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Virtual Table layout in memory?

Asked 2009-08-27T16:11:49.027
15

how are virtual tables stored in memory? their layout?

e.g. 

class A{
    public:
         virtual void doSomeWork();
};

class B : public A{
    public:
         virtual void doSomeWork();
};

How will be the layout of virtual tables of class A and class B in memory?

c++ vtable
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2 Answers

41

For GCC compiler in Linux run:

g++ -fdump-lang-class example.h

NOTE: Before GCC 8, the option is -fdump-class-hierarchy instead of -fdump-lang-class.

The output is:

Vtable for A
A::_ZTV1A: 3u entries
0     (int (*)(...))0
8     (int (*)(...))(& _ZTI1A)
16    (int (*)(...))A::doSomeWork

Class A
   size=8 align=8
   base size=8 base align=8
A (0x7fb76785a4e0) 0 nearly-empty
    vptr=((& A::_ZTV1A) + 16u)

Vtable for B
B::_ZTV1B: 3u entries
0     (int (*)(...))0
8     (int (*)(...))(& _ZTI1B)
16    (int (*)(...))B::doSomeWork

Class B
   size=8 align=8
   base size=8 base align=8
B (0x7fb7678510d0) 0 nearly-empty
    vptr=((& B::_ZTV1B) + 16u)
  A (0x7fb76785a540) 0 nearly-empty
      primary-for B (0x7fb7678510d0)

Also I've created the vtable-dumper tool to list contents of virtual tables in the shared objects. With this tool you don't need to compile headers, just run it on the object:

vtable-dumper SHLIB
answered 2012-07-13T07:37:45.303
13

As others have said, this is compiler dependant, and not something that you ever really need to think about in day-to-day use of C++. However, if you are simply curious about the issue, you should read Stan Lippman's book Inside the C++ Object Model.

answered 2009-08-27T16:24:31.013

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