[/============================================================================== Copyright (C) 2001-2008 Joel de Guzman Copyright (C) 2001-2009 Hartmut Kaiser Distributed under 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) ===============================================================================/] [section:lexer_static_model The /Static/ Lexer Model] The documentation of __lex__ so far mostly was about describing the features of the /dynamic/ 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 __spirit__ 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 analyers. The /static/ model strives to build upon the smooth integration with __spirit__ and C++, and reuses large parts of the __lex__ 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, it is possible reuse the token definition types developed using the /dynamic/ 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. Assuming you already built a dynamic lexer for your problem, there are two more steps needed to create a static lexical analyzer using __lex__: # generating the C++ code for the static analyzer (including the tokenization function and corresponding tables), and # modifying the dynamic lexical anlyzer to use the generated code. 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: [@../../example/lex/static_lexer/word_count_tokens.hpp the common token definition], [@../../example/lex/static_lexer/word_count_generate.cpp the code generator], [@../../example/lex/static_lexer/word_count_static.hpp the generated code], and [@../../example/lex/static_lexer/word_count_static.cpp the static lexical analyzer]). [import ../example/lex/static_lexer/word_count_tokens.hpp] [import ../example/lex/static_lexer/word_count_static.cpp] [import ../example/lex/static_lexer/word_count_generate.cpp] But first we provide the code snippets needed to understand the further 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. [wc_static_tokenids] 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. [wc_static_tokendef] 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 __class_lexertl_lexer__ template, where for the static model you will use the __class_lexertl_static_lexer__ type as the template parameter. 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 __qi__ 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. [wc_static_grammar] [heading Generating the Static Analyzer] 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 __lex__ library exposes a special API - the function __api_generate_static__. 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 prefix for the name of the generated function. All in all just a couple lines of code. [wc_static_generate_main] 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 [@../../example/lex/static_lexer/word_count_static.hpp here]). [heading Modifying the Dynamic Analyzer] 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 __class_lexertl_lexer__ template, we now need to change that to the __class_lexertl_static_lexer__ type. The second change is tightly related to the first one and involves correcting the corresponding `#include` statement to: [wc_static_include] Otherwise the main program is not different from an equivalent program using the dynamic model. This feature makes it really easy for instance to develop the lexer in dynamic mode and to switch to the static mode after the code has been stabilized. The simple generator application showed 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. [wc_static_main] [endsect]