retrieving string from boost::tuples::tuple

Question

After reading the article at https://www.boost.org/doc/libs/1_67_0/libs/tuple/doc/html/tuple_users_guide.html The following note is a problem for me. Note that extracting tuples with std::string or C-style string elements does not generally work, since the streamed tuple representation may not be unambiguously parseable. What type should I use to unambiguously parse strings from a stream into a tuple?

When retrieving a std::string from a tuple the string is devided by white space. Which is not desired! Setting a delimiter e.g., a number sign ( # ) does not help.

// typedef tuple
typedef std::string td_current_gmt, td_remote_endpoint, 
    td_request, td_response, td_elapsed_time;
typedef boost::tuples::tuple<td_current_gmt, td_remote_endpoint, 
    td_request, td_response, td_elapsed_time> tuple_logging;
// store in tuple
tuple_logging tl{ current_gmt, remote_endpoint, 
    request, response, elapsed_time };
// write tuple to file
tl = boost::tuples::make_tuple(current_gmt, remote_endpoint,
    request, response, elapsed_time);
boost::filesystem::path p = { "logging" };
boost::filesystem::ofstream ofs{ p };
ofs << /*boost::tuples::set_delimiter('#') <<*/ tl;
ofs.close();
// read tuple from file
tuple_logging tlin{ current_gmt, remote_endpoint,
    request, response, elapsed_time };
boost::filesystem::ifstream ifs{ p };
//ifs >> boost::tuples::set_delimiter('#');
ifs >> tlin;

The output is (Fri, 16 Aug 2019 06:28:05) But actually it has to be (Fri, 16 Aug 2019 06:28:05 GMT 192.168.178.14:52832 TRACE / HTTP/1.1 HTTP/1.1 200 OK 8.936800)

issue is solved last line in my code example should be: ifs >> boost::tuples::set_delimiter('#') >> tlin; output now is: (Fri, 192.168.178.14:52832 TRACE / HTTP/1.1 HTTP/1.1 200 OK 2.507500) — Martijn Klasing, Aug 16 '19 at 06:46
Jumping to a conclusion too fast. I now come to the conclusion that I cannot stream from a file directly into a tuple. So what I did is the following. Streaming tuples into a file, line by line. Reading the file, line by line, into a std::string. Isolate the tuple elements by string manipulation. With just old fashioned C-coding practice. — Martijn Klasing, Aug 18 '19 at 05:48

Martijn Klasing · Answer 1 · 2019-08-18T06:43:46.160

Here's the code.

void parse_logfile(
boost::filesystem::ifstream& ifs,
const boost::filesystem::path& p,
std::vector<tuple_logging>& vector_with_tuples

) { typedef std::string one_line_from_logging;

one_line_from_logging str;
tuple_logging tlin;

// clear vector
vector_with_tuples.clear();

// open log file for reading
ifs.open(p);

// read one line from log file, until eof
while (std::getline(ifs, str))
{
    size_t sBegin = 0, sEnd = 0;
    // 1
    // first character on a line is '(',
    // start at sBegin = 1
    sBegin = sEnd + 1;
    sEnd = str.find('#', sBegin);
    std::string current_gmt_ = str.substr(sBegin, sEnd - sBegin);
    // 2
    sBegin = sEnd + 1;
    sEnd = str.find('#', sBegin);
    std::string remote_endpoint_ = str.substr(sBegin, sEnd - sBegin);
    // 3
    sBegin = sEnd + 1;
    sEnd = str.find('#', sBegin);
    std::string request_ = str.substr(sBegin, sEnd - sBegin);
    // 4
    sBegin = sEnd + 1;
    sEnd = str.find('#', sBegin);
    std::string response_ = str.substr(sBegin, sEnd - sBegin);
    // 5
    sBegin = sEnd + 1;
    // last character on a line is ')'
    sEnd = str.find(')', sBegin);
    std::string elapsed_time_ = str.substr(sBegin, sEnd - sBegin);

    // create tuple from parsed log data out of file
    tlin = boost::tuples::make_tuple(
        current_gmt_,
        remote_endpoint_,
        request_,
        response_,
        elapsed_time_
    );

    // set tuple into vector
    vector_with_tuples.push_back(tlin);
}

// close log file
ifs.close();

}

retrieving string from boost::tuples::tuple

1 Answers1