order of destruction using virtual

Question

Can some one please help what the order of destruction is when I am using virtual functions. Does it start with the base class and then derived class?

Answer 1

Since I don't see how virtual function change any objects' destruction order, I assume you're referring to the order of destruction for base classes and data members in a virtual inheritance scenario.

Sub-objects are constructed

1. base classes are constructed from most base to most derived;
2. multiple base classes are constructed in the order of their declaration as base classes;
3. virtual base classes are constructed before all others, amongst themselves adhering to the above two rules;
4. data members are constructed before the enclosing object's constructor's body is executed, in order of their declaration.

Destruction is simply the opposite of construction, so you only need to memorize the above.

However, the above four rules are in that order because that makes sense, and if you understand why this order makes sense, you will not even have to memorize those four rules, but can infer them from your understanding (as I just did). So let's examine that order:

• You might want to use whatever service the base class provide from a derived class' constructor. Of course, you cannot use a (base) class object before it's actually constructed. Therefore, when a derived class is constructed, the base class needs to be already constructed. (Incidentally, this also explains why the virtual function dispatching doesn't fully work from within constructors: When a sub-object is constructed, only the sub-objects of base classes are already constructed; the derived classes' sub-objects are not yet constructed. Therefore a call to a virtual function must not be dispatched to a derived class. As always, destructors are the same, just backwards.)
• With multiple base classes being equal siblings, some order had to be picked arbitrarily. Ultimately, the order of declaration is the most simple one to use. Data members, which also are equal siblings, follow the same (more or less arbitrary) in-order-of-declaration rule.
• Virtual base classes are strange beasts. Because there will always only be one sub-object of a virtual base class, there's that special rule which says it always needs to be constructed first, right from the most derived class' constructor. (Which is why virtual base classes work best as abstract base classes with no data and only default constructors.)

Answer 2

First the derived, then the base. No difference wrt the non-virtual cases.

Additional note. When you have inheritance and virtual methods, you have to declare destructors as virtual, otherwise you can have undefined behavior at deletion.

Example, suppose Derived is derived from Base, and you allocate Derived with the following line:

Base *o = new Derived();
delete(o);

If this case occurs in your code, and Base has no virtual destructor, the resulting behavior is undefined. Typically, only the destructor of Base will be called. The destructor of Derived will not be called, because you are calling delete on a Base pointer. However, the program might crash instead. Once you are in the realm of undefined behavior, all bets are off and your running code is doomed. To prevent chaos the Base destructor must be virtual.