Class dispatcher

extlibs/fmt/include/fmt/format-inl.h:73–126

  FMT_ASSERT(buffer != nullptr && buffer_size != 0, "invalid buffer");

  class dispatcher {
   private:
    int error_code_;
    char*& buffer_;
    std::size_t buffer_size_;

    // A noop assignment operator to avoid bogus warnings.
    void operator=(const dispatcher&) {}

    // Handle the result of XSI-compliant version of strerror_r.
    int handle(int result) {
      // glibc versions before 2.13 return result in errno.
      return result == -1 ? errno : result;
    }

    // Handle the result of GNU-specific version of strerror_r.
    FMT_MAYBE_UNUSED
    int handle(char* message) {
      // If the buffer is full then the message is probably truncated.
      if (message == buffer_ && strlen(buffer_) == buffer_size_ - 1)
        return ERANGE;
      buffer_ = message;
      return 0;
    }

    // Handle the case when strerror_r is not available.
    FMT_MAYBE_UNUSED
    int handle(internal::null<>) {
      return fallback(strerror_s(buffer_, buffer_size_, error_code_));
    }

    // Fallback to strerror_s when strerror_r is not available.
    FMT_MAYBE_UNUSED
    int fallback(int result) {
      // If the buffer is full then the message is probably truncated.
      return result == 0 && strlen(buffer_) == buffer_size_ - 1 ? ERANGE
                                                                : result;
    }

#if !FMT_MSC_VER
    // Fallback to strerror if strerror_r and strerror_s are not available.
    int fallback(internal::null<>) {
      errno = 0;
      buffer_ = strerror(error_code_);
      return errno;
    }
#endif

   public:
    dispatcher(int err_code, char*& buf, std::size_t buf_size)
        : error_code_(err_code), buffer_(buf), buffer_size_(buf_size) {}

    int run() { return handle(strerror_r(error_code_, buffer_, buffer_size_)); }
  };
  return dispatcher(error_code, buffer, buffer_size).run();
}