Why does std::sort throw a segmentation fault on this code?

Question

Can someone explain why the sort below causes seg faults? Is this a known bug with g++ (sorting vector of pointers)? I am compiling with g++ 4.5.2.

#include <iostream>
#include <algorithm>
#include <vector>

using namespace std;

typedef vector<int> A;
bool face_cmp(const A *x, const A *y) {
  return x != y;
}

int main(int argc, char* argv[]) {

  vector<A *> vec;
  for (int i=0; i<100; i++) {
    vec.push_back( new vector<int>(i%100, i*i) );
  }

  vector<A *>::iterator it;
  sort(vec.begin(), vec.end(), face_cmp);

  return EXIT_SUCCESS;
}

Compiling on codepad gives:

/usr/local/lib/gcc/i686-pc-linux-gnu/4.1.2/../../../../include/c++/4.1.2/debug/safe_iterator.h:240:
    error: attempt to decrement a dereferenceable (start-of-sequence)     
    iterator.

Objects involved in the operation:
iterator "this" @ 0x0xbf4b0844 {
type = N11__gnu_debug14_Safe_iteratorIN9__gnu_cxx17__normal_iteratorIPPN15__gnu_debug_def6vectorIiSaIiEEEN10__gnu_norm6vectorIS7_SaIS7_EEEEENS4_IS7_SB_EEEE (mutable iterator);
  state = dereferenceable (start-of-sequence);
  references sequence with type `N15__gnu_debug_def6vectorIPNS0_IiSaIiEEESaIS3_EEE' @ 0x0xbf4b0844
}

Thank you for the all the quick replies. The original comp function was:

if (x == y) return false;
if (x->size() < y->size()) return true;
else if (x->size() > y->size()) return false;
else {
  for (register int i=0; i<x->size(); i++) {
    if ((*x)[i] < (*y)[i]) return true;
  }
  return false;
}

I just changed the first line and removed the rest. But it turns out it also suffers from not being a strict weak ordering (I forgot the case if (*x)[i] > (*y)[i]). I should probably have posted the entire function to begin with. Nevertheless, thanks again!!

Answer 1

The comparison function must define a strict weak ordering which means that a < b and b < a cannot be both true. Your comparison function does not have this property.

It does not define any "before-after" relationship, so it's no wonder that the algorithm relying on this property does not function properly.

Answer 2

Third argument of std::sort should be a function (or functional object) such that if compare(a, b) is true then compare(b, a) should be false , but your one isn't such. So your program is UB and can give any result.

Answer 3

No your code is wrong. Comparison functions for std::sort must use < or it's equivalent, using != is not correct. Probably you want this

bool face_cmp(const A *x, const A *y) {
  return *x < *y;
}

Answer 4

Define your comparison function as

bool face_cmp(const A *x, const A *y) {
  return x < y;
}