Why should I declare a virtual destructor for an abstract class in C++?

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, then what happens is up to the compiler.

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 in
   // which case you have a memory leak. Or it may crash
   delete p;
}