Can sizeof return 0 (zero)

Question

Is it possible for the sizeof operator to ever return 0 (zero) in C or C++? If it is possible, is it correct from a standards point of view?

Answer 1

typedef struct {
  int : 0;
} x;

x x1;
x x2;

Under MSVC 2010 (/Za /Wall):

sizeof(x) == 4
&x1 != &x2

Under GCC (-ansi -pedantic -Wall) :

sizeof(x) == 0
&x1 != &x2

i.e. Even though under GCC it has zero size, instances of the struct have distinct addresses.

ANSI C (C89 and C99 - I haven't looked at C++) says "It shall be possible to express the address of each individual byte of an object uniquely." This seems ambiguous in the case of a zero-sized object, since it arguably has no bytes.

Edit: "A bit-field declaration with no declarator, but only a colon and a width, indicates an unnamed bit-field. As a special case of this, a bit-field with a width of 0 indicates that no further bit-field is to be packed into the unit in which the previous bit-field, if any, was placed."