CODEWITHSUNDEEP
c++processvtable
Dirt
Dirt

Reputation: 331

do all instances of the same c++ class share a vtable or would each one get its own?

If Base is a base class and Derived a derived class and there are 25 instances of Derived, how are the vtables set up to be accessed by all the instances? Where are they loaded in the memory?

Upvotes: 6

Views: 1485

Answers (2)

sji
sji

Reputation: 1907

Compilers are allowed to implement dynamic dispatch however they want in c++, i don't think there is actually any requirement to even use a vtable at all, but it would be very unusual to find a compiler that didn't.

In most cases i think that each class (that contains some virtual methods) will own a single vtable (so if i had 5 instances of class A i will still only have 1 vtable), but this behaviour should not be relied upon in any way.

Non virtual classes have no need for vtables as far as i know.

Reading your question it seems as if you think that each object has its own copy of the code, I'm not sure and i don't want to accuse you of anything like that but just in case ...

Google something like: "what does a c++ object look like in memory"

Upvotes: 3

jcoder
jcoder

Reputation: 30035

There will be one vtable somewhere in memory, probably in the same place as the code. Each instance of the class will contain a single pointer to the vtable for that class, so in your case all 25 instances will contain a pointer to one copy of the vtable.

Multiple and virtual inheritance complicate things, but the principle is the same.

Upvotes: 5

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