A precondition is a condition that must hold before a function or operation is invoked. Make each function precondition explicit so invalid calls fail visibly rather than silently violating the algorithm. section 7.3
8 size_t gcd2(size_t a, size_t b) {
9 assert(a <= b);
10 if (!a) return b;
11 size_t rem = b % a;
12 return gcd2(rem, a);
13 }
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The gcd2 implementation asserts a <= b before handling the termination case, computing a remainder, and making its recursive call. Modulo arithmetic preserves that precondition for subsequent calls. section 7.3
Use different mechanisms for different kinds of preconditions:
- Use assert to abort execution with an informative message when a function receives arguments that violate its precondition. section 7.3
- Use #if and #error to abort compilation when required platform facilities are unavailable. section 8.1.5
- Use static_assert for more sophisticated compile-time conditions, such as required relationships between type sizes. section 8.1.5
