C++ 更改向量中的对象不起作用

Question

我想使用一个向量来实现一个 trie 来存储节点，但不知何故我的插入方法不起作用。我已经设法使用不同的实现来构建 trie 数据结构，但我想了解为什么我当前的实现不起作用。

作品（不是基于索引的子/引用存储）：

struct Trie {
   struct Trie *references[26];
   bool end; //It is true if node represents end of word.
};

不起作用（基于索引的子/引用存储）：

struct node {
    int references[26] = {0};
    bool end; 
};

由于插入功能错误，它不起作用。

void insert_word(string s){
    node *current_node = &trie[0];
    // current_node->references[4] = 9999 WORKS! Node in Trie is UPDATED
    for(int i=0;i<s.size();i++){
        print_trie();
        int letter_num = static_cast<int>(tolower(s[i])) - static_cast<int>('a');
        int next_index = current_node->references[letter_num];
        cout << "letter num: " << letter_num << " next index: " << next_index << endl;
        if(next_index == 0){
            node new_node;
            trie.push_back(new_node);
            current_node->references[letter_num] = trie.size()-1; // DOESN'T WORK! Node in Trie is NOT UPDATED
            cout << "new value: ";
            for(auto c:current_node->references)
                cout << c << " ";
            cout << endl;
            cout << "in for" << endl;
            print_trie();
            current_node = &trie.back();
        } else{
            current_node = &trie[next_index];
        }
    }
    current_node->end = true;
}

问题在于，当我访问current_node 作为对对象 ob 的引用时，我会更改它的值。 trie 向量中的对象/节点并不总是更新。它在第二行工作，但更进一步它以某种方式停止工作。我想知道为什么。

这是我为简化问题而编写的一个简短的调试程序。这里似乎一切正常。

    n1.references[0] = 1;
    n2.references[0] = 2;
    n3.references[0] = 3;

    trie.push_back(n1);
    trie.push_back(n2);
    trie.push_back(n3);

    node *n = &trie[0];
    n->references[0] = 10; // Tree is updated properly
    n = &trie[1];
    n->references[0] = 11; // Tree is updated properly

你能帮我理解为什么插入功能不能正常工作吗？

编辑：最小的工作示例

#include <vector>
#include <string>
#include <iostream>

using namespace std;

struct node
{
    int num_words;
    int references [26] = {0};
    bool end; 
};

vector<node> trie;
int n;


void print_trie(){
    cout << "#### NEW PRINT TRIE ##### " << endl;
    for(int i=0;i<trie.size();i++){
        cout << "node " << i << ": ";
        for(int j=0;j<26;j++)
            cout << trie[i].references[j] << " ";
        cout << endl;
    }
}

void insert_word(string s){
    node *current_node = &trie[0];
    // current_node->references[4] = 9999 WORKS! Node in Trie is UPDATED
    for(int i=0;i<s.size();i++){
        print_trie();
        int letter_num = static_cast<int>(tolower(s[i])) - static_cast<int>('a');
        int next_index = current_node->references[letter_num];
        cout << "letter num: " << letter_num << " next index: " << next_index << endl;
        if(next_index == 0){
            node new_node;
            trie.push_back(new_node);
            current_node->references[letter_num] = trie.size()-1; // DOESN'T WORK! Node in Trie is NOT UPDATED
            cout << "new reference value of node: ";
            for(auto c:current_node->references)
                cout << c << " ";
            cout << endl;
            current_node = &(trie[trie.size()-1]);
        } else{
            current_node = &trie[next_index];
        }
    }
    current_node->end = true;
}



int main()
{
    node root;
    trie.push_back(root);
    insert_word("hallohallo");
    return 0;
}

Answer 1

每当std::vector<T> 进行大小调整操作时，所有迭代器和指向元素的指针都会失效。以你的 mcve 为例说明它在哪里出轨，请考虑标记线：

void insert_word(string s){
    node *current_node = &trie[0];  // **HERE
    for(int i=0;i<s.size();i++){
        print_trie();
        int letter_num = static_cast<int>(tolower(s[i])) - static_cast<int>('a');
        int next_index = current_node->references[letter_num];
        cout << "letter num: " << letter_num << " next index: " << next_index << endl;
        if(next_index == 0){
            node new_node;
            trie.push_back(new_node); //** RESIZE
            current_node->references[letter_num] = trie.size()-1;
            cout << "new reference value of node: ";
            for(auto c:current_node->references)
                cout << c << " ";
            cout << endl;
            current_node = &(trie[trie.size()-1]); // **HERE
        } else{
            current_node = &trie[next_index]; // **HERE
        }
    }
    current_node->end = true;
}

在每个标有// **HERE 的位置中，您都在存储指向向量中托管的对象的指针。但是一旦达到容量，标有// **RESIZE 的行可以（并且将）通过复制/移动/等调整整个向量的大小。这意味着current_node 不再指向一个有效对象，它是一个悬空指针，但您的代码并不明智，并且会进入未定义的行为。

有几种方法可以解决这个问题。如果您提前知道它，您可以reserve 从一开始就知道它的容量，但是对于更强大的解决方案，不要一开始就使用指针。如果您通过 index 而不是指针进行枚举，则您的解决方案如下：

void insert_word(std::string s)
{
    size_t idx = 0;

    for(int i=0;i<s.size();i++){
        print_trie();
        int letter_num = static_cast<int>(tolower(s[i])) - static_cast<int>('a');
        size_t next_index = trie[idx].references[letter_num];
        std::cout << "letter num: " << letter_num << " next index: " << next_index << std::endl;
        if(next_index == 0){
            trie.emplace_back();
            trie[idx].references[letter_num] = trie.size()-1;
            std::cout << "new reference value of node: ";
            for(auto c : trie[idx].references)
                std::cout << c << ' ';
            std::cout << std::endl;
            idx = trie.size()-1;
        } else{
            idx = next_index;
        }
    }
    trie[idx].end = true;
}

注意current_node 的所有实例是如何被trie[idx] 替换的。而更改“当前节点”现在只需更改 idx 的值即可，即使基础向量调整大小也是如此。

Answer 2

这可能是由类型不匹配导致的int 被分配size_t
试试...= (int)trie.size()-1

#include <vector>
#include <iostream>
using namespace std;

struct node{
    int num_words;
    int references [26] = {};   //........... int
    bool end; 
};

vector<node> trie;
int n;

void print_trie(){
    cout << "#### NEW PRINT TRIE ##### " << endl;
    for(int i=0;i<trie.size();i++){
        cout << "node " << i << ": ";
        for(int j=0;j<26;j++)
                cout << trie[i].references[j] << " ";
        cout << endl;
    }
}

void insert_word(const string& s){
    node *current_node = &trie[0];
    // current_node->references[4] = 9999 WORKS! Node in Trie is UPDATED
    for(int i=0;i<s.size();i++){
        print_trie();
        int letter_num = int(tolower(s[i]) - 'a');
        int next_index = current_node->references[letter_num];
        cout << "letter num: " << letter_num << " next index: " << next_index << endl;
        if(next_index == 0){
                node new_node;
                trie.push_back(new_node);
                current_node->references[letter_num] = (int)trie.size()-1; //....size_t  DOESN'T WORK! Node in Trie is NOT UPDATED
                cout << "new reference value of node: ";
                for(auto c:current_node->references)
                    cout << c << " ";
                cout << endl;
                current_node = &(trie[trie.size()-1]);
        } else{
                current_node = &trie[next_index];
        }
    }
    current_node->end = true;
}



int main()
{
    node root;
    trie.push_back(root);
    insert_word("hallohallo");
    return 0;
}