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7c673cae FG |
1 | [#always_c] |
2 | [section always_c] | |
3 | ||
4 | [h1 Synopsis] | |
5 | ||
6 | template <char C, class T> | |
7 | struct always_c; | |
8 | ||
9 | This is a [link parser_combinator parser combinator]. | |
10 | ||
11 | [table Arguments | |
12 | [[Name] [Type]] | |
13 | [[`C`] [character]] | |
14 | [[`T`] [[link metaprogramming_value template metaprogramming value]]] | |
15 | ] | |
16 | ||
17 | [h1 Description] | |
18 | ||
19 | It accepts inputs beginning with the `C` character. It consumes that character | |
20 | and the result of parsing is `T`. Other inputs as rejected. | |
21 | ||
22 | [h1 Header] | |
23 | ||
24 | #include <boost/metaparse/always_c.hpp> | |
25 | ||
26 | [h1 Expression semantics] | |
27 | ||
28 | For any `c` character and `t` class the following are equivalent: | |
29 | ||
30 | always_c<c, t> | |
31 | ||
32 | always<lit_c<c>, t> | |
33 | ||
34 | [h1 Example] | |
35 | ||
36 | #include <boost/metaparse/always_c.hpp> | |
37 | #include <boost/metaparse/start.hpp> | |
38 | #include <boost/metaparse/string.hpp> | |
39 | #include <boost/metaparse/is_error.hpp> | |
40 | #include <boost/metaparse/get_result.hpp> | |
41 | ||
42 | #include <type_traits> | |
43 | ||
44 | using namespace boost::metaparse; | |
45 | ||
46 | using always13 = always_c<'x', std::integral_constant<int, 13>>; | |
47 | ||
48 | static_assert( | |
49 | !is_error<always13::apply<BOOST_METAPARSE_STRING("x"), start>>::type::value, | |
50 | "always13 should accept x" | |
51 | ); | |
52 | ||
53 | static_assert( | |
54 | std::is_same< | |
55 | get_result<always13::apply<BOOST_METAPARSE_STRING("x"), start>>::type, | |
56 | std::integral_constant<int, 13> | |
57 | >::type::value, | |
58 | "the result of parsing should be the integral_constant type" | |
59 | ); | |
60 | ||
61 | static_assert( | |
62 | is_error<always13::apply<BOOST_METAPARSE_STRING("y"), start>>::type::value, | |
63 | "always13 should reject characters other than x" | |
64 | ); | |
65 | ||
66 | [endsect] | |
67 |