get line from FILE*

get_line.cc

#include <string>
#include <cstring>
#include <cerrno>
#include <cstdio>
#include <cstdlib>
#include <stdexcept>
#include <boost/optional.hpp>

boost::optional<std::string> get_line(std::FILE* fp) {
  std::string s;

  for (;;) {
    char buf[256];

    if (!std::fgets(buf, sizeof(buf), fp)) {
      if (std::feof(fp)) {
        if (s.empty()) {
          return boost::none;
        } else {
          return std::move(s);
        }
      }
      throw std::runtime_error(std::strerror(errno));
    }

    std::size_t n = std::strlen(buf);
    if (n > 0 && buf[n-1] == '\n') {
      --n;
    }
    s.append(buf, n);

    if (n < sizeof(buf) - 1) {
      return std::move(s);
    }
  }
}

int main() {
  while (auto line_ = get_line(stdin)) {
    auto& line = *line_;
    std::puts(line.c_str());
  }
}