Re: "lang_statement_union_type and its usage?"

[Ian Lance Taylor <ian at zembu dot com>]

xinant@cognigine.com (Xinan Tang) writes:

> > All elements of lang_statement_union start with
> >   lang_statement_header_type header;
> > The first field in lang_statement_header_type is
> >   union lang_statement_union *next;
> > Therefore, in all cases, the first field in any instance of
> > lang_statement_union will be the next pointer.
> > This is a pretty common hack when implementing derived classes in C.
> > Ian
> 
>    I see. However, this may not work if some optimizing compilers
> would change the relative order of the `next' field. I.e., this scheme
> is compiler-dependent.

The ISO C standard forbids compilers from making the possible
optimization.  ISO C requires that structure members have addresses
that increase in the order in which they are declared.  ISO C also
says ``a pointer to a structure object, suitably converted, points to
its initial member'' and ``a pointer to a union object, suitably
converted, points to each of its members.''

This is required in order to make it possible for a C structure or
union to describe an area of memory used by hardware, a traditional
use of C in kernel driver implementations.

C is not a high level language.

Ian