Should every class have a virtual destructor?

No! Virtual destructors are used only when a object of a derived class is deleted through a base class pointer. If your class is not intended to serve as the base in this scenario, don't make the destructor virtual - you would be sending a wrong message.

The question is really, do you want to enforce rules about how your classes should be used? Why? If a class doesn't have a virtual destructor, anyone using the class knows that it is not intended to be derived from, and what limitations apply if you try it anyway. Isn't that good enough?

Or do you need the compiler to throw a hard error if anyone dares to do something you hadn't anticipated?

Give the class a virtual destructor if you intend for people to derive from it. Otherwise don't, and assume that anyone using your code is intelligent enough to use your code correctly.

Every abstract class should either have a,

• protected destructor, or,
• virtual destructor.

If you've got a public non-virtual destructor, that's no good, since it allows users to delete through that pointer a derived object. Since as we all know, that's undefined behavior.

For an abstract class, you already need a virtual-table pointer in the object, so making the destructor virtual doesn't (as far as I'm aware) have a high cost in terms of space or runtime performance. And it has the benefit that derived classes automatically have their destructors virtual (see @Aconcagua's comment). Of course, you can also make the destructor protected virtual for this case.

For a non-abstract class not intended to be deleted through a pointer to it, I don't think there's good reason to have a virtual destructor. It would waste resources, but more importantly it would give users a wrong hint. Just think about what weird sense it would make to give std::iterator a virtual destructor.