Question

I know it is a good practice to declare virtual destructors for base classes in C++, but is it always important to declare virtual destructors even for abstract classes that function as interfaces? Please provide some reasons and examples why.

Answer 1

It’s even more important for an interface. Any user of your class will probably hold a pointer to the interface, not a pointer to the concrete implementation. When they come to delete it, if the destructor is non-virtual, they will call the interface’s destructor (or the compiler-provided default, if you didn’t specify one), not the derived class’s destructor. Instant memory leak.

For example

class Interface {    virtual void doSomething() = 0; };  class Derived : public Interface {    Derived();    ~Derived()     {       // Do some important cleanup...    } };  void myFunc(void) {    Interface* p = new Derived();    // The behaviour of the next line is undefined. It probably     // calls Interface::~Interface, not Derived::~Derived    delete p;  }