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This standard recommends the inclusion of diagnostic tests into your program using the assert() macro or other mechanisms (see MSC11-A. Incorporate diagnostic tests using assertions). Static assertion is a new facility in the C++ 0X draft standard. This facility enables assertions to be made at compile time rather than runtime, providing meeting the following advantagesrequirements:

• All processing must be performed during compile time - no runtime cost in space or time is tolerable.
• Assertion failure must result in a meaningful and informative diagnostic error message.
• It can be used at file or block scope.
• Misuse does not result in silent malfunction but is diagnosed at compile time.

...

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

int mainfunc(void) {
  assert(offsetof(timer, DATA) == 4);
}

...

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

#if (offsetof(timer, DATA) != 4)
  #error "DATA must be at offset 4"
#endif

Using #error directives allows for clear diagnostic messages. Because this approach evaluates assertions at compile time, there is no run-time penalty.

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

#define static_assert(e, s) \
  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, "offsetof(struct timer, DATA) == 4");

The static_assert() macro accepts a constant expression e, which is evaluated as the first operand to the conditional operator. If e evaluates to nonzero, an array type with a dimension of 1 is defined; otherwise an array type with a dimension of -1 is defined. Because it is invalid to declare an array with a negative dimension, the resulting type definition will be flagged by the compiler. The name of the array is used to indicate the location of the failed assertion.

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