location of virtual function table pointer in object

Question

As I understand, the location of the virtual function pointer table in an object is compiler dependent.
Are there any pros/cons of placing this pointer at the beginning of the object vs at the end or vice-versa?

Tenth

possible duplicate of Retrieving vptr(pointer to virtual Table aka VTABLE)from the Objdump utility? – Absquatulate

Answer 1

The mere existence of a virtual function table is compiler dependent (but all compilers do), and the location is not mandated either... In all compilers of which I know the details, the vptr is stored in the beginning of the object. The reason is that it provides a uniform location. Consider a class hierarchy:

struct base {
   T data;
   virtual void f();
};
struct derived : base {
   T1 data;
   virtual void g();
};

If the vptr was stored at the end of the object, then it would be after sizeof(T) bytes for an object of complete type base. Now when you have an object of complete type derived, the layout of the base sub object must be compatible with the layout of a complete base object, so the vptr would still have to be sizeof(T) bytes inside the object, which would be somewhere in the middle of the derived object (sizeof(T) from the beginning, sizeof(T1) before the end). So it would no longer be at the end of the object.

Additionally, given a this pointer, a virtual call requires an indirection through the vtable, which basically is dereferencing the vptr, adding an offset and jumping to the memory location stored there. If the vptr was stored at the end of the object, for each virtual call there would be an extra addition to this before dereferencing the vptr.

Srinagar

In theory you could have vptr's both at the end of the base-class subobject and the full object. It would make some sense: have the Derived vptr for virtual functions that don't appear in Base. But the per-object overhead is not so nice. – Arnhem

Answer 2

Yes it is completely implementation dependent.
For a simple inheritance hierarchy it is located at the beginning of the object but for a complex hierarchy it won't be.
Anyhow, any source code you write should not rely on where it is located, in fact any code you write should not rely on even existence of a virtual table or a virtual table pointer.
The C++ Standard does not mandate that virtual dispatch be implemented through virtual table and pointer, an implementation is free to implement it using an other implementation method, However all mainstream compilers do implement this through table pointer mechanism the important point to note is they may differ in exact implementation of where the pointer is located etc.

Offshoot