undefined reference(compile error), plz help

Dear advance c++ programers:

  I tried to type in/test your code in chapter 2 of c++ primer (3rd
ed)
page 45, 46, 49
IntArray.h
IntArrayRC.h
(and add IntArray.cpp myself) /* they are all simple */

the following is my code copied from your book
----------------------------------------------
#include <iostream>
#include "IntArray.h"
#include "IntArrayRC.h"

using namespace std;

 void swap(IntArray&, int, int);

int main() {
  int array[ 4 ] = { 0, 1, 2, 3 };
  IntArray ia1( array, 4 );
  IntArrayRC ia2( array, 4 );

  // error: off-by-one: should be size - 1
  // not caught by IntArray object
   cout << "swap() with IntArray ia1\n";
  swap( ia1, 1, ia1.size() );

  // ok: caught by IntArrayRC object
  cout << "swap() with IntArrayRC ia2\n";
  swap( ia2, 1, ia2.size() );

  return 0;
}

  void swap( IntArray &ia, int i, int j )
    {
       int tmp = ia[ i ];
       ia[ i ] = ia[ j ];
       ia[ j ] = tmp;
    }
----------------------------------------------------
#ifndef IntArray_H
#define IntArray_H

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

   virtual int min() const; // #3a
   virtual int max() const; // #3b

   // if the value is found within the aray,
   // return the index of its first occurrence
   // otherwise, return -1

   virtual int find ( int value ) const; // #3c

  protected:
    // the private implementation
    static const int DefaultArraySize = 12;
    void init( int sz, int *array );

    int _size;
    int *ia;
};

#endif
----------------------------------------------
#ifndef IntArrayRC_H
#define IntArrayRC_H

#include "IntArray.h"

class IntArrayRC : public IntArray {
public:
  IntArrayRC( int sz = DefaultArraySize );
  IntArrayRC( int *array, int array_size );
  IntArrayRC( const IntArrayRC &rhs );
  virtual int& operator[]( int );

private:
  void check_range( int );
};

#endif
-------------------------------------------------
// IntArray.cpp

#include "IntArray.h"

IntArray::IntArray(int *iarray, int iarray_size)
{
  ia = iarray;
  _size = iarray_size;
}
  ------------------------------------------------------------
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'
/tmp/ccdxV8oM.o: In function `main':
pg49.cpp:(.text+0x5e): undefined reference to
`IntArrayRC::IntArrayRC(int*, int)'
/tmp/ccdxV8oM.o: In function `IntArray::~IntArray()':
pg49.cpp:(.text._ZN8IntArrayD2Ev[_ZN8IntArrayD5Ev]+0xb): undefined
reference to `vtable for IntArray'
/tmp/ccdxV8oM.o: In function `IntArrayRC::~IntArrayRC()':
pg49.cpp:(.text._ZN10IntArrayRCD2Ev[_ZN10IntArrayRCD5Ev]+0xb):
undefined reference to `vtable for IntArrayRC'
collect2: ld returned 1 exit status
eric@eric-laptop:~/CppPrimer3$
---------
you can see, in my IntArray.cpp, I already defined IntArray
constructor, but why I still got
In function IntArray::IntArray(int*, int) undefined reference to
'vtable for IntArray'
plz help, thank a lot in advance, Eric