Why can't I make the object?

[AL88]

I get an error at the build stage "undefined reference to LIST<int>::~List"
I'm trying to learn about templates and my book asked me to make the class into a template then declare the object.
Please show me what I am doing wrong and help me fix the error.Thanks.

  #include <iostream>

template <class T>
class List
{
private:
public:
    List():head(0),tail(0),theCount(0) {}
    virtual ~List();
    void Insert( int value );
    void append( int value );
    int is_present( int value ) const;
    int is_empty() const { return head == 0; }
    int count() const { return theCount; }
private:
    class ListCell
    {
    public:
        ListCell(int value, ListCell *cell = 0):val(value),next(cell){}
        int val;
        ListCell *next;
    };
    
    ListCell *head;
    ListCell *tail;
    int theCount;
};
  

int main()
{
List <int> myList;
}

[kbw]

You haven't defined the destructor, ~List<T>, or ~List<int> (as you've instantiated List<int>.

By defclaring myList, you're calling the default constructor and destructor.

BTW, are you sure the destructor should be virtual? I don't think it should be.

[AL88]

can you show me how I should define the destructor to get it to compile? thanks.

[user42]

It should not be virtual to start with.
Make sure to free any allocated memory in the destructor.

[AL88]

ive not used new anywhere, do the ponters count as allocated memory?

[AL88]

this works...

#include <iostream>

template <class T>
class List
{
private:
public:
    List():head(0),tail(0),theCount(0) {std::cout <<"List constructor called.\n";}
    virtual~List(){std::cout<<"List destructor called.\n";}
    void Insert( int value );
    void append( int value );
    int is_present( int value ) const;
    int is_empty() const { return head == 0; }
    int count() const { return theCount; }
private:
    class ListCell
    {
    public:
        ListCell(int value, ListCell *cell = 0):val(value),next(cell){}
        int val;
        ListCell *next;
    };
    
    ListCell *head;
    ListCell *tail;
    int theCount;
};
  

int main()
{
List <int> myList;
}

[Repeater]

When you write this in the declaration:
virtual ~List();

you're promising to the compiler that you will write the function
~List();

but you didn't.

Either don't make that promise (by not putting it in the declaration), or fulfil your promise by writing the function ~List();.