Why Does My `std::sort` Crash? (And How to Fix It)

Understanding Compiler Crashes with std::sort

The C Standard Library function, std::sort, plays a crucial role in sorting data structures. However, not all comparison functions behave as expected with std::sort, potentially leading to program crashes.

Consider the following code snippet:

#include <algorithm>

struct A
{
    int a;
};

bool compare(const A& a, const A& b)
{
    return a.a <= b.a; // Corrected from original code (<)
}

int main()
{
    A coll[8];
    std::sort(&coll[0], &coll[8]);
}

Problem:

In the original code, the comparison function compare uses a.a <= b.a to compare elements, which allows for equal elements to be considered sorted. This violates the strict weak ordering rule required by std::sort.

Solution:

According to the strict weak ordering rule, for any elements A, B, and C in a sequence, the following conditions must hold:

• If A < B, then B > A.
• If A < B and B < C, then A < C.
• A cannot be equal to itself (i.e., A != A).

The original comparison function compare does not satisfy this rule because it allows A to be equal to itself, leading to potential infinite loops within std::sort. To fix this, the comparison function needs to be replaced with the corrected version shown in the code snippet above.

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