private:
  // Appends the contents of message to message_.
  void AppendMessage(const Message& a_message) {
    if (message_.get() == NULL)
      message_.reset(new ::std::string);
    message_->append(a_message.GetString().c_str());
  }

  // Stores result of the assertion predicate.
  bool success_;
  // Stores the message describing the condition in case the expectation