Question

我基本上打算创建一个min heap和一些方法，比如remove min和insert。

当我用类的对象调用方法时，方法工作正常。但是，当我决定使用指针时，整个程序在开始时崩溃。

class TreeNode {
private:
  TreeNode *left;
  TreeNode *right;
  unsigned frequency;
  unsigned char val;

public:
  TreeNode *getLeft() { return left; }
  TreeNode *getRight() { return right; }
  TreeNode(unsigned char val, unsigned int frequency) {
    left = NULL;
    right = NULL;
    this->frequency = frequency;
    this->val = val;
  }
  unsigned getFrequency() { return frequency; }
  signed int setFrequency(int i) { this->frequency = i; }
  unsigned char getVal() { return val; }
};

class MinHeap {
private:
  int n;
  vector<TreeNode *> holder;

public:
  MinHeap() {
    n = 0;
    TreeNode *sentinel;
    sentinel->setFrequency(-100);
    holder.push_back(sentinel);
  }

  void insert(TreeNode *val) {
    n++;
    holder.push_back(val);
    restoreUp(n);
  }
  int getSize(); // returns size of heap
  ~MinHeap() {}

  void restoreUp(int b) {

    int k = holder.at(b)->getFrequency();
    TreeNode *newVal = holder.at(b);
    int iParent = floor(b / 2);

    while (holder.at(iParent)->getFrequency() > k && iParent > 0) {
      // std::swap(holder.at(b), holder.at(iParent));
      // cout<<iParent<<endl;
      holder.at(b) = holder.at(iParent);
      b = iParent;
      iParent = floor(b / 2);
    }
    holder.at(b) = newVal;
  }

  TreeNode *removeMin() {
    if (n == 0) {
      cout << "empty heap" << endl;
      return 0;
    } else if (n == 1) {
      // cout<<n<<endl;
      return holder.at(1);
    } else if (n == 2) {
      if (holder.at(1)->getFrequency() > holder.at(2)->getFrequency()) {
        return holder.at(2);
      }
      return holder.at(1);
    } else {
      TreeNode *minValue = holder.at(1);
      holder.at(0) = holder.at(n - 1);
      n--;
      restoreDown(1);
      return minValue;
    }
  }

  void restoreDown(int i) {
    TreeNode *node = holder.at(1);
    // int i = 1;
    int k = node->getFrequency();
    int leftIndex = 2 * i, rightIndex = (2 * i) + 1;

    while (rightIndex >= n) {
      if (k <= holder.at(leftIndex)->getFrequency() &&
          k <= holder.at(rightIndex)->getFrequency()) {
        holder.at(1) = node;
        return;
      }
    }
  }
};

int main(int argc, char **argv) {
  TreeNode *ttn = new TreeNode('t', frequency_table[uint8_t('t')]);

  /*MinHeap mh;
   mh.insert(ttn); This one works fine*/

  // but the next line crashes my code
  MinHeap *mh = new MinHeap();
  mh->insert(ttn);
}

这是一个特别的问题，因为测试用例使用指针而不是对象。对此有一个简单的纠正吗？

Answer 1

崩溃与将类用作普通对象或通过指针无关。在任何一种情况下，程序都有未定义的行为。

使用调试器时，首先要看到的是，构造函数内的setFrequency调用是一个名为sentinel的未初始化指针

MinHeap() {
    n = 0;
    TreeNode *sentinel; // not initialized
    sentinel->setFrequency(-100); // used
    holder.push_back(sentinel);
}

用指针调用我的方法的问题

1 个答案: