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According to the C++ 0X draft standard, the constant-expression in a static assert declaration is a constant expression that can be converted to bool at compile time. If the value of the converted expression is true, the declaration has no effect. Otherwise the program is ill-formed, and a diagnostic message (which includes the text of the string-literal) is issued at compile time. For example:

/* Passes */
static_assert(
  sizeof(int) <= sizeof(void*), 
  "sizeof(int) <= sizeof(void*)"
); 

/* PassesFails */
static_assert(
  sizeof(double) <= sizeof(int), 
  "sizeof(double) <= sizeof(int)"
); /* Fails */

Static assertion is not available in C99, but the facility is being considered for inclusion in C1X by the ISO/IEC WG14 international standardization working group.

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#define JOIN(x, y) JOIN_AGAIN(x, y)
#define JOIN_AGAIN(x, y) x ## y

#define static_assert(e) \
  typedef char JOIN(assertion_failed_at_line_, _LINE_) \
  [(e) ? 1 : -1]

struct timer {
  uint8_t MODE;
  uint32_t DATA;
  uint32_t COUNT;
};

static_assert(offsetof(struct timer, DATA) == 4);

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