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当我使用共享指针时，是否需要设置析构函数方法？

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我试着找到答案，但没有看到一个针对我的特定问题 . 我正在使用三元搜索树的共享指针（用于预测文本算法），并使用共享指针遇到一些问题 .

我离开C已经5年了，让我告诉你，Java并没有帮助你学习指针 . 我不得不重新学习在过去几天里我在学校学到的指针材料，并成功设法破坏了我的代码 .

这是我的大部分代码：

// TernarySearchTree.cc

#include "stdafx.h"
#include "ternary_search_tree.h"

//Constructor
TernarySearchTree::TernarySearchTree() {
  num_nodes_ = 0;
  size_in_memory_ = 0;
  root_node_ = nullptr;
}

TernarySearchTree::TernarySearchTree(const TernarySearchTree& other) {
  num_nodes_ = other.num_nodes_;
  size_in_memory_ = other.size_in_memory_;

  TernarySearchTreeNode node;
  node = *other.root_node_;
  root_node_.reset(&node);
}

//Destructor
TernarySearchTree::~TernarySearchTree() {

}

//operators
TernarySearchTree& TernarySearchTree::operator=(const TernarySearchTree& other) {

  //TODO: swap idiom - create a copy of the node then swap the new one with it
  //do this first to provide exception safety
  TernarySearchTreeNode node;
  node = *other.root_node_;

  root_node_.reset(&node);
  num_nodes_ = other.num_nodes_;
  size_in_memory_ = other.size_in_memory_;

  return *this;
}

//Convert from string to c-style string
std::vector<char> TernarySearchTree::ConvertStringToCString(std::string str) {

  std::vector<char> wordCharacters (str.begin(), str.end());
  //remove newlines or tabs
  if (wordCharacters.back() == '\n' || wordCharacters.back() == '\t') {
    wordCharacters.pop_back();
  }
  wordCharacters.push_back('\0');
  return wordCharacters;
}

//Insert a node
TernarySearchTreeNode TernarySearchTree::InsertNode(TernarySearchTreeNode &currentNode, 
                                                    char character,
                                                    NodePosition position,
                                                    bool isRoot) {

  TernarySearchTreeNode newNode;
  newNode.set_character(character);

  if (!isRoot) {
    switch (position) {
    case NODE_POS_LEFT:
      currentNode.set_left_node(newNode);
      break;
    case NODE_POS_CENTRE:
      currentNode.set_centre_node(newNode);
      break;
    case NODE_POS_RIGHT:
      currentNode.set_right_node(newNode);
      break;
    default:
      break;
    }
  }

  return newNode;
}

//Insert a word
void TernarySearchTree::InsertWord(std::string word) {

  std::vector<char> characters = ConvertStringToCString(word);
  std::shared_ptr<TernarySearchTreeNode> currentNode = 0;
  bool isFirstCharacter = true;

  //Add each character to a node while traversing
  //Base case where there is no root node
  if (!root_node_) {

    for(std::vector<char>::iterator it = characters.begin(); it != characters.end(); ++it) {

      if (*it != '\0') {        
        //if it is the first character
        //root_node_ is equal to the address of new node
        if (isFirstCharacter) {
          std::cout << "HIHI";
          TernarySearchTreeNode node = InsertNode(*currentNode, *it, NODE_POS_CENTRE, true);
          root_node_.reset(&node);
          currentNode.reset(&node);
          isFirstCharacter = false;

        } else {
          TernarySearchTreeNode node = InsertNode(*currentNode, *it, NODE_POS_CENTRE, false);
          std::cout << std::endl << node.get_character();
          currentNode.reset(&node);
        }
      }
    }
    //If not base case, then we need to compare each character
  } else {
    currentNode = root_node_;
    for(std::vector<char>::iterator it = characters.begin(); it != characters.end(); ++it) {
      if (*it != '\0') {
        currentNode.reset(&SetNextNode(*currentNode, *it, *std::next(it, 1)));
      } else {
        currentNode->set_end_of_word(true);
      }
    }
  }
}

//Recursive function for obtaining/adding the next node when inserting a word
TernarySearchTreeNode TernarySearchTree::SetNextNode(TernarySearchTreeNode &currentNode, const char currentChar, const char nextChar) {

  //If characters match
  if (currentChar == currentNode.get_character()) {

    //if centre node exists
    if (currentNode.get_centre_node()) {
      return *(currentNode.get_centre_node());

      //Otherwise, create a new node and recall method on that node
    } else {

      //If not the end of the word, make a new node with the next letter
      if (nextChar != '\0') {
        return InsertNode(currentNode, nextChar, NODE_POS_CENTRE, false);

      } else {
        return currentNode;
      }
    }
    //If it is less, follow node on the left
  } else if (currentChar < currentNode.get_character()) {

    //if left node exists, recursive call
    if (currentNode.get_left_node()) {
      return SetNextNode(*(currentNode.get_left_node()), currentChar, nextChar);

      //Otherwise, create a new node and recall method on that node
    } else {
      return SetNextNode(InsertNode(currentNode, currentChar, NODE_POS_LEFT, false), currentChar, nextChar);
    }
    //Otherwise it is bigger, so take right path
  } else {

    //if right node exists, recursive call
    if (currentNode.get_right_node()) {
      return SetNextNode(*(currentNode.get_right_node()), currentChar, nextChar);

      //Otherwise, create a new node and recall method on that node
    } else {
      return SetNextNode(InsertNode(currentNode, currentChar, NODE_POS_RIGHT, false), currentChar, nextChar);
    }
  }
}

//Populate the TST from a word list/file
void TernarySearchTree::PopulateTreeFromTextFile(std::string fileName) {

  std::ifstream file;
  std::string line;
  file.open(fileName);

  if (file.is_open()) {
    //Assume text file has one word per line
    while (std::getline(file, line)) {
      InsertWord(line);
    }     
  }
}

//Search
bool TernarySearchTree::SearchForWord(std::string word) {
  return false;
}

int _tmain(int argc, _TCHAR* argv[])
{

  //Test
  TernarySearchTree tst;
  //Open file
  tst.PopulateTreeFromTextFile("simple.txt");

  //start at root and follow some paths
 std::cout << tst.get_root_node();


  /**std::vector<char> vec;
  vec.push_back('a');
  vec.push_back('c');
  std::vector<char>::iterator it = vec.begin();
  std::cout << *std::next(vec.begin(), 1);
  std::cout << (*it < 'c');
  it++;
  std::cout << *std::next(it, 0);
  std::cout <<  (*it < 'c');
  **/
  return 0;
}

并为节点：

/*TST node methods */
#include <iostream>
#include "ternary_search_tree_node.h"

/** ADD COPY CONSTRUCTOR*/
//Constructors
TernarySearchTreeNode::TernarySearchTreeNode() {

  character_ = '\0';
  end_of_word_ = false;
  left_node_ = nullptr;
  centre_node_ = nullptr;
  right_node_ = nullptr;
}

TernarySearchTreeNode::TernarySearchTreeNode(const TernarySearchTreeNode& other) {

  character_ = other.character_;
  end_of_word_ = other.end_of_word_;

  TernarySearchTreeNode leftNode;
  leftNode = *other.left_node_;
  left_node_.reset(&leftNode);

  TernarySearchTreeNode centreNode;
  centreNode = *other.centre_node_;
  centre_node_.reset(&centreNode);

  TernarySearchTreeNode rightNode;
  rightNode = *other.right_node_;
  right_node_.reset(&rightNode);
}

TernarySearchTreeNode::TernarySearchTreeNode(char character, bool end_of_word,
                                             TernarySearchTreeNode left_node, 
                                             TernarySearchTreeNode centre_node, 
                                             TernarySearchTreeNode right_node) {

  character_ = character;
  end_of_word_ = end_of_word;
  left_node_.reset(&left_node);
  centre_node_.reset(&centre_node);
  right_node_.reset(&right_node);
}

//Destructor
TernarySearchTreeNode::~TernarySearchTreeNode() {

  left_node_.reset();
  centre_node_.reset();
  right_node_.reset();

}

//operators
TernarySearchTreeNode& TernarySearchTreeNode::operator=(const TernarySearchTreeNode& other) {

  if (&other) {
    TernarySearchTreeNode leftNode;
    leftNode = *other.left_node_;
    TernarySearchTreeNode centreNode;
    centreNode = *other.centre_node_;
    TernarySearchTreeNode rightNode;
    rightNode = *other.right_node_;

    left_node_.reset(&leftNode);
    centre_node_.reset(&centreNode);
    right_node_.reset(&rightNode);

    character_ = other.character_;
    end_of_word_ = other.end_of_word_;
  }

  return *this;
}

//printing
std::ostream& operator<<(std::ostream& os, const TernarySearchTreeNode& obj)
{
  // write obj to stream
  char c = obj.get_character();
  bool b = obj.is_end_of_word();

  os << c << "\t is end of word: " << b;
  return os;
}

当我在调试模式（Visual Studios）中运行时，它能够设置根节点，但是当它输入第二个节点时，当currentNode在函数的else语句中调用.reset（＆node）时，它崩溃尝试删除"stuff" InsertWord . 我在复制构造函数或operator =方法或析构函数中做错了吗？它上方的cout行确实打印了正确的字母，因此看起来节点正在正确创建 .

调试调用堆栈显示：

TernarySearchTree.exe！std :: _ Ref_count_base :: _ Decref（）Line 118 C TernarySearchTree.exe！std :: _ Ptr_base :: _ Decref（）Line 347 C TernarySearchTree.exe！std :: shared_ptr :: ~shared_ptr（）Line 624 C TernarySearchTree.exe！std :: shared_ptr :: reset（）行649 C TernarySearchTree.exe！TernarySearchTreeNode ::〜TernarySearchTreeNode（）第50行C TernarySearchTree.exe！TernarySearchTreeNode ::`标量删除析构函数'（unsigned int）C TernarySearchTree.exe ！std :: _ Ref_count :: _ Destroy（）行161 C TernarySearchTree.exe！std :: _ Ref_count_base :: _ Decref（）行120 C TernarySearchTree.exe！std :: _ Ptr_base :: _ Decref（）行347 C TernarySearchTree.exe！std :: shared_ptr :: ~shared_ptr（）第624行第3行TernarySearchTree.exe！std :: shared_ptr :: reset（）行649 C TernarySearchTree.exe！TernarySearchTreeNode :: ~TernarySearchTreeNode（）第50行C TernarySearchTree.exe！TernarySearchTree :: InsertWord （std :: basic_string，std :: allocator word）第105行C TernarySearchTree.exe！TernarySearchTree :: PopulateTreeFromTextFile（s td :: basic_string，std :: allocator fileName）行182 C TernarySearchTree.exe！wmain（int argc，wchar_t * * argv）行200 C TernarySearchTree.exe！__ tmainCRTStartup（）行533 C TernarySearchTree.exe！wmainCRTStartup（）行377 C kernel32.dll！7592338a（）未知[下面的框架可能不正确和/或缺少，没有为kernel32.dll加载符号] ntdll.dll！77599f72（）未知ntdll.dll！77599f45（）未知

感谢您的任何帮助，您可以提供！如果有任何其他你需要我提供的话，请告诉我（我正在阅读的文本文件中只有单词 corn ） .

c++ pointers visual-c++

1 回答

  • 5

    您的问题是您在C中使用Java样式 . 与Java中的所有内容本质上都是指针不同，在C中，您必须考虑值，引用，指针和对象生存期之间的差异 .

    这个功能不好：

    TernarySearchTreeNode::TernarySearchTreeNode(char character, bool end_of_word,
                                             TernarySearchTreeNode left_node, 
                                             TernarySearchTreeNode centre_node, 
                                             TernarySearchTreeNode right_node) {

  character_ = character;
  end_of_word_ = end_of_word;
  left_node_.reset(&left_node);
  centre_node_.reset(&centre_node);
  right_node_.reset(&right_node);
}

    您按值获取 TernarySearchTreeNode 个对象，然后将其地址放入 shared_ptr . shared_ptr to获取动态分配对象（使用 new 创建的对象）的所有权，并在引用计数变为零时删除它 . 上面的对象（left_node等）是堆栈对象，它们将在函数末尾超出范围 . 当您将其地址放入 shared_ptr 时，它将稍后尝试删除这些对象，但它们不再存在 .

    至于建议如何解决这个问题，这里有很多假设刚刚结束 . 例如，子节点可以有多个父节点吗？复制节点真的有意义吗？

    我现在假设复制节点是有意义的，所以使用 shared_ptr 是合理的 . 那样的话我们可以从这里开始：

    TernarySearchTreeNode TernarySearchTree::InsertNode(std::shared_ptr<TernarySearchTreeNode currentNode>, 
                                                    char character,
                                                    NodePosition position,
                                                    bool isRoot) {

  auto newNode = std::make_shared<TernarySearchTreeNode>();
  newNode->set_character(character);

  if (!isRoot) {
    switch (position) {
    case NODE_POS_LEFT:
      currentNode->set_left_node(newNode);

    然后像 set_left_node 这样的所有函数也应该将 std::shared_ptr<TernarySearchNode> 作为参数 . 您不应该调用 reset() ，它允许 shared_ptr 获取空闲指针的所有权（refcount == 1） . shared_ptr 通过在析构函数中增加复制和解除引用的引用计数来工作 . 当您取消引用指针然后获取地址时，您正在使用shared_ptr .

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