function<int(int,int)> sum = [&](int a, int b)
        {
            ++a;
            return a+b;
        };
        int a = 2, b = 3;
        int s = sum(a, b);
        cout << a << endl;// why it's 2 i have passed 'a' with refrence. how to pass it with refrence.

        function<int(int)> get2;
        
        cout << get2(3);

        get2 = [&](int a)
        {
            return a;
        };