C++循环期间字符串大小会发生变化

我的程序假设读取一个字符串，然后将每个字符插入堆栈中。我注意到当我打印length时，这是单词的大小，它变成了一些高数字。例如：word = "hello"长度一开始将 = 5，但最终更改为 = 111 。此外，当我使用 2 个字母时，我总是遇到分段错误。是什么原因造成的？为什么单词的长度会发生变化？

#include <iostream>
#include <string>
#include "Stack.h"
using namespace std;
int main()
{
    Stack stack;
    string word;
    cout << "Enter word: ";
    getline(cin, word);
    cout << word << "|"  << endl;
    int length = word.size();
    for (int i = 0; i < length; i++) {
        cout << "i: " << i << "tlength: " << length << endl;
        stack.push(word[i]);
        cout << "TOP: "  << stack.top() << endl;
    }   

    while (!stack.isEmpty())
    {
        cout << stack.pop();
    }
    cout << endl;   
    return 0;
}

#include <iostream>
#include <string>
#define STACK_CAPACITY 1000
using namespace std;
class Stack
{
    private:
        int topIndex;
        char arr[];
    public:
        // Constructor
        Stack()
        {
            arr[STACK_CAPACITY];
            topIndex = -1;
        }
        // adds elements to "top" of array
        void push(char c)
        {
            // if stack is full, do not add
            if (isFull())
            {
                cout << "Push on full Stack" << endl;
                // terminate function
            }
            topIndex++;
            arr[topIndex] = c;
        }
        // Removes last inserted (push) element from the stack and returns it
        char pop()
        {
            // checks if Stack is empty
            if (isEmpty())
            {
                cout << "Pop on empty Stack" << endl;
                return '@';
            }
            // if not empty, remove and return last element inserted
            char temp = arr[topIndex];
            arr[topIndex--] = ' ';
            return temp;
        }
        // Returns but does not remove last inserted (push) element
        char top() { return arr[topIndex]; }
        // Utilities 
        bool isEmpty() { return topIndex == -1; } 
        bool isFull() { return topIndex == STACK_CAPACITY - 1; }
        int size() { return topIndex + 1; }
        // Destructor
        ~Stack() 
        {
        }
}