Synopsis

template <class P1, class P2, class P3>
struct middle_of;

This is a parser combinator.

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.

Header

#include <boost/metaparse/middle_of.hpp>

Expression semantics

For any p1, p2 and p3 parsers

middle_of<p1, p2, p3>

is equivalent to

nth_of<1, p1, p2, p3>

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"
);