title: convert string to long long int
body: <p>I want to read the 3rd line from a text file in as a <code>string</code>, convert it to a <code>long long int</code>, and return that value.</p>
  
  <p>The data on the 3rd line of the text file is <code>1234567890123456</code></p>
  
  <pre><code>long long int File::Getline3(int user1, int user3)
  {
      std::string filename = std::to_string(user1);
  
      std::ifstream fin(filename + ".txt");
      fin.getline (line1, 5);
      fin.getline (line2, 5);
      fin.getline (line3, 20);
      fin.close();
  
      // convert line 3 to a string called str
      const char *line3;
      std::string str(line3);
  
      // convert str to long long int called user3
      long long int strtoll(const char *nptr, char **endptr, int base);
      char* endptr = NULL;
      user3 = strtoll(str.c_str(), &amp;endptr, 10);
  
      return user3;
  }
  </code></pre>
  
  <p>The comments are in to show what I think I'm doing, however I am probably wrong (I'm new to pointers).</p>
  
  <p>I get an "unresolved external" error when I try to build my program.</p>