Clear stringStream before use

int main() {

	string s = "My error message\n My error message\n My error message\n My error message";
	stringstream ss;

	cout << for_each(s.begin(), s.end(), hexify<char>(ss)) << endl;
	cout << for_each(s.begin(), s.end(), hexify<char>(ss)) << endl;
	// Or, as in your case:
	//
	// log4cpp::Category::getRoot() <<  log4cpp::Priority::DEBUG << for_each(s.begin(), s.end(), hexify<char>(ss)) << endl;

	return 0;
}

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