73ef4ff3
 Hu Chunming
提交三方库
|
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
|
[#always]
[section always]
[h1 Synopsis]
template <class P, class T>
struct always;
This is a [link parser_combinator parser combinator].
[table Arguments
[[Name] [Type]]
[[`P`] [[link parser parser]]]
[[`T`] [[link metaprogramming_value template metaprogramming value]]]
]
[h1 Description]
It accepts an input if and only if `P` accepts it, but the result of parsing
will be `T` instead of the result `P` returned.
[h1 Header]
#include <boost/metaparse/always.hpp>
[h1 Expression semantics]
For any `p` parser and `t` class the following are equivalent:
always<p, t>
last_of<p, return_<t>>
[h1 Example]
#include <boost/metaparse/always.hpp>
#include <boost/metaparse/lit_c.hpp>
#include <boost/metaparse/start.hpp>
#include <boost/metaparse/string.hpp>
#include <boost/metaparse/is_error.hpp>
#include <boost/metaparse/get_result.hpp>
#include <type_traits>
using namespace boost::metaparse;
using always13 = always<lit_c<'x'>, std::integral_constant<int, 13>>;
static_assert(
!is_error<always13::apply<BOOST_METAPARSE_STRING("x"), start>>::type::value,
"always13 should accept x"
);
static_assert(
std::is_same<
get_result<always13::apply<BOOST_METAPARSE_STRING("x"), start>>::type,
std::integral_constant<int, 13>
>::type::value,
"the result of parsing should be the integral_constant type"
);
static_assert(
is_error<always13::apply<BOOST_METAPARSE_STRING("y"), start>>::type::value,
"always13 should reject characters other than x"
);
[endsect]
|
