Hello,

In 9.5/1 the standard says:
 >>
If a union contains a static data member, or a member of reference type,=20
the pro=ADgram
is ill-formed.
<<

Why are static data members disallowed in unions? They've been disallowed=
=20
at least since the ARM.  They would make reasonable sense to me.  I'd use=
=20
them for the same reasons that I use static data members in a class.

union U {
 U() : i(0) { ++n; recent =3D this; }
 ~U() { --n; if (recent =3D=3D this) recent =3D 0; }
 static int num() { return n; }
 int i;
 static int n;
 static U* recent;
};
int U::n;
U* U::recent;

I guess there could be squeamishness about the fact that union's non-stat=
ic=20
data members are overlapped and the static data members would be=20
distinct.  But this doesn't seem difficult to grasp to me (and if you wan=
t=20
the static data members overlapped you could put them in an union).

Instead I have to do:

int U_n;
union U;
U* U_recent;
union U {
 U() : i(0) { ++U_n; U_recent =3D this; }
 ~U() { --U_n; if (U_recent =3D=3D this) U_recent =3D 0; }
 static int num() { return U_n; }
 int i;
};

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