CODEWITHSUNDEEP
c++
sasieightynine
sasieightynine

Reputation: 444

C++ freeing memory allocation of array with sub arrays

Theoretically, is it enough if I only delete arrays and results but don't delete sub_array inside of arrays in the following code snippet, before return 0; or do I have to first delete all the sub_array before I can safely delete arrays and results.

int main() {

    int subarrays, queries;

    cin >> subarrays >> queries;

    int** arrays = new int* [subarrays]();

    int* results = new int[queries];

    for (int i = 0; i < subarrays; i++) {

        int length;
        cin >> length;

        int* sub_array = new int[length];

        for (int j = 0; j < length; j++) {

            int element;
            cin >> element;

            sub_array[j] = element;
        }

        arrays[i] = sub_array;
    }

    for (int i = 0; i < queries; i++) {

        int query_from, arr_index;
        cin >> query_from >> arr_index;

        results[i] = arrays[query_from][arr_index];
    }

    for (int i = 0; i < queries; i++) {

        cout << results[i] << endl;
    }

    return 0;
}

Upvotes: 2

Views: 724

Answers (2)

andresantacruz
andresantacruz

Reputation: 1724

You shouldn't delete sub_array because the buffer it points to is stored in arrays[n]. As the only way you could delete sub_array is inside the block scope it was declared (which is the for loop), if you do delete it then arrays[n] will be a dangling pointer (a pointer that points to a freed or invalid memory block) and dangling pointers are evil.

What you should do about arrays and sub_array is when they are not needed anymore, you first iterate through all elements of arrays, deleting each one of them and after this, delete arrays itself. Something like:

// when you are done with them
for(auto i = 0; i < subarrays; i++)
{
    delete[] arrays[i];
}

delete[] arrays;

As a side-note you should consider using smart pointers in this code.

Upvotes: 5

Edward Aung
Edward Aung

Reputation: 3512

Yes, you have to first delete all the sub_array before I can safely delete arrays and results to prevent memory leak.

You can employ a loop to go through the sub-arrays in the array.

Upvotes: 1

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