Warning GNU compiler warning “class has virtual functions but non-virtual destructor”

class GenericSymbolGenerator {
   protected:                     // <== ok 
    ~GenericSymbolGenerator(void) {}

   public:
    virtual GenericSymbolTableCollection* generateSymbolTableCollection(GenericSymbolTableCollection *gst) = 0;
    GenericSymbolGenerator(void) {}
    // ~GenericSymbolGenerator(void) {} // <== warning if used
};

class PascalPredefinedSymbolGenerator : public GenericSymbolGenerator {
   protected:
    ~PascalPredefinedSymbolGenerator(void) {} // <== ok

   public:
    GenericSymbolTableCollection* generateSymbolTableCollection(GenericSymbolTableCollection *pst); // initializes *pst
    PascalPredefinedSymbolGenerator(void) {}
    // ~PascalPredefinedSymbolGenerator(void) {} <== warning if used
};

class PascalSymbolGenerator : public GenericSymbolGenerator {
    protected:
         ~PascalSymbolGenerator(void) {} // <== ok

    public:
     GenericSymbolTableCollection* generateSymbolTableCollection(GenericSymbolTableCollection *st); // initializes st
     PascalSymbolGenerator(void) {}
     // ~PascalSymbolGenerator(void) {} // <== warning if used
};

GenericSymbolGenerator *ptr = new PascalPredefinedSymbolGenerator();
delete ptr; // behavior is undefined, we tried to call the base class destructor