boost_1_45_0/libs/spirit/doc/qi/actions.qbk

[/==============================================================================
    Copyright (C) 2001-2010 Joel de Guzman
    Copyright (C) 2001-2010 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 Semantic Actions]

The example in the previous section was very simplistic. It only recognized
data, but did nothing with it. It answered the question: "Did the input match?".
Now, we want to extract information from what was parsed. For example, we would
want to store the parsed number after a successful match. To do this, you will 
need ['semantic actions].

Semantic actions may be attached to any point in the grammar specification. 
These actions are C++ functions or function objects that are called whenever a 
part of the parser successfully recognizes a portion of the input. Say you have 
a parser `P`, and a C++ function `F`. You can make the parser call `F` whenever 
it matches an input by attaching `F`:

    P[F]

The expression above links `F` to the parser, `P`.

The function/function object signature depends on the type of the parser to
which it is attached. The parser `double_` passes the parsed number. Thus, if we
were to attach a function `F` to `double_`, we need `F` to be declared as:

    void F(double n);

There are actually 2 more arguments being passed (the parser context and a
reference to a boolean 'hit' parameter). We don't need these, for now, but we'll
see more on these other arguments later. Spirit.Qi allows us to bind a single
argument function, like above. The other arguments are simply ignored.

[heading Examples of Semantic Actions]

Presented are various ways to attach semantic actions:

* Using plain function pointer
* Using simple function object
* Using __boost_bind__ with a plain function
* Using __boost_bind__ with a member function
* Using __boost_lambda__

[import ../../example/qi/actions.cpp]

Given:

[tutorial_semantic_action_functions]

Take note that with function objects, we need to have an `operator()` with 3
arguments. Since we don't care about the other two, we can use `unused_type` for
these. We'll see more of `unused_type` elsewhere. `unused_type` is a Spirit 
supplied support class.

All examples parse inputs of the form:

    "{integer}"

An integer inside the curly braces.

The first example shows how to attach a plain function:

[tutorial_attach_actions1]

What's new? Well `int_` is the sibling of `double_`. I'm sure you can guess
what this parser does.

The next example shows how to attach a simple function object:

[tutorial_attach_actions2]

We can use __boost_bind__ to 'bind' member functions:

[tutorial_attach_actions4]

Likewise, we can also use __boost_bind__ to 'bind' plain functions:

[tutorial_attach_actions3]

Yep, we can also use __boost_lambda__:

[tutorial_attach_actions5]

There are more ways to bind semantic action functions, but the examples above
are the most common. Attaching semantic actions is the first hurdle one has
to tackle when getting started with parsing with Spirit. Familiarize yourself
with this task and get intimate with the tools behind it such as __boost_bind__
and __boost_lambda__.

The examples above can be found here: [@../../example/qi/actions.cpp]

[heading Phoenix]

__phoenix__, a companion library bundled with Spirit, is specifically suited
for binding semantic actions. It is like __boost_lambda__ on steroids, with
special custom features that make it easy to integrate semantic actions with
Spirit. If your requirements go beyond simple to moderate parsing, it is 
suggested that you use this library. All the following examples in this tutorial
will use __phoenix__ for semantic actions.

[important There are different ways to write semantic actions for __qi__: 
           using plain functions, __boost_bind__, __boost_lambda__, or 
           __phoenix__. The latter three allow you to use special placeholders
           to control parameter placement (`_1`, `_2`, etc.). Each of those
           libraries has it's own implementation of the placeholders, all
           in different namespaces. You have to make sure not to mix 
           placeholders with a library they don't belong to and not to 
           use different libraries while writing a semantic action.

           Generally, for __boost_bind__, use `::_1`, `::_2`, etc. (yes, these
           placeholders are defined in the global namespace). 

           For __boost_lambda__ use the placeholders defined in the namespace 
           `boost::lambda`. 

          For semantic actions written using __phoenix__ use the placeholders 
          defined in the namespace `boost::spirit`. Please note that all 
          existing placeholders for your convenience are also available from 
          the namespace `boost::spirit::qi`.]

[endsect]