Re: undefined reference(compile error), plz help

class IntArray {
public:
   // equality and inequality operations: #2b
   explicit IntArray( int size = DefaultArraySize );
   IntArray( int *array, int array_size );
   IntArray( const IntArray &rhs );

   // virtual destructor!
   virtual ~IntArray() { delete [] ia; }

   // equality and inequality operations:
   bool operator==( const IntArray& ) const;
   bool operator!=( const IntArray& ) const;

   // assignment operator: #2a
   IntArray& operator=( const IntArray& );
   int size() const { return _size; }

// we've removed the check on the index...
   virtual int& operator[](int index) { return ia[index]; }
   // int size() const; // #1
   virtual void sort(); // #4

but my compile result( errors) is
--
eric@eric-laptop:~/CppPrimer3$ g++ IntArray.cpp pg49.cpp
/tmp/ccOnEojJ.o: In function `IntArray::IntArray(int*, int)':
IntArray.cpp:(.text+0x8): undefined reference to `vtable for IntArray'