Is what I am doing wrong?

I have written the following code to reverse the individual words in a string. This code works like a charm and generates the required output; the only caveat being it appends a blank space at the end of the original string. E.g.: if the original string is abcd efgh then the output would be dcba hgfe.

class Solution {
public:
    string rev(string& str) {
        int left=0, right=str.size()-1;
        while(left<right) {
            swap(str[left], str[right]);
            left++;
            right--;
        }

        return str;
    }

    string reverseWords(string s) {
        string result = "";
        string str;
        stringstream iss(s);

        while(iss>>str) {
            result+=rev(str);
            result+=" ";
        }

        //Removing last space added at the end of the string
        result[result.size()-1]=result[result.size()];
        return result;
    }
};

I am aware why this extra spaces crops up. This answer also represented how to overcome it. But I am doing it in the following way:

result[result.size()-1]=result[result.size()];

By this, I intend to overwrite the terminating space with the NULL character \0. But is it correct? I am doubtful because I think I am accessing an element that is out of bounds. Could someone please confirm?

Thank you!