How to watch the vtable in gdb in a C++ program?

Question

I have an interesting problem in one of my C++ programs. Apperantly the vtable of one of my classes is/becomes messed up during program execution. In a gdb session I found out that if I call a method of the object directly it succeeds, but if I use a pointer or reference I end up in the destructor of a completely unrelated class which isn't going to be instantiated anytime soon. Without the this -Pointer changing of course.

Assuming my observations are correct, how do I watch the vtable of an object in gdb? I'm on Linux using gcc, my gdb version is GNU gdb (Ubuntu/Linaro 7.3-0ubuntu2) 7.3-2011.08 .

Answer 1

You can use the -fdump-class-hierarchy option of gcc which will give you the vtable information, however the output can be very verbose and hard to read.

For instance, given the following trivial classes:

class Base {                                                                            
    public:                                                                             
        virtual int method() = 0;                                                       
};                                                                                      

class Derived : public Base {                                                           
    public:                                                                             
        int method() {                                                                  
            return 10;                                                                  
        }                                                                               
};  

the relevant output is

Vtable for Base
Base::_ZTV4Base: 3u entries
0     (int (*)(...))0
8     (int (*)(...))(& _ZTI4Base)
16    (int (*)(...))__cxa_pure_virtual

Class Base
   size=8 align=8
   base size=8 base align=8
Base (0x7f14c308ccc0) 0 nearly-empty
    vptr=((& Base::_ZTV4Base) + 16u)

Vtable for Derived
Derived::_ZTV7Derived: 3u entries
0     (int (*)(...))0
8     (int (*)(...))(& _ZTI7Derived)
16    (int (*)(...))Derived::method

Class Derived
   size=8 align=8
   base size=8 base align=8
Derived (0x7f14c2ee7208) 0 nearly-empty
    vptr=((& Derived::_ZTV7Derived) + 16u)
  Base (0x7f14c308cd20) 0 nearly-empty
      primary-for Derived (0x7f14c2ee7208)

This should give you an idea which address ranges to expect during debuggng etc.

Answer 2

Unless you're hacking, I doubt your vtable gets messed up. Are you calling the virtual function from the costructor?

It might also be that the debugger is messing with you. Compiling with optimization, you might get the same address for functions that do the same thing, as to not have duplicated code. I ran into this under Windows, where Visual Studio was also jumping in different functions that actually did the same thing. Try outputting something instead of walking through the code with the debugger... This might be the cause.