为什么 std::sort 会在此代码上引发分段错误?

Why does std::sort throw a segmentation fault on this code?(为什么 std::sort 会在此代码上引发分段错误?)

问题描述

有人可以解释为什么下面的排序会导致段错误吗?这是 g++(指针排序向量)的已知错误吗?我正在使用 g++ 4.5.2 进行编译.

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;
}

在键盘上编译给出:

/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
}

感谢您的所有快速回复.原来的 comp 函数是:

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;
}

我只是更改了第一行并删除了其余部分.但事实证明，它也不是严格的弱排序(我忘记了如果 (*x)[i] > (*y)[i] 的情况).我可能应该一开始就发布整个功能.不过，再次感谢！

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!!

推荐答案

比较函数必须定义严格的弱排序，这意味着 a <b 和 b <a 不能同时为真.您的比较函数没有此属性.

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.

它没有定义任何前后"关系，因此依赖这个属性的算法不能正常运行也就不足为奇了.

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

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