libs/spirit/doc/lex/lexer_quickstart2.qbk
[/==============================================================================
Copyright (C) 2001-2011 Joel de Guzman
Copyright (C) 2001-2011 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_quickstart2 Quickstart 2 - A better word counter using __lex__]
People familiar with __flex__ will probably complain about the example from the
section __sec_lex_quickstart_1__ as being overly complex and not being
written to leverage the possibilities provided by this tool. In particular the
previous example did not directly use the lexer actions to count the lines,
words, and characters. So the example provided in this step of the tutorial will
show how to use semantic actions in __lex__. Even though this examples still
counts textual elements, the purpose is to introduce new concepts and
configuration options along the lines (for the full example code
see here: [@../../example/lex/word_count_lexer.cpp word_count_lexer.cpp]).
[import ../example/lex/word_count_lexer.cpp]
[heading Prerequisites]
In addition to the only required `#include` specific to /Spirit.Lex/ this
example needs to include a couple of header files from the __phoenix__
library. This example shows how to attach functors to token definitions, which
could be done using any type of C++ technique resulting in a callable object.
Using __phoenix__ for this task simplifies things and avoids adding
dependencies to other libraries (__phoenix__ is already in use for
__spirit__ anyway).
[wcl_includes]
To make all the code below more readable we introduce the following namespaces.
[wcl_namespaces]
To give a preview at what to expect from this example, here is the flex program
which has been used as the starting point. The useful code is directly included
inside the actions associated with each of the token definitions.
[wcl_flex_version]
[heading Semantic Actions in __lex__]
__lex__ uses a very similar way of associating actions with the token
definitions (which should look familiar to anybody knowledgeable with
__spirit__ as well): specifying the operations to execute inside of a pair of
`[]` brackets. In order to be able to attach semantic actions to token
definitions for each of them there is defined an instance of a `token_def<>`.
[wcl_token_definition]
The semantics of the shown code is as follows. The code inside the `[]`
brackets will be executed whenever the corresponding token has been matched by
the lexical analyzer. This is very similar to __flex__, where the action code
associated with a token definition gets executed after the recognition of a
matching input sequence. The code above uses function objects constructed using
__phoenix__, but it is possible to insert any C++ function or function object
as long as it exposes the proper interface. For more details on please refer
to the section __sec_lex_semactions__.
[heading Associating Token Definitions with the Lexer]
If you compare this code to the code from __sec_lex_quickstart_1__ with regard
to the way how token definitions are associated with the lexer, you will notice
a different syntax being used here. In the previous example we have been
using the `self.add()` style of the API, while we here directly assign the token
definitions to `self`, combining the different token definitions using the `|`
operator. Here is the code snippet again:
this->self
= word [++ref(w), ref(c) += distance(_1)]
| eol [++ref(c), ++ref(l)]
| any [++ref(c)]
;
This way we have a very powerful and natural way of building the lexical
analyzer. If translated into English this may be read as: The lexical analyzer
will recognize ('`=`') tokens as defined by any of ('`|`') the token
definitions `word`, `eol`, and `any`.
A second difference to the previous example is that we do not explicitly
specify any token ids to use for the separate tokens. Using semantic actions to
trigger some useful work has freed us from the need to define those. To ensure
every token gets assigned a id the __lex__ library internally assigns unique
numbers to the token definitions, starting with the constant defined by
`boost::spirit::lex::min_token_id`.
[heading Pulling everything together]
In order to execute the code defined above we still need to instantiate an
instance of the lexer type, feed it from some input sequence and create a pair
of iterators allowing to iterate over the token sequence as created by the
lexer. This code shows how to achieve these steps:
[wcl_main]
[endsect]
