title: Output error when input isn't a number. C++
body: <p>I am making a function that takes a number from the user's input and finds the absolute value of it.  I want to make it return an error if the user inputs anything other than just a number.  How would I go about doing that?</p>
  
  <p><em>(I know that this is probably an easy question for a lot of you, but I'm taking my first programming class in C++ so I know very little. Any help would be greatly appreciated.)</em></p>