Deleting dynamic char** in C++

Question

Disclosure: I'm trying to solve a challenge with strict time and memory limits. I would normally use vectors and strings, but here I need the fastest and smallest solution (with vectors it actually ran above the time limit), so I turned to dynamic arrays of char*. The relevant parts of my code:

char** substrings(string s, int* n){
    *n = 0;
    ...
    ////////////////////////////////
    char** strings = new char*[*n];
    ////////////////////////////////
    for (int i = 0; i < s.length(); i++){
        for (int j = 1; j < s.length() - i + 1; j++){
            ...
            strings[si] = tmp;
            ...
        }
    }
    return strings;
}

int main(){
    ...
    for (int ti = 0; ti < t; ti++){
        cin >> s;
        char** substr = substrings(s, &n);
        ...

        for (int i = 0; i < n; i++){
            delete substr[i];
        }
    }
    return 0;
}

Everything runs just fine without deleting the array (of arrays), but that is unacceptable, so how do I go about this? I've tried a lot of variations that seemed logical but I get runtime errors.

Answer 1

It is similar to allocating, but in reverse order, and using delete[] instead of new[] :

for(int i = 0; i <  LENGTH; i++) 
    delete[] strings[i]; // delete each pointer in char** strings
delete[] strings; // finally delete the array of pointers

I assumed here that LENGTH is the length of the array of pointers to char* . So it looks that you only perform the first round of de-allocation

for (int i = 0; i < n; i++){
        delete substr[i]; // need delete[] substr[i] here

but with delete instead of delete[] , you need delete[] substr[i] instead, as my guess is that substr[i] is a char* pointer pointing the first element of an array of char s allocated by new[] . You finally need the additional

delete[] substr;