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<div class="titlepage"><div><div><h4 class="title">
<a name="spirit.lex.abstracts.lexer_static_model"></a><a class="link" href="lexer_static_model.html" title="The Static Lexer Model"> The <span class="emphasis"><em>Static</em></span>
Lexer Model</a>
</h4></div></div></div>
<p>
The documentation of <span class="emphasis"><em>Spirit.Lex</em></span> so far mostly was
about describing the features of the <span class="emphasis"><em>dynamic</em></span> model,
where the tables needed for lexical analysis are generated from the regular
expressions at runtime. The big advantage of the dynamic model is its flexibility,
and its integration with the <a href="http://boost-spirit.com" target="_top">Spirit</a>
library and the C++ host language. Its big disadvantage is the need to
spend additional runtime to generate the tables, which especially might
be a limitation for larger lexical analyzers. The <span class="emphasis"><em>static</em></span>
model strives to build upon the smooth integration with <a href="http://boost-spirit.com" target="_top">Spirit</a>
and C++, and reuses large parts of the <span class="emphasis"><em>Spirit.Lex</em></span>
library as described so far, while overcoming the additional runtime requirements
by using pre-generated tables and tokenizer routines. To make the code
generation as simple as possible, the static model reuses the token definition
types developed for the <span class="emphasis"><em>dynamic</em></span> model without any
changes. As will be shown in this section, building a code generator based
on an existing token definition type is a matter of writing 3 lines of
code.
</p>
<p>
Assuming you already built a dynamic lexer for your problem, there are
two more steps needed to create a static lexical analyzer using <span class="emphasis"><em>Spirit.Lex</em></span>:
</p>
<div class="orderedlist"><ol class="orderedlist" type="1">
<li class="listitem">
generating the C++ code for the static analyzer (including the tokenization
function and corresponding tables), and
</li>
<li class="listitem">
modifying the dynamic lexical anlyzer to use the generated code.
</li>
</ol></div>
<p>
Both steps are described in more detail in the two sections below (for
the full source code used in this example see the code here: <a href="../../../../../example/lex/static_lexer/word_count_tokens.hpp" target="_top">the
common token definition</a>, <a href="../../../../../example/lex/static_lexer/word_count_generate.cpp" target="_top">the
code generator</a>, <a href="../../../../../example/lex/static_lexer/word_count_static.hpp" target="_top">the
generated code</a>, and <a href="../../../../../example/lex/static_lexer/word_count_static.cpp" target="_top">the
static lexical analyzer</a>).
</p>
<p>
But first we provide the code snippets needed to further understand the
descriptions. Both, the definition of the used token identifier and the
of the token definition class in this example are put into a separate header
file to make these available to the code generator and the static lexical
analyzer.
</p>
<p>
</p>
<pre class="programlisting"><span class="keyword">enum</span> <span class="identifier">tokenids</span>
<span class="special">{</span>
<span class="identifier">IDANY</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">lex</span><span class="special">::</span><span class="identifier">min_token_id</span> <span class="special">+</span> <span class="number">1</span><span class="special">,</span>
<span class="special">};</span>
</pre>
<p>
</p>
<p>
The important point here is, that the token definition class is not different
from a similar class to be used for a dynamic lexical analyzer. The library
has been designed in a way, that all components (dynamic lexical analyzer,
code generator, and static lexical analyzer) can reuse the very same token
definition syntax.
</p>
<p>
</p>
<pre class="programlisting"><span class="comment">// This token definition class can be used without any change for all three
</span><span class="comment">// possible use cases: a dynamic lexical analyzer, a code generator, and a
</span><span class="comment">// static lexical analyzer.
</span><span class="keyword">template</span> <span class="special"><</span><span class="keyword">typename</span> <span class="identifier">BaseLexer</span><span class="special">></span>
<span class="keyword">struct</span> <span class="identifier">word_count_tokens</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">lex</span><span class="special">::</span><span class="identifier">lexer</span><span class="special"><</span><span class="identifier">BaseLexer</span><span class="special">></span>
<span class="special">{</span>
<span class="identifier">word_count_tokens</span><span class="special">()</span>
<span class="special">:</span> <span class="identifier">word_count_tokens</span><span class="special">::</span><span class="identifier">base_type</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">lex</span><span class="special">::</span><span class="identifier">match_flags</span><span class="special">::</span><span class="identifier">match_not_dot_newline</span><span class="special">)</span>
<span class="special">{</span>
<span class="comment">// define tokens and associate them with the lexer
</span> <span class="identifier">word</span> <span class="special">=</span> <span class="string">"[^ \t\n]+"</span><span class="special">;</span>
<span class="keyword">this</span><span class="special">-></span><span class="identifier">self</span> <span class="special">=</span> <span class="identifier">word</span> <span class="special">|</span> <span class="char">'\n'</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">lex</span><span class="special">::</span><span class="identifier">token_def</span><span class="special"><>(</span><span class="string">"."</span><span class="special">,</span> <span class="identifier">IDANY</span><span class="special">);</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">lex</span><span class="special">::</span><span class="identifier">token_def</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">word</span><span class="special">;</span>
<span class="special">};</span>
</pre>
<p>
</p>
<p>
The only thing changing between the three different use cases is the template
parameter used to instantiate a concrete token definition. Fot the dynamic
model and the code generator you probably will use the <code class="computeroutput"><span class="identifier">lex</span><span class="special">::</span><span class="identifier">lexertl</span><span class="special">::</span><span class="identifier">lexer</span><span class="special"><></span></code> template, where for the static
model you will use the <code class="computeroutput"><span class="identifier">lex</span><span class="special">::</span><span class="identifier">lexertl</span><span class="special">::</span><span class="identifier">static_lexer</span><span class="special"><></span></code> type as the template parameter.
</p>
<p>
This example not only shows how to build a static lexer, but it additionally
demonstrates how such a lexer can be used for parsing in conjunction with
a <span class="emphasis"><em>Spirit.Qi</em></span> grammar. For completeness, we provide
the simple grammar used in this example. As you can see, this grammar does
not have any dependencies on the static lexical analyzer, and for this
reason it is not different from a grammar used either without a lexer or
using a dynamic lexical analyzer as described before.
</p>
<p>
</p>
<pre class="programlisting"><span class="comment">// This is an ordinary grammar definition following the rules defined by
</span><span class="comment">// Spirit.Qi. There is nothing specific about it, except it gets the token
</span><span class="comment">// definition class instance passed to the constructor to allow accessing the
</span><span class="comment">// embedded token_def<> instances.
</span><span class="keyword">template</span> <span class="special"><</span><span class="keyword">typename</span> <span class="identifier">Iterator</span><span class="special">></span>
<span class="keyword">struct</span> <span class="identifier">word_count_grammar</span> <span class="special">:</span> <span class="identifier">qi</span><span class="special">::</span><span class="identifier">grammar</span><span class="special"><</span><span class="identifier">Iterator</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">TokenDef</span><span class="special">></span>
<span class="identifier">word_count_grammar</span><span class="special">(</span><span class="identifier">TokenDef</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">tok</span><span class="special">)</span>
<span class="special">:</span> <span class="identifier">word_count_grammar</span><span class="special">::</span><span class="identifier">base_type</span><span class="special">(</span><span class="identifier">start</span><span class="special">)</span>
<span class="special">,</span> <span class="identifier">c</span><span class="special">(</span><span class="number">0</span><span class="special">),</span> <span class="identifier">w</span><span class="special">(</span><span class="number">0</span><span class="special">),</span> <span class="identifier">l</span><span class="special">(</span><span class="number">0</span><span class="special">)</span>
<span class="special">{</span>
<span class="keyword">using</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">phoenix</span><span class="special">::</span><span class="identifier">ref</span><span class="special">;</span>
<span class="keyword">using</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">phoenix</span><span class="special">::</span><span class="identifier">size</span><span class="special">;</span>
<span class="comment">// associate the defined tokens with the lexer, at the same time
</span> <span class="comment">// defining the actions to be executed
</span> <span class="identifier">start</span> <span class="special">=</span> <span class="special">*(</span> <span class="identifier">tok</span><span class="special">.</span><span class="identifier">word</span> <span class="special">[</span> <span class="special">++</span><span class="identifier">ref</span><span class="special">(</span><span class="identifier">w</span><span class="special">),</span> <span class="identifier">ref</span><span class="special">(</span><span class="identifier">c</span><span class="special">)</span> <span class="special">+=</span> <span class="identifier">size</span><span class="special">(</span><span class="identifier">_1</span><span class="special">)</span> <span class="special">]</span>
<span class="special">|</span> <span class="identifier">lit</span><span class="special">(</span><span class="char">'\n'</span><span class="special">)</span> <span class="special">[</span> <span class="special">++</span><span class="identifier">ref</span><span class="special">(</span><span class="identifier">l</span><span class="special">),</span> <span class="special">++</span><span class="identifier">ref</span><span class="special">(</span><span class="identifier">c</span><span class="special">)</span> <span class="special">]</span>
<span class="special">|</span> <span class="identifier">qi</span><span class="special">::</span><span class="identifier">token</span><span class="special">(</span><span class="identifier">IDANY</span><span class="special">)</span> <span class="special">[</span> <span class="special">++</span><span class="identifier">ref</span><span class="special">(</span><span class="identifier">c</span><span class="special">)</span> <span class="special">]</span>
<span class="special">)</span>
<span class="special">;</span>
<span class="special">}</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span> <span class="identifier">c</span><span class="special">,</span> <span class="identifier">w</span><span class="special">,</span> <span class="identifier">l</span><span class="special">;</span> <span class="comment">// counter for characters, words, and lines
</span> <span class="identifier">qi</span><span class="special">::</span><span class="identifier">rule</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="special">};</span>
</pre>
<p>
</p>
<a name="spirit.lex.abstracts.lexer_static_model.generating_the_static_analyzer"></a><h6>
<a name="id1179582"></a>
<a class="link" href="lexer_static_model.html#spirit.lex.abstracts.lexer_static_model.generating_the_static_analyzer">Generating
the Static Analyzer</a>
</h6>
<p>
The first additional step to perform in order to create a static lexical
analyzer is to create a small standalone program for creating the lexer
tables and the corresponding tokenization function. For this purpose the
<span class="emphasis"><em>Spirit.Lex</em></span> library exposes a special API - the function
<code class="computeroutput"><span class="identifier">generate_static_dfa</span><span class="special">()</span></code>
. It implements the whole code generator, no further code is needed. All
what it takes to invoke this function is to supply a token definition instance,
an output stream to use to generate the code to, and an optional string
to be used as a suffix for the name of the generated function. All in all
just a couple lines of code.
</p>
<p>
</p>
<pre class="programlisting"><span class="keyword">int</span> <span class="identifier">main</span><span class="special">(</span><span class="keyword">int</span> <span class="identifier">argc</span><span class="special">,</span> <span class="keyword">char</span><span class="special">*</span> <span class="identifier">argv</span><span class="special">[])</span>
<span class="special">{</span>
<span class="comment">// create the lexer object instance needed to invoke the generator
</span> <span class="identifier">word_count_tokens</span><span class="special"><</span><span class="identifier">lex</span><span class="special">::</span><span class="identifier">lexertl</span><span class="special">::</span><span class="identifier">lexer</span><span class="special"><></span> <span class="special">></span> <span class="identifier">word_count</span><span class="special">;</span> <span class="comment">// the token definition
</span>
<span class="comment">// open the output file, where the generated tokenizer function will be
</span> <span class="comment">// written to
</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">ofstream</span> <span class="identifier">out</span><span class="special">(</span><span class="identifier">argc</span> <span class="special"><</span> <span class="number">2</span> <span class="special">?</span> <span class="string">"word_count_static.hpp"</span> <span class="special">:</span> <span class="identifier">argv</span><span class="special">[</span><span class="number">1</span><span class="special">]);</span>
<span class="comment">// invoke the generator, passing the token definition, the output stream
</span> <span class="comment">// and the name suffix of the tables and functions to be generated
</span> <span class="comment">//
</span> <span class="comment">// The suffix "wc" used below results in a type lexertl::static_::lexer_wc
</span> <span class="comment">// to be generated, which needs to be passed as a template parameter to the
</span> <span class="comment">// lexertl::static_lexer template (see word_count_static.cpp).
</span> <span class="keyword">return</span> <span class="identifier">lex</span><span class="special">::</span><span class="identifier">lexertl</span><span class="special">::</span><span class="identifier">generate_static_dfa</span><span class="special">(</span><span class="identifier">word_count</span><span class="special">,</span> <span class="identifier">out</span><span class="special">,</span> <span class="string">"wc"</span><span class="special">)</span> <span class="special">?</span> <span class="number">0</span> <span class="special">:</span> <span class="special">-</span><span class="number">1</span><span class="special">;</span>
<span class="special">}</span>
</pre>
<p>
</p>
<p>
The shown code generator will generate output, which should be stored in
a file for later inclusion into the static lexical analzyer as shown in
the next topic (the full generated code can be viewed <a href="../../../../../example/lex/static_lexer/word_count_static.hpp" target="_top">here</a>).
</p>
<div class="note"><table border="0" summary="Note">
<tr>
<td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../images/note.png"></td>
<th align="left">Note</th>
</tr>
<tr><td align="left" valign="top"><p>
The generated code will have compiled in the version number of the current
<span class="emphasis"><em>Spirit.Lex</em></span> library. This version number is used
at compilation time of your static lexer object to ensure this is compiled
using exactly the same version of the <span class="emphasis"><em>Spirit.Lex</em></span>
library as the lexer tables have been generated with. If the versions
do not match you will see an compilation error mentioning an <code class="computeroutput"><span class="identifier">incompatible_static_lexer_version</span></code>.
</p></td></tr>
</table></div>
<a name="spirit.lex.abstracts.lexer_static_model.modifying_the_dynamic_analyzer"></a><h6>
<a name="id1181346"></a>
<a class="link" href="lexer_static_model.html#spirit.lex.abstracts.lexer_static_model.modifying_the_dynamic_analyzer">Modifying
the Dynamic Analyzer</a>
</h6>
<p>
The second required step to convert an existing dynamic lexer into a static
one is to change your main program at two places. First, you need to change
the type of the used lexer (that is the template parameter used while instantiating
your token definition class). While in the dynamic model we have been using
the <code class="computeroutput"><span class="identifier">lex</span><span class="special">::</span><span class="identifier">lexertl</span><span class="special">::</span><span class="identifier">lexer</span><span class="special"><></span></code>
template, we now need to change that to the <code class="computeroutput"><span class="identifier">lex</span><span class="special">::</span><span class="identifier">lexertl</span><span class="special">::</span><span class="identifier">static_lexer</span><span class="special"><></span></code> type. The second change is tightly
related to the first one and involves correcting the corresponding <code class="computeroutput"><span class="preprocessor">#include</span></code> statement to:
</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">lex_static_lexertl</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span>
</pre>
<p>
</p>
<p>
Otherwise the main program is not different from an equivalent program
using the dynamic model. This feature makes it easy to develop the lexer
in dynamic mode and to switch to the static mode after the code has been
stabilized. The simple generator application shown above enables the integration
of the code generator into any existing build process. The following code
snippet provides the overall main function, highlighting the code to be
changed.
</p>
<p>
</p>
<pre class="programlisting"><span class="keyword">int</span> <span class="identifier">main</span><span class="special">(</span><span class="keyword">int</span> <span class="identifier">argc</span><span class="special">,</span> <span class="keyword">char</span><span class="special">*</span> <span class="identifier">argv</span><span class="special">[])</span>
<span class="special">{</span>
<span class="comment">// Define the token type to be used: 'std::string' is available as the type
</span> <span class="comment">// of the token value.
</span> <span class="keyword">typedef</span> <span class="identifier">lex</span><span class="special">::</span><span class="identifier">lexertl</span><span class="special">::</span><span class="identifier">token</span><span class="special"><</span>
<span class="keyword">char</span> <span class="keyword">const</span><span class="special">*,</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">mpl</span><span class="special">::</span><span class="identifier">vector</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="special">></span> <span class="identifier">token_type</span><span class="special">;</span>
<span class="comment">// Define the lexer type to be used as the base class for our token
</span> <span class="comment">// definition.
</span> <span class="comment">//
</span> <span class="comment">// This is the only place where the code is different from an equivalent
</span> <span class="comment">// dynamic lexical analyzer. We use the `lexertl::static_lexer<>` instead of
</span> <span class="comment">// the `lexertl::lexer<>` as the base class for our token defintion type.
</span> <span class="comment">//
</span> <span class="comment">// As we specified the suffix "wc" while generating the static tables we
</span> <span class="comment">// need to pass the type lexertl::static_::lexer_wc as the second template
</span> <span class="comment">// parameter below (see word_count_generate.cpp).
</span> <span class="keyword">typedef</span> <span class="identifier">lex</span><span class="special">::</span><span class="identifier">lexertl</span><span class="special">::</span><span class="identifier">static_lexer</span><span class="special"><</span>
<span class="identifier">token_type</span><span class="special">,</span> <span class="identifier">lex</span><span class="special">::</span><span class="identifier">lexertl</span><span class="special">::</span><span class="identifier">static_</span><span class="special">::</span><span class="identifier">lexer_wc</span>
<span class="special">></span> <span class="identifier">lexer_type</span><span class="special">;</span>
<span class="comment">// Define the iterator type exposed by the lexer.
</span> <span class="keyword">typedef</span> <span class="identifier">word_count_tokens</span><span class="special"><</span><span class="identifier">lexer_type</span><span class="special">>::</span><span class="identifier">iterator_type</span> <span class="identifier">iterator_type</span><span class="special">;</span>
<span class="comment">// Now we use the types defined above to create the lexer and grammar
</span> <span class="comment">// object instances needed to invoke the parsing process.
</span> <span class="identifier">word_count_tokens</span><span class="special"><</span><span class="identifier">lexer_type</span><span class="special">></span> <span class="identifier">word_count</span><span class="special">;</span> <span class="comment">// Our lexer
</span> <span class="identifier">word_count_grammar</span><span class="special"><</span><span class="identifier">iterator_type</span><span class="special">></span> <span class="identifier">g</span> <span class="special">(</span><span class="identifier">word_count</span><span class="special">);</span> <span class="comment">// Our parser
</span>
<span class="comment">// Read in the file into memory.
</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span> <span class="identifier">str</span> <span class="special">(</span><span class="identifier">read_from_file</span><span class="special">(</span><span class="number">1</span> <span class="special">==</span> <span class="identifier">argc</span> <span class="special">?</span> <span class="string">"word_count.input"</span> <span class="special">:</span> <span class="identifier">argv</span><span class="special">[</span><span class="number">1</span><span class="special">]));</span>
<span class="keyword">char</span> <span class="keyword">const</span><span class="special">*</span> <span class="identifier">first</span> <span class="special">=</span> <span class="identifier">str</span><span class="special">.</span><span class="identifier">c_str</span><span class="special">();</span>
<span class="keyword">char</span> <span class="keyword">const</span><span class="special">*</span> <span class="identifier">last</span> <span class="special">=</span> <span class="special">&</span><span class="identifier">first</span><span class="special">[</span><span class="identifier">str</span><span class="special">.</span><span class="identifier">size</span><span class="special">()];</span>
<span class="comment">// Parsing is done based on the the token stream, not the character stream.
</span> <span class="keyword">bool</span> <span class="identifier">r</span> <span class="special">=</span> <span class="identifier">lex</span><span class="special">::</span><span class="identifier">tokenize_and_parse</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">word_count</span><span class="special">,</span> <span class="identifier">g</span><span class="special">);</span>
<span class="keyword">if</span> <span class="special">(</span><span class="identifier">r</span><span class="special">)</span> <span class="special">{</span> <span class="comment">// success
</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="string">"lines: "</span> <span class="special"><<</span> <span class="identifier">g</span><span class="special">.</span><span class="identifier">l</span> <span class="special"><<</span> <span class="string">", words: "</span> <span class="special"><<</span> <span class="identifier">g</span><span class="special">.</span><span class="identifier">w</span>
<span class="special"><<</span> <span class="string">", characters: "</span> <span class="special"><<</span> <span class="identifier">g</span><span class="special">.</span><span class="identifier">c</span> <span class="special"><<</span> <span class="string">"\n"</span><span class="special">;</span>
<span class="special">}</span>
<span class="keyword">else</span> <span class="special">{</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span> <span class="identifier">rest</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">cerr</span> <span class="special"><<</span> <span class="string">"Parsing failed\n"</span> <span class="special"><<</span> <span class="string">"stopped at: \""</span>
<span class="special"><<</span> <span class="identifier">rest</span> <span class="special"><<</span> <span class="string">"\"\n"</span><span class="special">;</span>
<span class="special">}</span>
<span class="keyword">return</span> <span class="number">0</span><span class="special">;</span>
<span class="special">}</span>
</pre>
<p>
</p>
<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>
The generated code for the static lexer contains the token ids as they
have been assigned, either explicitly by the programmer or implicitely
during lexer construction. It is your responsibility to make sure that
all instances of a particular static lexer type use exactly the same
token ids. The constructor of the lexer object has a second (default)
parameter allowing it to designate a starting token id to be used while
assigning the ids to the token definitions. The requirement above is
fullfilled by default as long as no <code class="computeroutput"><span class="identifier">first_id</span></code>
is specified during construction of the static lexer instances.
</p></td></tr>
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<td align="left"></td>
<td align="right"><div class="copyright-footer">Copyright © 2001-2010 Joel de Guzman, Hartmut Kaiser<p>
Distributed under the Boost Software License, Version 1.0. (See accompanying
file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
</p>
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