使用共享指針時是否需要設置析構函數方法？

Question

我試圖找到一個答案，但沒有找到針對我特定問題的答案。 我正在為三元搜索樹使用共享指針（用於預測文本算法），並且使用共享指針遇到了一些問題。

我離開C ++已經5年了，讓我告訴你，Java不能幫助您學習指針。 在過去的幾天里，我不得不重新學習了5-6年前在學校學習的指針材料，並且成功地破壞了我的代碼。

這是我擁有的大多數代碼：

// 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;
}

對於節點：

/*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;
}

當我在調試模式（Visual Studios）中運行時，它能夠設置根節點，但是當它輸入第二個節點時，當currentNode在else語句中調用.reset（＆node）時，嘗試刪除“東西”會崩潰。函數InsertWord 。 我在復制構造函數或operator =方法或析構函數中做錯什么了嗎？ 它上方的cout行確實會打印正確的字母，因此看起來該節點已正確創建。

調試調用堆棧顯示：

TernarySearchTree.exe！std :: _ Ref_count_base :: _ Decref（）第118行C ++ TernarySearchTree.exe！std :: _ Ptr_base :: _ Decref（）第347 C ++ TernarySearchTree.exe！std :: shared_ptr ::〜shared_ptr（）第624行C ++ TernarySearchTree .exe！std :: shared_ptr :: reset（）第649行C ++ TernarySearchTree.exe！TernarySearchTreeNode ::〜TernarySearchTreeNode（）第50行C ++ TernarySearchTree.exe！TernarySearchTreeNode：`標量刪除析構函數（unsigned int）C ++ TernarySearchTree.exe！ std :: _ Ref_count :: _ Destroy（）第161行C ++ TernarySearchTree.exe！std :: _ Ref_count_base :: _ Decref（）第120行C ++ TernarySearchTree.exe！std :: _ Ptr_base :: _ Decref（）第347行C ++ TernarySearchTree.exe！std： ：shared_ptr :: ~~ shared_ptr（）第624行C ++ TernarySearchTree.exe！std :: shared_ptr :: reset（）第649行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（std :: basic_string，std :: allocator fileName）第182行C ++ TernarySearchTree.exe！ wmain（int argc，wchar_t * * argv）第200行C ++ TernarySearchTree.exe！__ tmainCRTStartup（）第533行C TernarySearchTree.exe！wmainCRTStartup（）第377 C C kernel32.dll！7592338a（）未知[下面的幀可能不正確和/或丟失，沒有為kernel32.dll加載任何符號]
ntdll.dll！77599f72（）未知ntdll.dll！77599f45（）未知

感謝您的任何幫助，您可以提供！ 並且讓我知道您是否還有其他需要我提供的內容（我正在閱讀的文本文件中包含corn這個詞）。

Answer 1

您的問題是您在C ++中使用Java樣式。 與Java中的一切本質上都是指針不同，在Java中，您必須考慮值，引用，指針和對象生存期之間的差異。

此功能不好：

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TernarySearchTreeNode 值獲取TernarySearchTreeNode對象，然后將其地址放入shared_ptr 。 shared_ptr獲取動態分配的對象（使用new創建的對象）的所有權並在引用計數為零時將其刪除的點。 上面的對象（left_node等）是堆棧對象，將在函數末尾超出范圍。 當將它們的地址放入shared_ptr ，它將稍后嘗試刪除那些對象，但它們不再存在。

至於如何解決此問題的建議，這里有很多假設尚在進行中。 例如，一個子節點可以有多個父節點嗎？ 復制節點實際上有意義嗎？

我現在假設復制節點是有意義的，因此使用shared_ptr是合理的。 在這種情況下，我們可以從這里開始：

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，所有函數（例如set_left_node也應將std::shared_ptr<TernarySearchNode>作為參數。 您不應該調用reset() ，它存在是為了允許shared_ptr獲取自由指針的所有權（refcount == 1）。 shared_ptr通過增加副本上的引用計數並在析構函數中取消引用來工作。 當取消引用指針然后獲取地址時，您正在使用shared_ptr。