为什么我的代码不能反转c++中的字符串？

#include <iostream>
#include <string>

std::string reverseString(const std::string& str) {
    std::string reversedStr;
    for (int i = str.length() - 1; i >= 0; --i) {
        reversedStr += str[i];
    }
    return reversedStr;
}

int main() {
    std::string str = "Hello, World!";
    std::string reversedStr = reverseString(str);
    std::cout << reversedStr << std::endl;
    return 0;
}

#include <iostream>
#include <string>

void reverseString(std::string& str) {
    int start = 0;
    int end = str.length() - 1;
    while (start < end) {
        std::swap(str[start], str[end]);
        start++;
        end--;
    }
}

int main() {
    std::string str = "Hello, World!";
    reverseString(str);
    std::cout << str << std::endl;
    return 0;
}

#include <iostream>
#include <string>
#include <algorithm>

int main() {
    std::string str = "Hello, World!";
    std::reverse(str.begin(), str.end());
    std::cout << str << std::endl;
    return 0;
}