1[#nth_of] 2[section nth_of] 3 4[h1 Synopsis] 5 6 template <class N, class... Ps> 7 struct nth_of; 8 9This is a [link parser_combinator parser combinator]. 10 11[table Arguments 12 [[Name] [Type]] 13 [[`N`] [[link boxed_value boxed] integer value in the range `[0..sizeof...(Ps)]`]] 14 [[`Ps`] [[link parser parser]s]] 15] 16 17[h1 Description] 18 19`nth_of` applies the `Ps...` parsers in sequence. It accepts an input when all 20of these parsers accept it. The result of parsing is the result of the `N`. 21parser. 22 23On compilers, which are not C++11-compliant, the maximum number of parsers 24`nth_of` accepts can be specified with the `BOOST_METAPARSE_LIMIT_SEQUENCE_SIZE` 25macro. Its default value is `5`. 26 27[h1 Header] 28 29 #include <boost/metaparse/nth_of.hpp> 30 31[h1 Expression semantics] 32 33For any `p0`, ..., `pn` parsers and `k` boxed integer value the following are 34equivalent 35 36 nth_of<k, p0, ..., pn> 37 38 nth_of_c<k::type::value, p0, ..., pn> 39 40[h1 Example] 41 42 #include <boost/metaparse/nth_of.hpp> 43 #include <boost/metaparse/int_.hpp> 44 #include <boost/metaparse/lit_c.hpp> 45 #include <boost/metaparse/token.hpp> 46 #include <boost/metaparse/start.hpp> 47 #include <boost/metaparse/string.hpp> 48 #include <boost/metaparse/is_error.hpp> 49 #include <boost/metaparse/get_result.hpp> 50 51 #include <type_traits> 52 53 using namespace boost::metaparse; 54 55 using int_token = token<int_>; 56 using left_paren_token = token<lit_c<'('>>; 57 using right_paren_token = token<lit_c<')'>>; 58 59 using int_in_parens = 60 nth_of< 61 std::integral_constant<int, 1>, 62 left_paren_token, int_token, right_paren_token 63 >; 64 65 static_assert( 66 get_result< 67 int_in_parens::apply<BOOST_METAPARSE_STRING("(13)"), start> 68 >::type::value == 13, 69 "it should return the result of the second parser" 70 ); 71 72 static_assert( 73 is_error< 74 int_in_parens::apply<BOOST_METAPARSE_STRING("13"), start> 75 >::type::value, 76 "it should reject the input when there are no parens" 77 ); 78 79 static_assert( 80 is_error< 81 int_in_parens::apply<BOOST_METAPARSE_STRING("(13"), start> 82 >::type::value, 83 "it should reject the input when there is no closing paren" 84 ); 85 86[endsect] 87 88