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Wiki MarkupThis 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 gives the ability to make assertions at compile time rather than runtime, providing the following advantages:

• 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 rather is diagnosed at compile time

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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 0, an array type with a dimension of 1 is defined; otherwise an array type with a dimension of -1 is defined. Because it is illegal 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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\[[ISO/IEC 9899-1999|AA. C References#ISO/IEC 9899-1999]\] Section 6.10.1, "Conditional inclusion," and Section 6.10.3.3, "The ## operator"
[Klarer 04] R. Klarer, J. Maddock, B. Dawes, and H. Hinnant. "Proposal to Add Static Assertions to the Core Language (Revision 3)" (ISO C++ committee paper ISO/IEC JTC1/SC22/WG21/N1720, October 2004). This document is available online at http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2004/n1720.html.

\[Saks 08] Dan Saks, Stephen C. Dewhurst. Presentation. Sooner Rather Than Later: Static Programming Techniques for C++.
\[Saks 05] Dan Saks. [_Catching errors early with compile-time assertions|http://www.embedded.com/columns/programmingpointers/164900888?_requestid=287187]. Embedded Systems Design.  June, 2005.
\[Eckel 2007] Bruce Eckel. [_Thinking in C++ - Volume 2_|http://bruce-eckel.developpez.com/livres/cpp/ticpp/v2/]. January 25, 2007.

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