multibyte compliant substr

gistfile1.cpp

std::string substr(const std::string& str, size_t nb){
    std::stringstream result;
    auto it = str.begin();
    unsigned char c;
    size_t count = 0;
    while(it != str.end() && count < nb){
        c = *it;
        result << c;
        if(c >= 0xc0){
            ++it;
            c = *it;
            while((c & 0xc0) == 0x80 && it != str.end()){
                result << c;
                ++it;
                c = *it;
            }
        }else{
            ++it;
        }
        ++count;
    }
    return result.str();
}