[#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]