title: error: no match for 'operator*' (operand types are 'std: :string {aka std basic_string<char>}' and {aka std basic_string<char>}')
body: <p>So I'm new to programming and this is part of a university engineering program. I've decided to have fun and write something outside of class. I'm trying to write a program in c++ that allows the user to enter 3 values for length, width, and height of a rectangular prism and the program will use the 3 values to calculate the surface area and volume of the prism.</p>
  
  <p>This is what I have so far. (Written in Vocareum)</p>
  
  <pre><code>//---------------------------------------------------------------------------------------// 
  //Calculate rectangular prism//        
  //---------------------------------------------------------------------------------------//
  #include &lt;string&gt;
  #include &lt;iostream&gt;  
  using namespace std;
  
  int main() 
  {
  string length; // length of prism in cm 
  string width; // width of prism in cm 
  string height; // height of prism in cm
  
  cout &lt;&lt; "Please enter the length, width, and height of the rectangular prism." &lt;&lt; endl; 
  
  cin &gt;&gt; length; //length of prism in cm
  cin &gt;&gt; width; //width of prism in cm
  cin &gt;&gt; height; //height of prism in cm
  
  double volume; //volume of prism in cm^3
  double sa; //surface area of prism in cm^2
  
  volume = length * width * height;
  sa = (length * width * 2) + (width * height * 2) + (height * length * 2);
  
  cout &lt;&lt; "The volume of the rectangular prism is " &lt;&lt; volume &lt;&lt; " cm^3." &lt;&lt; endl;
  cout &lt;&lt; "The surface area of the rectangular prism is " &lt;&lt; sa &lt;&lt; " cm^2." &lt;&lt; endl;
  
  return 0;
  }
  
  //Whenever I try to compile, I'll get 4 error messages that reads 
  //"error: no match for 'operator*' (operand types are 'std: :string {aka std basic_string&lt;char&gt;}' and {aka std basic_string&lt;char&gt;}')
  //ps these 3 comments aren't in the code
  </code></pre>
  
  <p>How do I fix it?</p>