December 2003

Generic<Programming>: Exception Safety Analysis

Andrei Alexandrescu and David B. Held, p 40
Programming is understanding and exceptions are no exception

(wacky guarantee)
• basic guarantee
• strong guarantee
• nothrow guarantee

void Fun() {
  Gun();
  Hun();
}

Gun and Hum strong. What about Fun? May be from strong to wacky...
But if Gun is strong and pure, then Fun stays strong.

Conclusion

• In a sequence of function calls, you obtain the strong guarantee if you write your code as a sequence of pure calls, followed by no more than one strong call, followed by a sequence of nothrow calls.
• Purity is context dependent; you can transform a strong function into a pure function by having it operate on local automatic data. Note that pure functions are always strong.
• Alternatively, you can use try/catch with purity restoration in the catch clause. A variant of that uses ScopeGuard for automatically restoring purity in case of an exception.
• When the system has broken invariants, always restore them with nothrow calls.
• To determine a function's exception-safety behavior, analyze it with the algorithm in the previous section, having at hand the summaries of the functions it invokes.

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