Sunday, June 26, 2011

is my dtor virtual if I don't declare it virtual?(my take

Q: is my dtor virtual if I don't declare it virtual?

A lot of implicit rules, but there's a simple rule -- see below

Here's my long answer
- If you are a top-level class (i.e. not inheriting), and don't declare a dtor ---> non-virtual
+ if you declare it "virtual" -----> virtual
+ if you inherit and declare it "virtual" -----> virtual
= if you inherit but don't declare a dtor, then synthesized dtor is ------> as virtual as your parent's
= if you inherit and declare without "virtual", then your dtor is still -----> as virtual as your parent's. http://www.parashift.com/c++-faq-lite/virtual-functions.html#faq-20.7

Rule -- once ancestor's dtor is declared virtual, descendants have no way of losing the virtuality.

It's illuminating to visualize the memory layout. Physically, a subclass real estate always encloses a base instance. Since the base real estate has 32 bits for a vptr, every descendant instance has exactly 32 bits for a vptr too, not more no less. Java simply puts the 32 bit footprint into Object.java, so every java object has 1 and only 1 vptr.

If you go through dtor virtuality down a family tree, you may see NV -> NV -> NV ...-> V -> V -> V. Once you get a Virtual, you can only get more Virtuals.

As an analogy, a 3-generation warrior is made a king. All his descendants become royal.

As an analogy, a 3-generation farmer becomes a landlord in 1949. All children and grand children are considered by communists as landlords.

Reason? As described in http://www.parashift.com/c++-faq-lite/virtual-functions.html#faq-20.4, once a base class gets a (class-level) vtbl, its subclass always gets its own vtbl. I believe every descendant dtor is always on a vtbl.

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