Static Members in C++ classes

Tags: cpp oop

If we want to keep a data member or member function commonly accessible to all objects of the class (without being compiled separately for each object), we can make them static

Consider below example for computing the Nth Padovan number in this GFG question. We pre-compute the ans vector only once and use that commonly for all objects of Solution class

Static member variables must be defined outside of the class definition to allocate memory for them. Notice the vector<int> Solution::ans; line

class Solution {
    static vector<int> ans;

  public:
    static void init () {
        // Pre-compute ans[] ONLY ONCE and use it everywhere
        if (ans.empty()) {
            ans.resize(1e6 + 1, 0);
            ans[2] = ans[1] = ans[0] = 1;
            for (int i = 3; i <= 1e6; i++) {
                ans[i] = ans[i - 2] + ans[i - 3];
            }
        }
    }

    Solution () {
        init();
    }

    int padovanSequence (int n) {
        return ans[n];
    }
};

// Allocating memory for the static member
vector<int> Solution::ans;

int main () {
    int t;
    cin >> t;
    while (t--) {
        int n;
        cin >> n;
        Solution ob;
        cout << "p(" << n << ") = " << ob.padovanSequence(n) << endl;
    }
    return 0;
}