Access private members using object

I saw the following example somewhere and in that, the objects of the same class were able to access private members. I couldn't understand the logic behind it.

#include <iostream>

using namespace std;

class CTime
{
    int hours;
    int minutes;
public:
    void getTime(int h, int m)
    {
        hours = h;
        minutes = m;
    }
    void putTime(void)
    {
        cout << hours << "hours and ";
        cout << minutes << " minutes" << "\n";
    }
    void sum(CTime t1, CTime t2);
};

// ---------- vvvv ---------------
//  --------- Here ---------------
void CTime::sum(CTime t1, CTime t2)
{
    minutes  = t1.minutes + t2.minutes;
    hours = minutes/60;
    minutes = minutes%60;
    hours = hours + t1.hours + t2.hours;
}

int main()
{
    CTime T1, T2, T3;

    T1.getTime(2, 45);
    T2.getTime(3, 30);

    T3.sum(T1, T2);

    cout << "T1 = ";
    T1.putTime();

    cout << "T2 = ";
    T2.putTime();

    cout << "T3 = ";
    T3.putTime();

    return 0;
}

In sum(CTime, CTime) function, the objects of CTime are able to access the private members. In which scenario is it possible. Kindly clarify. Thank you.