Unresolved external LNK errors

Hello Everyone. I am trying to work through an example cpp program that uses exception handling to detect non-number user entries, but am stuck on a few LNK errors.

The errors are as followed (tied together I believe):

• error LNK2001: unresolved external symbol "public: virtual int__thiscall NonNumber::What(void)" (?What@NonNumber@@UAEHXZ)
  
• error LNK1120: 1 unresolved externals.

Can anyone tell me what is going on? My file is coded below:

numberverifier.cpp

// numberverifier.cpp
#include <iostream>
#include <cmath>
#include <string>
#include <stdexcept>
using namespace std; 

// class NonNumber definition
class NonNumber : public runtime_error
{
public:

   // constructor
   NonNumber()
      : runtime_error( "non-integer detected" ) 
   {
      // empty
   } // end class NonNumber definition

   virtual int What();

private: 
   string message;

}; // end class NonNumber

// function castInput definition
int castInput( string input )
{
   int result = 0;
   int negative = 1;

   // check for minus sign
   if ( input[ 0 ] == '-' )
     negative = -1;

   for ( int i = input.length() - 1, j = 0; i >= 0; i--, j++ )
   {
      if ( negative == -1 && i == 0 )
         continue;

      if ( input[ i ] >= '0' && input[ i ] <= '9' )
         result += static_cast< int >( input[ i ] - '0' ) * pow( 10.0, j );   
      else 
		 throw NonNumber();
   } // end for

   return result * negative;

} // end function castInput

int main()
{
   string input;
   int convert; 

   cout << "Please enter a number (end-of-file to terminate): ";

   while ( cin >> input )
   {
	   try
	   {
		   convert = castInput(input);
		   cout << "The number entered was: " << convert << endl;
	   } // end try
		catch (NonNumber &e)
	   {
		   cout << "INVALID INPUT: " << e.What() << endl;
	   } // end catch

      cout << "\n\nPlease enter a number (end-of-file to terminate): ";
   } // end while

   cout << endl;
} // end main

Any help as always is much appreciated! I'm still stumbling through the basics of C++ I professionally develop in .NET so C++ is not being all that cooperative with me.