virtual destructor for pure abstract class [duplicate]

Question

Based on what I found here and on other links on stackoverflow, we should always define a virtual destructor in the base class if we plan to use it polymorphically. I want to know if there is an exception to this rule.

I have seen production code that does not define virtual destructor for the pure abstract base classes and in one of cppcon 2014 video Accept no visitor, around 10:06 the BoolExp struct defined is a pure abstract class and has no virtual destructor.

So for a pure abstract class defined like this

  class Base {
      public:
         virtual foo() = 0;
         virtual bar() = 0;
     }

My question is it absolutely must that we define a virtual destructor for "Base" class, even though it does have any data members? Are there any exceptions to the virtual destructor rule?

Thanks in advance.

Best, RG

Answer 1

My question is it absolutely must that we define a virtual destructor for "Base" class, even though it does have any data members?

It depends. If you have a case like

base * foo = new child(stuff);
// doing stuff
delete foo;

then you absolutely must have a virtual destructor. Without it you'll never destroy the child part.

If you have a case like

child * foo = new child(stuff);
// doing stuff
delete foo;

Then you do not need a virtual destructor, as child's will be called.

So the rule is if you delete polymorphically, you need a polymorphic (virtual) destructor, if not, then you don't

Answer 2

The exception to the rule is if you never delete an object through a pointer to the base class. In that case the base class destructor does not need to be virtual.

But if you ever delete an object via a base class pointer, then the base class destructor must be virtual or your program has Undefined Behaviour.