Using a base class called Collection, I need to implement my derived class, VariableArray. However, I keep getting the error message

undefined reference to `Collection::Collection()'

I am not certain what I need to do here. I have a constructor defined(none virtual in my base class), along with a copy constructor, and a virtual destructor. In the derived class definition file, I included the code below, but I still get the error message.

VariableArray::VariableArray():Collection()
  {         }

Please any help will be appreciated.

BASE CLASS

#include <iostream>
using namespace std;

class Collection
{
    public:
                Collection();
                Collection( const Collection & );
            virtual ~Collection()=0;
            virtual bool add(const int x) =0;
            virtual int& remove() = 0; 
            virtual Collection& operator=(const Collection& rhs)=0;
            virtual int& operator[](int index) =0;   
        virtual const int& operator[](int index)const =0; 
        Collection& copy(); 
                int size_();

    protected:

    private:
               int _size;  

};