1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
|
[/==============================================================================
Copyright (C) 2001-2010 Joel de Guzman
Copyright (C) 2001-2005 Dan Marsden
Copyright (C) 2001-2010 Thomas Heller
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 Adding an expression]
This is not a toy example. This is actually part of the library. Remember the
[link phoenix.modules.statement.while__statement `while`] lazy statement? Putting together
everything we've learned so far, we eill present it here in its entirety
(verbatim):
BOOST_PHOENIX_DEFINE_EXPRESSION(
(boost)(phoenix)(while_)
, (meta_grammar) // Cond
(meta_grammar) // Do
)
namespace boost { namespace phoenix
{
struct while_eval
{
typedef void result_type;
template <typename Cond, typename Do, typename Context>
result_type
operator()(Cond const& cond, Do const& do_, Context & ctx) const
{
while(eval(cond, ctx))
{
eval(do_, ctx);
}
}
};
template <typename Dummy>
struct default_actions::when<rule::while_, Dummy>
: call<while_eval, Dummy>
{};
template <typename Cond>
struct while_gen
{
while_gen(Cond const& cond) : cond(cond) {}
template <typename Do>
typename expression::while_<Cond, Do>::type const
operator[](Do const& do_) const
{
return expression::while_<Cond, Do>::make(cond, do_);
}
Cond const& cond;
};
template <typename Cond>
while_gen<Cond> const
while_(Cond const& cond)
{
return while_gen<Cond>(cond);
}
}}
`while_eval` is an example of how to evaluate an expression. It gets called in
the `rule::while` action. `while_gen` and `while_` are the expression template
front ends. Let's break this apart to undestand what's happening. Let's start at
the bottom. It's easier that way.
When you write:
while_(cond)
we generate an instance of `while_gen<Cond>`, where `Cond` is the type of `cond`.
`cond` can be an arbitrarily complex actor expression. The `while_gen` template
class has an `operator[]` accepting another expression. If we write:
while_(cond)
[
do_
]
it will generate a proper composite with the type:
expression::while_<Cond, Do>::type
where `Cond` is the type of `cond` and `Do` is the type of `do_`. Notice how we are using Phoenix's
[link phoenix.inside.expression Expression] mechanism here
template <typename Do>
typename expression::while_<Cond, Do>::type const
operator[](Do const& do_) const
{
return expression::while_<Cond, Do>::make(cond, do_);
}
Finally, the `while_eval` does its thing:
while(eval(cond, ctx))
{
eval(do_, ctx);
}
`cond` and `do_`, at this point, are instances of [link phoenix.inside.actor Actor]. `cond` and `do_` are the [link phoenix.inside.actor Actors]
passed as parameters by `call`, ctx is the [link phoenix.inside.actor Context]
[endsect]
|
