Method on_number_part

test/basic_parser.cpp:1526–1530

            bool on_string_part( string_view, std::size_t, system::error_code& ) { return true; }
            bool on_string( string_view, std::size_t, system::error_code& ) { return true; }
            bool on_number_part( string_view sv, system::error_code&)
            {
                captured.append(sv.data(), sv.size());
                return true;
            }
            bool on_int64( std::int64_t, string_view sv, system::error_code& )
            {
                captured.append(sv.data(), sv.size());