In C, assert is a runtime assertion macro from <assert.h>. It evaluates a Boolean expression whenever execution reaches it, unless NDEBUG is defined during compilation. If the expression is false, assert prints a diagnostic message and calls abort. section 8.8
Checking a function precondition
A recursive greatest-common-divisor function begins with assert(a <= b). This makes its required argument relationship explicit: an invalid initial call produces an informative failure instead of silently continuing. Recursive calls preserve the condition through the modulus operation. section 7.3
inline size_t gcd2(size_t a, size_t b) [[__unsequenced__]] { 1
assert(a <= b); 2
if (!a) return b; 3
size_t r = b % a; 4
return gcd2(r, a);
}
Inspect the assertion at the start of gcd2, alongside its termination check, remainder calculation, and recursive call. section 7.3
Use assertions according to these rules:
- Assert properties that are supposed to hold at a particular runtime point, including dynamic conditions and function preconditions. section 8.8
- Keep assertions active during debugging so false assumptions about variable values lead to diagnostic output and a failure location. section 8.8
- Define NDEBUG only for a production compilation when assertions should not evaluate expressions or call abort; therefore, do not rely on assert expressions for required program effects. section 8.8
- Prefer static_assert for compile-time Boolean checks; use assert for conditions that must be tested during execution. section 8.8
