80 lines
1.8 KiB
Plaintext
80 lines
1.8 KiB
Plaintext
[#middle_of]
|
|
[section middle_of]
|
|
|
|
[h1 Synopsis]
|
|
|
|
template <class P1, class P2, class P3>
|
|
struct middle_of;
|
|
|
|
This is a [link parser_combinator parser combinator].
|
|
|
|
[table Arguments
|
|
[[Name] [Type]]
|
|
[[`P1`] [[link parser parser]]]
|
|
[[`P2`] [[link parser parser]]]
|
|
[[`P3`] [[link parser parser]]]
|
|
]
|
|
|
|
[h1 Description]
|
|
|
|
`middle_of` applies `P1`, `P2` and `P3` in sequence. It accepts an input when
|
|
all of these three parsers accept it. The result of parsing is the result of
|
|
`P2`.
|
|
|
|
[h1 Header]
|
|
|
|
#include <boost/metaparse/middle_of.hpp>
|
|
|
|
[h1 Expression semantics]
|
|
|
|
For any `p1`, `p2` and `p3` parsers
|
|
|
|
middle_of<p1, p2, p3>
|
|
|
|
is equivalent to
|
|
|
|
nth_of<1, p1, p2, p3>
|
|
|
|
[h1 Example]
|
|
|
|
#include <boost/metaparse/middle_of.hpp>
|
|
#include <boost/metaparse/int_.hpp>
|
|
#include <boost/metaparse/lit_c.hpp>
|
|
#include <boost/metaparse/token.hpp>
|
|
#include <boost/metaparse/start.hpp>
|
|
#include <boost/metaparse/string.hpp>
|
|
#include <boost/metaparse/is_error.hpp>
|
|
#include <boost/metaparse/get_result.hpp>
|
|
|
|
using namespace boost::metaparse;
|
|
|
|
using int_token = token<int_>;
|
|
using left_paren_token = token<lit_c<'('>>;
|
|
using right_paren_token = token<lit_c<')'>>;
|
|
|
|
using int_in_parens = middle_of<left_paren_token, int_token, right_paren_token>;
|
|
|
|
static_assert(
|
|
get_result<
|
|
int_in_parens::apply<BOOST_METAPARSE_STRING("(13)"), start>
|
|
>::type::value == 13,
|
|
"it should return the result of the middle parser"
|
|
);
|
|
|
|
static_assert(
|
|
is_error<
|
|
int_in_parens::apply<BOOST_METAPARSE_STRING("13"), start>
|
|
>::type::value,
|
|
"it should reject the input when there are no parens"
|
|
);
|
|
|
|
static_assert(
|
|
is_error<
|
|
int_in_parens::apply<BOOST_METAPARSE_STRING("(13"), start>
|
|
>::type::value,
|
|
"it should reject the input when there is no closing paren"
|
|
);
|
|
|
|
[endsect]
|
|
|