If the newline is expected to be at the end of the string, then:
if (!s.empty() && s[s.length()-1] == '\n') {
s.erase(s.length()-1);
}
If the string can contain many newlines anywhere in the string:
std::string::size_type i = 0;
while (i < s.length()) {
i = s.find('\n', i);
if (i == std::string:npos) {
break;
}
s.erase(i);
}