The Power of Ten

Rule Five

 Use minmally two assertions per function on average.

The assertion density of the code should average to a minimum of two assertions per function. Assertions are used to check for anomalous conditions that should never happen in real-life executions. Assertions must always be side-effect free and should be defined as Boolean tests. When an assertion fails, an explicit recovery action must be taken, e.g., by returning an error condition to the caller of the function that executes the failing assertion. Any assertion for which a static checking tool can prove that it can never fail or never hold violates this rule. This means that it is not possible to satisfy the rule by adding unhelpful assert(true) statements.

Rationale

 Statistics for industrial coding efforts indicate that unit tests often find at least one defect per 10 to 100 lines of code written. The odds of intercepting defects increase with assertion density. Use of assertions is often also recommended as part of a strong defensive coding strategy. Assertions can be used to verify pre- and post-conditions of functions, parameter values, return values of functions, and loop-invariants. Because assertions are side-effect free, they can be selectively disabled after testing in performance-critical code. A typical use of an assertion would be as follows: 
	if (!c_assert(p >= 0) == true) {
		return ERROR;
	}
with the assertion defined as follows: 
	#define c_assert(e)	((e) ? (true) : \
		tst_debugging("%s,%d: assertion '%s' failed\n", \
		__FILE__, __LINE__, #e), false)
In this definition, __FILE__ and __LINE__ are predefined by the macro preprocessor to produce the filename and line-number of the failing assertion. The syntax #e turns the assertion condition e into a string that is printed as part of the error message. In code destined for an embedded processor there is of course no place to print the error message itself -- in that case, the call to tst_debugging is turned into a no-op, and the assertion turns into a pure Boolean test that enables error recovery from anomolous behavior.
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