Displaying C++ vtables

A vtable is a mapping that allows your C++ application properly reconcile the function pointers for the base classes that have virtual methods and the child classes that override those methods (or do not override them). A class that does not have virtual methods will not have a vtable.

A vtable is pointed to by a pointer (“vpointer”) at the top of each object, usually, where the vtable is the same for all objects of a particular class.

Though you can derive the pointer yourself, you can use gdb, ddd, etc.. to display it:

Source code:

class BaseClass
{
    public:

    virtual int call_me1()
    {
        return 5;
    }

    virtual int call_me2()
    {
        return 10;
    }

    int call_me3()
    {
        return 15;
    }
};

class ChildClass : public BaseClass
{
    public:

    int call_me1()
    {
        return 20;
    }

    int call_me2()
    {
        return 25;
    }
};

Compile this with:

g++ -fdump-class-hierarchy -o vtable_example vtable_example.cpp

This emits a “.class” file that has the following (I’ve skipped some irrelevant information at the top, about other types:

Vtable for BaseClass
BaseClass::_ZTV9BaseClass: 4u entries
0     (int (*)(...))0
4     (int (*)(...))(& _ZTI9BaseClass)
8     (int (*)(...))BaseClass::call_me1
12    (int (*)(...))BaseClass::call_me2

Class BaseClass
   size=4 align=4
   base size=4 base align=4
BaseClass (0x0xb6a09230) 0 nearly-empty
    vptr=((& BaseClass::_ZTV9BaseClass) + 8u)

Vtable for ChildClass
ChildClass::_ZTV10ChildClass: 4u entries
0     (int (*)(...))0
4     (int (*)(...))(& _ZTI10ChildClass)
8     (int (*)(...))ChildClass::call_me1
12    (int (*)(...))ChildClass::call_me2

Class ChildClass
   size=4 align=4
   base size=4 base align=4
ChildClass (0x0xb76fdc30) 0 nearly-empty
    vptr=((& ChildClass::_ZTV10ChildClass) + 8u)
  BaseClass (0x0xb6a092a0) 0 nearly-empty
      primary-for ChildClass (0x0xb76fdc30)

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