Virtual destructor - when it is necessary

Question

Let's say that I have a base class X with only one virtual function.

I extend it to Y and override that function.

I didn't provide explicit destructor for X and Y.

What happens during the destruction in this statement.

Y y;
X *x= &y;
return;
.
.
.
X*x = new Y;
delete x;
return;

Which will cause memory leak..

Neither? Also I don't see how these two code snippets on their own are related to virtual destructors? You'd need to at least show the class definitions. — UnholySheep, Nov 16 '16 at 16:26
1) No problem. x is just a pointer, but y gets destructed and frees all resources from super class too. 2) Resources of Y get lost, because just the destructor of X is called on an instance of Y in memory -> possible memory leak (or worse). — Youka, Nov 16 '16 at 16:27
@UnholySheep there is nothing special about class definition. I mentioned the statement. The second one seems to cause a memory leak — Yashwanth CB, Nov 16 '16 at 16:37

score 0 · Answer 1 · answered Nov 16 '16 at 16:27

Y y;
X *x = &y;
return;

Nothing bad happens here. The object y is (fully) destroyed at the end of its scope.

X*x = new Y;
delete x;
return;

This will cause a memory leak if you're lucky. The behavior is undefined, meaning anything bad could happen.

It's generally a good idea to declare a virtual destructor whenever the class is intended to be a base class for other classes.

1 Answers1