Re: Inheritance and offsetof
On Sep 21, 8:20 pm, Marcel M=FCller <news.5.ma...@spamgourmet.org>
wrote:
When I switch an existing project to gcc I get dozens of
warnings concerning offsetof. The code runs fine with and
without gcc. Of course this means nothing.
Example:
#include <stddef.h>
#define structoffsetof(type, base) ((int)(static_cast<base*>((type*)64))-=
64)
Note that there's a reinterpret_cast in there (the C style cast)
with very much implementation defined behavior (and an
implementation may define it to be undefined behavior).
struct A
{
int m1;
int m2;
};
struct B : A
{
int m3;
};
int main()
{
A a;
a.m1 = offsetof(B, m3) - structoffsetof(B, A);
And the offsetof here is undefined behavior. On recent versions
of g++, I think it fails to compile (but I don't have access to
any machine with a recent version of g++ installed to check).
return 0;
}
test.cpp: In function `int main()':
test.cpp:16: warning: invalid access to non-static data member `A::m1'
of NULL object
test.cpp:16: warning: (perhaps the `offsetof' macro was used incorrectly)
Of course, it is because B is no longer a C style POD type
because of the inheritance. But even if the memory layout is
up to the implementation the offset of m3 should be stable.
Maybe. Who knows? Using offsetof on anything but a POD is
undefined behavior, and I would expect a good implementation to
reject it at compile time.
I use this in a wrapper to a C library which takes struct A*
as type and requires relative offsets to custom components. I
would prefer not to use aggregation because A has many members
and this would significantly blow up the code.
Is it undefined behavior or is it only a warning?
Totally undefined behavior.
--
James Kanze