Actions</a>
</h4></div></div></div>
<p>
The main task of a lexer normally is to recognize tokens in the input.
Traditionally this has been complemented with the possibility to execute
arbitrary code whenever a certain token has been detected. <span class="emphasis"><em>Spirit.Lex</em></span>
has been designed to support this mode of operation as well. We borrow
from the concept of semantic actions for parsers (<span class="emphasis"><em>Spirit.Qi</em></span>)
and generators (<span class="emphasis"><em>Spirit.Karma</em></span>). Lexer semantic actions
may be attached to any token definition. These are C++ functions or function
objects that are called whenever a token definition successfully recognizes
a portion of the input. Say you have a token definition <code class="computeroutput"><span class="identifier">D</span></code>,
and a C++ function <code class="computeroutput"><span class="identifier">f</span></code>, you
can make the lexer call <code class="computeroutput"><span class="identifier">f</span></code>
whenever it matches an input by attaching <code class="computeroutput"><span class="identifier">f</span></code>:
</p>
<pre class="programlisting"><span class="identifier">D</span><span class="special">[</span><span class="identifier">f</span><span class="special">]</span>
</pre>
<p>
The expression above links <code class="computeroutput"><span class="identifier">f</span></code>
to the token definition, <code class="computeroutput"><span class="identifier">D</span></code>.
The required prototype of <code class="computeroutput"><span class="identifier">f</span></code>
is:
</p>
<pre class="programlisting"><span class="keyword">void</span> <span class="identifier">f</span> <span class="special">(</span><span class="identifier">Iterator</span><span class="special">&</span> <span class="identifier">start</span><span class="special">,</span> <span class="identifier">Iterator</span><span class="special">&</span> <span class="identifier">end</span><span class="special">,</span> <span class="identifier">pass_flag</span><span class="special">&</span> <span class="identifier">matched</span><span class="special">,</span> <span class="identifier">Idtype</span><span class="special">&</span> <span class="identifier">id</span><span class="special">,</span> <span class="identifier">Context</span><span class="special">&</span> <span class="identifier">ctx</span><span class="special">);</span>
</pre>
<div class="variablelist">
<p class="title"><b>where:</b></p>
<dl>
<dt><span class="term"><code class="computeroutput"><span class="identifier">Iterator</span><span class="special">&</span>
<span class="identifier">start</span></code></span></dt>
<dd><p>
This is the iterator pointing to the begin of the matched range in
the underlying input sequence. The type of the iterator is the same
as specified while defining the type of the <code class="computeroutput"><span class="identifier">lexertl</span><span class="special">::</span><span class="identifier">actor_lexer</span><span class="special"><...></span></code> (its first template parameter).
The semantic action is allowed to change the value of this iterator
influencing, the matched input sequence.
</p></dd>
<dt><span class="term"><code class="computeroutput"><span class="identifier">Iterator</span><span class="special">&</span>
<span class="identifier">end</span></code></span></dt>
<dd><p>
This is the iterator pointing to the end of the matched range in
the underlying input sequence. The type of the iterator is the same
as specified while defining the type of the <code class="computeroutput"><span class="identifier">lexertl</span><span class="special">::</span><span class="identifier">actor_lexer</span><span class="special"><...></span></code> (its first template parameter).
The semantic action is allowed to change the value of this iterator
influencing, the matched input sequence.
</p></dd>
<dt><span class="term"><code class="computeroutput"><span class="identifier">pass_flag</span><span class="special">&</span>
<span class="identifier">matched</span></code></span></dt>
<dd><p>
This value is pre/initialized to <code class="computeroutput"><span class="identifier">pass_normal</span></code>.
If the semantic action sets it to <code class="computeroutput"><span class="identifier">pass_fail</span></code>
this behaves as if the token has not been matched in the first place.
If the semantic action sets this to <code class="computeroutput"><span class="identifier">pass_ignore</span></code>
the lexer ignores the current token and tries to match a next token
from the input.
</p></dd>
<dt><span class="term"><code class="computeroutput"><span class="identifier">Idtype</span><span class="special">&</span>
<span class="identifier">id</span></code></span></dt>
<dd><p>
This is the token id of the type Idtype (most of the time this will
be a <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span></code>) for the matched token.
The semantic action is allowed to change the value of this token
id, influencing the if of the created token.
</p></dd>
<dt><span class="term"><code class="computeroutput"><span class="identifier">Context</span><span class="special">&</span>
<span class="identifier">ctx</span></code></span></dt>
<dd><p>
This is a reference to a lexer specific, unspecified type, providing
the context for the current lexer state. It can be used to access
different internal data items and is needed for lexer state control
from inside a semantic action.
</p></dd>
</dl>
</div>
<p>
When using a C++ function as the semantic action the following prototypes
are allowed as well:
</p>
<pre class="programlisting"><span class="keyword">void</span> <span class="identifier">f</span> <span class="special">(</span><span class="identifier">Iterator</span><span class="special">&</span> <span class="identifier">start</span><span class="special">,</span> <span class="identifier">Iterator</span><span class="special">&</span> <span class="identifier">end</span><span class="special">,</span> <span class="identifier">pass_flag</span><span class="special">&</span> <span class="identifier">matched</span><span class="special">,</span> <span class="identifier">Idtype</span><span class="special">&</span> <span class="identifier">id</span><span class="special">);</span>
<span class="keyword">void</span> <span class="identifier">f</span> <span class="special">(</span><span class="identifier">Iterator</span><span class="special">&</span> <span class="identifier">start</span><span class="special">,</span> <span class="identifier">Iterator</span><span class="special">&</span> <span class="identifier">end</span><span class="special">,</span> <span class="identifier">pass_flag</span><span class="special">&</span> <span class="identifier">matched</span><span class="special">);</span>
<span class="keyword">void</span> <span class="identifier">f</span> <span class="special">(</span><span class="identifier">Iterator</span><span class="special">&</span> <span class="identifier">start</span><span class="special">,</span> <span class="identifier">Iterator</span><span class="special">&</span> <span class="identifier">end</span><span class="special">);</span>
<span class="keyword">void</span> <span class="identifier">f</span> <span class="special">();</span>
</pre>
<div class="important"><table border="0" summary="Important">
<tr>
<td rowspan="2" align="center" valign="top" width="25"><img alt="[Important]" src="../../../images/important.png"></td>
<th align="left">Important</th>
</tr>
<tr><td align="left" valign="top"><p>
In order to use lexer semantic actions you need to use type <code class="computeroutput"><span class="identifier">lexertl</span><span class="special">::</span><span class="identifier">actor_lexer</span><span class="special"><></span></code>
as your lexer class (instead of the type <code class="computeroutput"><span class="identifier">lexertl</span><span class="special">::</span><span class="identifier">lexer</span><span class="special"><></span></code> as described in earlier examples).
</p></td></tr>
</table></div>
<a name="spirit.lex.abstracts.lexer_semantic_actions.the_context_of_a_lexer_semantic_action"></a><h6>
<a name="id1176393"></a>
<a class="link" href="lexer_semantic_actions.html#spirit.lex.abstracts.lexer_semantic_actions.the_context_of_a_lexer_semantic_action">The
context of a lexer semantic action</a>
</h6>
<p>
The last parameter passed to any lexer semantic action is a reference to
an unspecified type (see the <code class="computeroutput"><span class="identifier">Context</span></code>
type in the table above). This type is unspecified because it depends on
the token type returned by the lexer. It is implemented in the internals
of the iterator type exposed by the lexer. Nevertheless, any context type
is expected to expose a couple of functions allowing to influence the behavior
of the lexer. The following table gives an overview and a short description
of the available functionality.
</p>
<div class="table">
<a name="id1176414"></a><p class="title"><b>Table 8. Functions exposed by any context passed to a lexer semantic action</b></p>
<div class="table-contents"><table class="table" summary="Functions exposed by any context passed to a lexer semantic action">
<colgroup>
<col>
<col>
</colgroup>
<thead><tr>
<th>
<p>
Name
</p>
</th>
<th>
<p>
Description
</p>
</th>
</tr></thead>
<tbody>
<tr>
<td>
<p>
<code class="computeroutput"><span class="identifier">Iterator</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">get_eoi</span><span class="special">()</span> <span class="keyword">const</span></code>
</p>
</td>
<td>
<p>
The function <code class="computeroutput"><span class="identifier">get_eoi</span><span class="special">()</span></code> may be used by to access the
end iterator of the input stream the lexer has been initialized
with
</p>
</td>
</tr>
<tr>
<td>
<p>
<code class="computeroutput"><span class="keyword">void</span> <span class="identifier">more</span><span class="special">()</span></code>
</p>
</td>
<td>
<p>
The function <code class="computeroutput"><span class="identifier">more</span><span class="special">()</span></code> tells the lexer that the next
time it matches a rule, the corresponding token should be appended
onto the current token value rather than replacing it.
</p>
</td>
</tr>
<tr>
<td>
<p>
<code class="computeroutput"><span class="identifier">Iterator</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">less</span><span class="special">(</span><span class="identifier">Iterator</span>
<span class="keyword">const</span><span class="special">&</span>
<span class="identifier">it</span><span class="special">,</span>
<span class="keyword">int</span> <span class="identifier">n</span><span class="special">)</span></code>
</p>
</td>
<td>
<p>
The function <code class="computeroutput"><span class="identifier">less</span><span class="special">()</span></code> returns an iterator positioned
to the nth input character beyond the current token start iterator
(i.e. by passing the return value to the parameter <code class="computeroutput"><span class="identifier">end</span></code> it is possible to return
all but the first n characters of the current token back to the
input stream.
</p>
</td>
</tr>
<tr>
<td>
<p>
<code class="computeroutput"><span class="keyword">bool</span> <span class="identifier">lookahead</span><span class="special">(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span>
<span class="identifier">id</span><span class="special">)</span></code>
</p>
</td>
<td>
<p>
The function <code class="computeroutput"><span class="identifier">lookahead</span><span class="special">()</span></code> can be used to implement lookahead
for lexer engines not supporting constructs like flex' <code class="computeroutput"><span class="identifier">a</span><span class="special">/</span><span class="identifier">b</span></code> (match <code class="computeroutput"><span class="identifier">a</span></code>,
but only when followed by <code class="computeroutput"><span class="identifier">b</span></code>).
It invokes the lexer on the input following the current token
without actually moving forward in the input stream. The function
returns whether the lexer was able to match a token with the
given token-id <code class="computeroutput"><span class="identifier">id</span></code>.
</p>
</td>
</tr>
<tr>
<td>
<p>
<code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span> <span class="identifier">get_state</span><span class="special">()</span> <span class="keyword">const</span></code>
and <code class="computeroutput"><span class="keyword">void</span> <span class="identifier">set_state</span><span class="special">(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span>
<span class="identifier">state</span><span class="special">)</span></code>
</p>
</td>
<td>
<p>
The functions <code class="computeroutput"><span class="identifier">get_state</span><span class="special">()</span></code> and <code class="computeroutput"><span class="identifier">set_state</span><span class="special">()</span></code> may be used to introspect and
change the current lexer state.
</p>
</td>
</tr>
<tr>
<td>
<p>
<code class="computeroutput"><span class="identifier">token_value_type</span> <span class="identifier">get_value</span><span class="special">()</span>
<span class="keyword">const</span></code> and <code class="computeroutput"><span class="keyword">void</span> <span class="identifier">set_value</span><span class="special">(</span><span class="identifier">Value</span>
<span class="keyword">const</span><span class="special">&)</span></code>
</p>
</td>
<td>
<p>
The functions <code class="computeroutput"><span class="identifier">get_value</span><span class="special">()</span></code> and <code class="computeroutput"><span class="identifier">set_value</span><span class="special">()</span></code> may be used to introspect and
change the current token value.
</p>
</td>
</tr>
</tbody>
</table></div>
</div>
<br class="table-break"><a name="spirit.lex.abstracts.lexer_semantic_actions.lexer_semantic_actions_using_phoenix"></a><h6>
<a name="id1177768"></a>
<a class="link" href="lexer_semantic_actions.html#spirit.lex.abstracts.lexer_semantic_actions.lexer_semantic_actions_using_phoenix">Lexer
Semantic Actions Using Phoenix</a>
</h6>
<p>
Even if it is possible to write your own function object implementations
(i.e. using Boost.Lambda or Boost.Bind), the preferred way of defining
lexer semantic actions is to use <a href="../../../../../phoenix/doc/html/index.html" target="_top">Boost.Phoenix</a>.
In this case you can access the parameters described above by using the
predefined <a href="http://boost-spirit.com" target="_top">Spirit</a> placeholders:
</p>
<div class="table">
<a name="id1177791"></a><p class="title"><b>Table 9. Predefined Phoenix placeholders for lexer semantic actions</b></p>
<div class="table-contents"><table class="table" summary="Predefined Phoenix placeholders for lexer semantic actions">
<colgroup>
<col>
<col>
</colgroup>
<thead><tr>
<th>
<p>
Placeholder
</p>
</th>
<th>
<p>
Description
</p>
</th>
</tr></thead>
<tbody>
<tr>
<td>
<p>
<code class="computeroutput"><span class="identifier">_start</span></code>
</p>
</td>
<td>
<p>
Refers to the iterator pointing to the beginning of the matched
input sequence. Any modifications to this iterator value will
be reflected in the generated token.
</p>
</td>
</tr>
<tr>
<td>
<p>
<code class="computeroutput"><span class="identifier">_end</span></code>
</p>
</td>
<td>
<p>
Refers to the iterator pointing past the end of the matched input
sequence. Any modifications to this iterator value will be reflected
in the generated token.
</p>
</td>
</tr>
<tr>
<td>
<p>
<code class="computeroutput"><span class="identifier">_pass</span></code>
</p>
</td>
<td>
<p>
References the value signaling the outcome of the semantic action.
This is pre-initialized to <code class="computeroutput"><span class="identifier">lex</span><span class="special">::</span><span class="identifier">pass_flags</span><span class="special">::</span><span class="identifier">pass_normal</span></code>.
If this is set to <code class="computeroutput"><span class="identifier">lex</span><span class="special">::</span><span class="identifier">pass_flags</span><span class="special">::</span><span class="identifier">pass_fail</span></code>,
the lexer will behave as if no token has been matched, if is
set to <code class="computeroutput"><span class="identifier">lex</span><span class="special">::</span><span class="identifier">pass_flags</span><span class="special">::</span><span class="identifier">pass_ignore</span></code>, the lexer will
ignore the current match and proceed trying to match tokens from
the input.
</p>
</td>
</tr>
<tr>
<td>
<p>
<code class="computeroutput"><span class="identifier">_tokenid</span></code>
</p>
</td>
<td>
<p>
Refers to the token id of the token to be generated. Any modifications
to this value will be reflected in the generated token.
</p>
</td>
</tr>
<tr>
<td>
<p>
<code class="computeroutput"><span class="identifier">_val</span></code>
</p>
</td>
<td>
<p>
Refers to the value the next token will be initialized from.
Any modifications to this value will be reflected in the generated
token.
</p>
</td>
</tr>
<tr>
<td>
<p>
<code class="computeroutput"><span class="identifier">_state</span></code>
</p>
</td>
<td>
<p>
Refers to the lexer state the input has been match in. Any modifications
to this value will be reflected in the lexer itself (the next
match will start in the new state). The currently generated token
is not affected by changes to this variable.
</p>
</td>
</tr>
<tr>
<td>
<p>
<code class="computeroutput"><span class="identifier">_eoi</span></code>
</p>
</td>
<td>
<p>
References the end iterator of the overall lexer input. This
value cannot be changed.
</p>
</td>
</tr>
</tbody>
</table></div>
</div>
<br class="table-break"><p>
The context object passed as the last parameter to any lexer semantic action
is not directly accessible while using <a href="../../../../../phoenix/doc/html/index.html" target="_top">Boost.Phoenix</a>
expressions. We rather provide predefined Phoenix functions allowing to
invoke the different support functions as mentioned above. The following
table lists the available support functions and describes their functionality:
</p>
<div class="table">
<a name="id1178080"></a><p class="title"><b>Table 10. Support functions usable from Phoenix expressions inside lexer semantic
actions</b></p>
<div class="table-contents"><table class="table" summary="Support functions usable from Phoenix expressions inside lexer semantic
actions">
<colgroup>
<col>
<col>
<col>
</colgroup>
<thead><tr>
<th>
<p>
Plain function
</p>
</th>
<th>
<p>
Phoenix function
</p>
</th>
<th>
<p>
Description
</p>
</th>
</tr></thead>
<tbody>
<tr>
<td>
<p>
<code class="computeroutput"><span class="identifier">ctx</span><span class="special">.</span><span class="identifier">more</span><span class="special">()</span></code>
</p>
</td>
<td>
<p>
<code class="computeroutput"><span class="identifier">more</span><span class="special">()</span></code>
</p>
</td>
<td>
<p>
The function <code class="computeroutput"><span class="identifier">more</span><span class="special">()</span></code> tells the lexer that the next
time it matches a rule, the corresponding token should be appended
onto the current token value rather than replacing it.
</p>
</td>
</tr>
<tr>
<td>
<p>
<code class="computeroutput"><span class="identifier">ctx</span><span class="special">.</span><span class="identifier">less</span><span class="special">()</span></code>
</p>
</td>
<td>
<p>
<code class="computeroutput"><span class="identifier">less</span><span class="special">(</span><span class="identifier">n</span><span class="special">)</span></code>
</p>
</td>
<td>
<p>
The function <code class="computeroutput"><span class="identifier">less</span><span class="special">()</span></code> takes a single integer parameter
<code class="computeroutput"><span class="identifier">n</span></code> and returns
an iterator positioned to the nth input character beyond the
current token start iterator (i.e. by assigning the return value
to the placeholder <code class="computeroutput"><span class="identifier">_end</span></code>
it is possible to return all but the first <code class="computeroutput"><span class="identifier">n</span></code>
characters of the current token back to the input stream.
</p>
</td>
</tr>
<tr>
<td>
<p>
<code class="computeroutput"><span class="identifier">ctx</span><span class="special">.</span><span class="identifier">lookahead</span><span class="special">()</span></code>
</p>
</td>
<td>
<p>
<code class="computeroutput"><span class="identifier">lookahead</span><span class="special">(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span><span class="special">)</span></code>
or <code class="computeroutput"><span class="identifier">lookahead</span><span class="special">(</span><span class="identifier">token_def</span><span class="special">)</span></code>
</p>
</td>
<td>
<p>
The function <code class="computeroutput"><span class="identifier">lookahead</span><span class="special">()</span></code> takes a single parameter specifying
the token to match in the input. The function can be used for
instance to implement lookahead for lexer engines not supporting
constructs like flex' <code class="computeroutput"><span class="identifier">a</span><span class="special">/</span><span class="identifier">b</span></code>
(match <code class="computeroutput"><span class="identifier">a</span></code>, but
only when followed by <code class="computeroutput"><span class="identifier">b</span></code>).
It invokes the lexer on the input following the current token
without actually moving forward in the input stream. The function
returns whether the lexer was able to match the specified token.
</p>
</td>
</tr>
</tbody>
</table></div>
</div>
<br class="table-break">
</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>
</div></td>
</tr></table>
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