typecheck code

[Sri Ram Vemulpali]

Hi all,

/*
 * Check at compile time that something is of a particular type.
 * Always evaluates to 1 so you may use it easily in comparisons.
 */
 #define typecheck(type,x) \
 ({      type __dummy; \
        typeof(x) __dummy2; \
        (void)(&__dummy == &__dummy2); \
        1; \
 })

#define typecheck_fn(type,function) \
({      typeof(type) __tmp = function; \
       (void)__tmp; \
})

Can anyone help me, explain the above code typecheck. How does
(void)(&__dummy == &__dummy2) evaluates to 1

I appreciate any explain.

-- 
Regards,
Sri.