cppreference.com > C++ Strings > substr substr Syntax:
      #include <string>
      string substr( size_type index, size_type num = npos );
The substr() function returns a substring of the current string, starting at index, and num characters long. If num is omitted, it will default to string::npos, and the substr() function will simply return the remainder of the string starting at index. For example:
       string s("What we have here is a failure to communicate");
       string sub = s.substr(21);
       cout << "The original string is " << s << endl;
       cout << "The substring is " << sub << endl;
displays
       The original string is What we have here is a failure to communicate
       The substring is a failure to communicate
Related topics: copy