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Today, while sorting a vector using a comparison function in C++, I encountered a crash.
The error message was a segment fault, providing no clear indication of the exact issue in the code.
After some investigation, I finally pinpointed it, the issue was with the comparison function:

bool comp(int a, int b){
    return a <= b;
}

Using a <= b is incorrect, because it results in ambiguous comparisons.
After change it to a < b, the problem was solved.

Comparison Function Rules:

Rule Description Example/Explanation
Asymmetry If comp(a, b) is true, then comp(b, a) must be false. If a < b is true, then b < a should be false.
Transitivity If comp(a, b) and comp(b, c) are true, then comp(a, c) must be true. If a < b and b < c, then a < c must hold.
Irreflexivity comp(a, a) must always return false. An element is never less than itself.
Return Type The comparison function should return a boolean value. bool comp(const T& a, const T& b);
Consistency The function should provide consistent results for the same inputs. Results should not change when called multiple times with the same values.
No Modification The comparison function should not modify its arguments. It should take const references to avoid modification.
Strict Weak Order The function must impose a strict weak ordering on the elements. It should behave like a < operator or similar comparison.
No Equality Comparison Equal elements should not return true for either direction. If a == b, both comp(a, b) and comp(b, a) should be false.
Contents
  1. 1. Comparison Function Rules: