c++ 将指针推入指针优先级队列会导致立即 valgrind 错误

Posted 2023-02-16

【中文标题】c++ 将指针推入指针优先级队列会导致立即 valgrind 错误

【英文标题】：c++ pushing pointer onto pointer priority queue causes immediate valgrind errors

【发布时间】：2021-10-22 02:48:47

【问题描述】：

我正在用 C++ 编写 Huffman 代码实现，但是在构造过程中，将指针推送到类指针的优先级队列中会导致如下所示的几种 valgrind 错误：

==1158==    at 0x40508E: void std::__push_heap<__gnu_cxx::__normal_iterator<Node**, std::vector<Node*, std::allocator<Node*> > >, long, Node*, nodeCompare>(__gnu_cxx::__normal_iterator<Node**, std::vector<Node*, std::allocator<Node*> > >, long, long, Node*, nodeCompare) (stl_heap.h:182)
==1158==    by 0x4034B5: void std::push_heap<__gnu_cxx::__normal_iterator<Node**, std::vector<Node*, std::allocator<Node*> > >, nodeCompare>(__gnu_cxx::__normal_iterator<Node**, std::vector<Node*, std::allocator<Node*> > >, __gnu_cxx::__normal_iterator<Node**, std::vector<Node*, std::allocator<Node*> > >, nodeCompare) (stl_heap.h:221)
==1158==    by 0x4027AB: std::priority_queue<Node*, std::vector<Node*, std::allocator<Node*> >, nodeCompare>::push(Node* const&) (stl_queue.h:499)
==1158==    by 0x40177A: HuffmanCode::HuffmanCode(std::map<char, int, std::less<char>, std::allocator<std::pair<char const, int> > >) (huffmanCodes.cpp:117)
==1158==    by 0x401F78: main (huffmanCodes.cpp:188)
==1158==  Uninitialised value was created by a stack allocation
==1158==    at 0x404EF3: void std::__push_heap<__gnu_cxx::__normal_iterator<Node**, std::vector<Node*, std::allocator<Node*> > >, long, Node*, nodeCompare>(__gnu_cxx::__normal_iterator<Node**, std::vector<Node*, std::allocator<Node*> > >, long, long, Node*, nodeCompare) (stl_heap.h:178)

这是我认为与问题相关的代码：

struct nodeCompare
  bool operator()(Node const& n1, Node const& n2)
    return n1.getFreq() > n2.getFreq();
  
;

// huffman code constructor, takes in character frequency map
HuffmanCode::HuffmanCode(map<char, int> m)
  // creates priority_queue, creates leafs and adds them to the queue
  priority_queue<Node*, vector<Node*>, nodeCompare> PQ;
  for(auto item: m)
    Node* temp = new Node(item.first, item.second, true);
    PQ.push(temp); // causes error
  

  // builds rest of tree upward until there is only a single element left (the root)
  while(PQ.size() > 1)
    Node* temp = new Node(false);
    Node* left = PQ.top(); // causes error
    PQ.pop();
    Node* right = PQ.top();
    PQ.pop();

    temp->setLChild(left);
    temp->setRChild(right);
    temp->setFreq(left->getFreq() + right->getFreq());
    PQ.push(temp); // causes error
  

项目中有更多代码，但我认为这是导致问题的原因。任何帮助/见解将不胜感激。

【参考方案1】：

应该在 nodeCompare 中使用指针

struct nodeCompare
  bool operator()(Node* n1, Node* n2)
    return n1->getFreq() > n2->getFreq();
  
;

【讨论】：

这...实际上就是它。我不知道为什么在这么简单的修复过程中错误如此无用。