Question

I have a base class Media and several derived classes, namely DVD, Book, etc…
The base class is written as:

class Media{
    private:
        int id;
        string title;
        int year;
    public:
        Media(){ id = year = 0; title = ""; }
        Media(int _id, string _title, int _year): id(_id), title(_title), year(_year) {}
//      virtual ~Media() = 0;
        void changeID(int newID){ id = newID; }
        virtual void print(ostream &out);
};

The thing is: without the destructor, GCC gives me a bunch of warnings class has virtual functions but non-virtual destructor, but still compiles and my program works fine. Now I want to get rid of those annoying warnings so I satisfy the compiler by adding a virtual destructor, the result is: it doesn’t compile, with the error:

undefined reference to `Media::~Media()`

Making the destructor pure virtual doesn’t solve the problem.
So what has gone wrong?

Answer 1

You need to also define the virtual destructor, not only add it.

//Media.h
class Media{
    //....
    virtual ~Media() = 0;
};

//Media.cpp
#include "Media.h"
//....
Media::~Media() {};

The reason you get the warnings is that all classes that will be derived from should have a virtual or protected (credit @Steve) destructor, otherwise deleting an instance via a pointer to a base class results in undefined behavior.

Note you HAVE TO provide a definition for destructors, even if they are pure virtual.