1[#nth_of_c] 2[section nth_of_c] 3 4[h1 Synopsis] 5 6 template <int N, class... Ps> 7 struct nth_of_c; 8 9This is a [link parser_combinator parser combinator]. 10 11[table Arguments 12 [[Name] [Type]] 13 [[`N`] [`int` value in the range `[0..sizeof...(Ps)]`]] 14 [[`Ps`] [[link parser parser]s]] 15] 16 17[h1 Description] 18 19`nth_of_c` applies the `Ps...` parsers in sequence. It accepts an input 20when all of these parsers accept it. The result of parsing is the result of the 21`N`. parser. 22 23On compilers, which are not C++11-compliant, the maximum number of parsers 24`nth_of_c` accepts can be specified with the 25`BOOST_METAPARSE_LIMIT_SEQUENCE_SIZE` macro. Its default value is `5`. 26 27[h1 Header] 28 29 #include <boost/metaparse/nth_of.hpp> 30 31[h1 Expression semantics] 32For any `p0`, ..., `pn` parsers and `k` integer value, where `0 <= k < n` the 33following are equivalent 34 35 nth_of_c<k, p0, ..., pn> 36 37 transform<sequence<p0, ..., pn>, boost::mpl::at_c<boost::mpl::_1, k>> 38 39[h1 Example] 40 41 #include <boost/metaparse/nth_of_c.hpp> 42 #include <boost/metaparse/int_.hpp> 43 #include <boost/metaparse/lit_c.hpp> 44 #include <boost/metaparse/token.hpp> 45 #include <boost/metaparse/start.hpp> 46 #include <boost/metaparse/string.hpp> 47 #include <boost/metaparse/is_error.hpp> 48 #include <boost/metaparse/get_result.hpp> 49 50 using namespace boost::metaparse; 51 52 using int_token = token<int_>; 53 using left_paren_token = token<lit_c<'('>>; 54 using right_paren_token = token<lit_c<')'>>; 55 56 using int_in_parens = 57 nth_of_c<1, left_paren_token, int_token, right_paren_token>; 58 59 static_assert( 60 get_result< 61 int_in_parens::apply<BOOST_METAPARSE_STRING("(13)"), start> 62 >::type::value == 13, 63 "it should return the result of the second parser" 64 ); 65 66 static_assert( 67 is_error< 68 int_in_parens::apply<BOOST_METAPARSE_STRING("13"), start> 69 >::type::value, 70 "it should reject the input when there are no parens" 71 ); 72 73 static_assert( 74 is_error< 75 int_in_parens::apply<BOOST_METAPARSE_STRING("(13"), start> 76 >::type::value, 77 "it should reject the input when there is no closing paren" 78 ); 79 80[endsect] 81 82