Re: Is the vptr at the beginning of an object?
* Paul N:
On 2 Apr, 11:07, James Kanze <james.ka...@gmail.com> wrote:
On Apr 2, 9:56 am, "DaveB" <DBu...@usenet.net> wrote:
In practice, across most compilers, is the vptr at offset 0 of
a class object? I've found some posts that say GCC puts it at
the end, but they were very old posts and I don't use it now
so can't test it.
Who cares? What does it matter where the vptr is placed? (And
of course, many objects have more than one vptr, and not all of
them can be at offset 0.)
Just out of curiosity, I thought the compiler was obliged to put the
first member at offset zero?
Only for a POD. For a POD it's guaranteed by reinterpret_cast to suitable type
yielding a pointer to the first member (and vice versa).
If so (and it could well be I'm wrong
here...) then it must put the vptr (assuming there is one) somewhere
else?
When the first data member is at offset 0 a vptr, if the implementation uses
that scheme (but all do), must be put somewhere else, yes.
Generally this is only relevant when considering binary layout compatibility.
However, the language supports compilers that put a vpointer at offset 0, e.g.
when introducing a virtual member function in a derived class, because given a
pointer to some class type object a static_cast up or down an inheritance chain
can change the pointer value as given by a conversion to void*.
Cheers & hth.,
- Alf