1[#middle_of] 2[section middle_of] 3 4[h1 Synopsis] 5 6 template <class P1, class P2, class P3> 7 struct middle_of; 8 9This is a [link parser_combinator parser combinator]. 10 11[table Arguments 12 [[Name] [Type]] 13 [[`P1`] [[link parser parser]]] 14 [[`P2`] [[link parser parser]]] 15 [[`P3`] [[link parser parser]]] 16] 17 18[h1 Description] 19 20`middle_of` applies `P1`, `P2` and `P3` in sequence. It accepts an input when 21all of these three parsers accept it. The result of parsing is the result of 22`P2`. 23 24[h1 Header] 25 26 #include <boost/metaparse/middle_of.hpp> 27 28[h1 Expression semantics] 29 30For any `p1`, `p2` and `p3` parsers 31 32 middle_of<p1, p2, p3> 33 34is equivalent to 35 36 nth_of<1, p1, p2, p3> 37 38[h1 Example] 39 40 #include <boost/metaparse/middle_of.hpp> 41 #include <boost/metaparse/int_.hpp> 42 #include <boost/metaparse/lit_c.hpp> 43 #include <boost/metaparse/token.hpp> 44 #include <boost/metaparse/start.hpp> 45 #include <boost/metaparse/string.hpp> 46 #include <boost/metaparse/is_error.hpp> 47 #include <boost/metaparse/get_result.hpp> 48 49 using namespace boost::metaparse; 50 51 using int_token = token<int_>; 52 using left_paren_token = token<lit_c<'('>>; 53 using right_paren_token = token<lit_c<')'>>; 54 55 using int_in_parens = middle_of<left_paren_token, int_token, right_paren_token>; 56 57 static_assert( 58 get_result< 59 int_in_parens::apply<BOOST_METAPARSE_STRING("(13)"), start> 60 >::type::value == 13, 61 "it should return the result of the middle parser" 62 ); 63 64 static_assert( 65 is_error< 66 int_in_parens::apply<BOOST_METAPARSE_STRING("13"), start> 67 >::type::value, 68 "it should reject the input when there are no parens" 69 ); 70 71 static_assert( 72 is_error< 73 int_in_parens::apply<BOOST_METAPARSE_STRING("(13"), start> 74 >::type::value, 75 "it should reject the input when there is no closing paren" 76 ); 77 78[endsect] 79 80