To quote from C99, structs within a union that share initial common members are treated as a special case.
Originally Posted by
c99
With one exception, if the value of a member of a union object is used when the most recent store to the object was to a different member, the behavior is implementation-defined.70)
One special guarantee is made in order to simplify the use of unions: If a union contains several structures that share a common initial sequence (see below), and if the union object currently contains one of these structures, it is permitted to inspect the common initial part of any of them anywhere that a declaration of the completed type of the union is visible. Two structures share a common initial sequence if corresponding members have compatible types (and, for bit-fields, the same widths) for a sequence of one or more initial members.
The example being
Code:
union {
struct {
int alltypes;
}n;
struct {
int type;
int intnode;
} ni;
struct {
int type;
double doublenode;
} nf;
}u;
u.nf.type = 1;
u.nf.doublenode = 3.14;
/* ... */
if (u.n.alltypes == 1)
if (sin(u.nf.doublenode) == 0.0)
/* ... */
I would think that the C++ standard would inherit a similar clause.