I was trying to solve LeetCode Problem 30 in C++ and was getting an error which should not have been possible in the given code below.
The problem arises when it is while (r <= s.size()-w) and works fine if it is while (r+w <= s.size()). Its technically the same thing then how is 1st part invalid and 2nd part valid??

class Solution {
    vector<int> indices;
    unordered_map<string, int> fm;
public:
    vector<int> findSubstring(string s, vector<string>& words) {
        for (const string &word: words)
            fm[word]++;
        const int w= words[0].size();
        const int len= w*words.size();
        for (int l, r, i= 0; i < w; i++) {
            unordered_map<string, int> curr;
            l= i, r= i;
            while (r+w <= s.size()) {            //Error if it is: while (r <= s.size()-w) {
                string word= s.substr(r, w);
                cout << r << '.' << word << ' ';
                r+= w;
                if (fm.find(word) == fm.end()) {
                    curr.clear();
                    l= r;
                    continue;
                }
                curr[word]++;
                while (curr[word] > fm[word]) {
                    curr[s.substr(l, w)]--;
                    l+= w;
                }
                if (r-l == len)
                    indices.push_back(l);
            }
        }
        return indices;
    }
};

The specific error i was getting while running the testcase: s= "mississippi", words= ["mississippis"] and the error was: terminate called after throwing an instance of 'std::out_of_range' what(): basic_string::substr: __pos (which is 12) > this->size() (which is 11) which is not possible as the loop will never execute!
Can anyone please explain??

I was trying to simply solve the problem but was confused on how while loop could execute even when the condition is FALSE!