using offsetof with g++

[Gabriel Dos Reis]

Trevor Blackwell <tlb@anybots.com> writes:

| > If you can persuade the C++ committee that offsetof is important not
| > only for legacy C code but also for new C++ code, then there's a good
| > chance that this restriction will be loosened in a future version of the
| > language.  On the other hand, you can also expect to be asked why
| > you want to use offsetof instead of pointers-to-member.  (And if you can
| > show that there are things you can't do with pointers to members, you
| > might find that people will prefer to solve the problem by removing
| > whatever limitation pointers to members have.)
| 
| I've been having the same problem with offsetof. The thing I want to do
| that can't be done with member pointers is:
| 
| struct B {
|   int y;
| };
| 
| struct A {
|   int x;
|   B b;
| };
| 
| int A::*myptr1 = &A::x; // fine
| int A::*myptr2 = &A::b.y; // error

   B* A::*myptr2 = &A::b;  // fine

there has been a proposal to allow composition of point-to-member and
generic pointer to data member.  And there still is a suggestion
to complete the set of operations on pointer to members.

myptr2 is an index into B.  So if you get that index, then you can
also compose &A::b with &B::y and get the relative of B::y inside A.

-- Gaby