What is a virtual destructor?
In general, a virtual fn means to start at the class of the object itself, not the type of the pointer/ref (“do the right thing based on the actual class of” is a good way to remember it). Virtual destructors (dtors) are no different: start the destruction process “down” at the object’s actual class, rather than “up” at the ptr’s class (ie: “destroy yourself using the *correct* destruction routine”). Virtual destructors are so valuable that some people want compilers to holler at you if you forget them. In general there’s only one reason *not* to make a class’ dtor virtual: if that class has no virtual fns, the introduction of the first virtual fn imposes typically 4 bytes overhead in the size of each object (there’s a bit of magic for how C++ “does the right thing”, and it boils down to an extra ptr per object called the “virtual table pointer” or “vptr”).
