title: error: cannot convert const string to const char* for argument 1 to size_t strlen(const char*)
body: <p>I'm pretty new to C++ and am very confused as to what is happening here. The error is the line <code>int len = strlen(strin);</code>. Any suggestions how to fix this would be much appreciated.   </p>
  
  <pre><code>BigNum::BigNum(const std::string&amp; strin)
  {
      digits = NULL;
  
      int len = strlen(strin);
  
      if (len == 0)
      {
          BigNum zero;
          *this = zero;
          return;
      }
  
      used = len;
      positive = true;
  
      int i = 0;
  
      if(strin[i] == '-')  
      {
          positive = false;
          i = 1;
          used--;
      }
      else if(strin[i] == '+')  
      {
          i = 1;
          used--;
      }
  
      capacity = double_up_default(used);
  
      digits = new unsigned int[capacity];
  
      for(unsigned int k = 0; k &lt; used; ++k) 
      {
          digits[used - k - 1] = strin[i++] - '0';
      }
  
      trim();
  }
  </code></pre>