actions.qbk

来自「Boost provides free peer-reviewed portab」· QBK 代码 · 共 104 行

[/==============================================================================
    Copyright (C) 2001-2008 Joel de Guzman
    Copyright (C) 2001-2008 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]

Our parser above is really nothing but a recognizer. It answers the question
"did the input match our grammar?", but it does not do anything other than that.
It does not extract any information from what was parsed. For example, whenever
we parse a real number, we wish to store the parsed number after a successful
match.

Enter 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.

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. Get used to it. `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 sibbling 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__ in steroids, with
special custom features that make it easy to integrate semantic actions with
Spirit. If your requirements go beyond simple to moderate parsing, I suggest you
use this library. Examples presented henceforth shall be using the library
exclusively

[endsect]