How to safely read a line from an std::istream?

You could write your own version of std::getline with a maximum number of characters read parameter, something called getline_n or something.

#include <string>
#include <iostream>

template<typename CharT, typename Traits, typename Alloc>
auto getline_n(std::basic_istream<CharT, Traits>& in, std::basic_string<CharT, Traits, Alloc>& str, std::streamsize n) -> decltype(in) {
    std::ios_base::iostate state = std::ios_base::goodbit;
    bool extracted = false;
    const typename std::basic_istream<CharT, Traits>::sentry s(in, true);
    if(s) {
        try {
            str.erase();
            typename Traits::int_type ch = in.rdbuf()->sgetc();
            for(; ; ch = in.rdbuf()->snextc()) {
                if(Traits::eq_int_type(ch, Traits::eof())) {
                    // eof spotted, quit
                    state |= std::ios_base::eofbit;
                    break;
                }
                else if(str.size() == n) {
                    // maximum number of characters met, quit
                    extracted = true;
                    in.rdbuf()->sbumpc();
                    break;
                }
                else if(str.max_size() <= str.size()) {
                    // string too big
                    state |= std::ios_base::failbit;
                    break;
                }
                else {
                    // character valid
                    str += Traits::to_char_type(ch);
                    extracted = true;
                }
            }
        }
        catch(...) {
            in.setstate(std::ios_base::badbit);
        }
    }

    if(!extracted) {
        state |= std::ios_base::failbit;
    }

    in.setstate(state);
    return in;
}

int main() {
    std::string s;
    getline_n(std::cin, s, 10); // maximum of 10 characters
    std::cout << s << '\n';
}

Might be overkill though.