Using a constructor with three parameters in a C++ class

Question

I am making a class called Time in C++ and the class has three integers as private member variables. I am pretty new at using classes in C++ and am trying to figure out how to solve this particular problem. The problem is that when I try to do this:

cout << "Almost midnight: " << Time(0,0,0) - Time(0,0,1) << endl;

I get a compiler error, and I think it is because the constructor that takes three parameters needs to be coded differently, because the private variables in the class take the value from the first constructor and then tries to subtract from the value from the second constructor so the first values get lost (I believe). So how does the constructor need to be in order to keep the old values, so I can do the subtraction without storing it in a named variable.

This is my code:

#include <iostream>
#include <cctype>
#include <cstdlib>
using namespace std;

class Time
{
    private:
        int hours;
        int minutes;
        int seconds;
        void normalize();

    public:
        Time() {hours = minutes = seconds = 0; normalize();};
        Time(int x, int y, int z);
        friend Time operator + (const Time& t1, const Time& t2);
        friend Time operator - (const Time& t1, const Time& t2);
        friend bool operator < (const Time& t1, const Time& t2);
        friend istream& operator >>(istream& ins, Time& t1);
        friend ostream& operator <<(ostream& out, Time& t1);
};

Time::Time(int x, int y, int z)
{
    hours = x;
    minutes = y;
    seconds = z;

    normalize();
}

void Time::normalize()
{
    int s = seconds;
    int m = minutes;
    int h = hours;

    while (s < 0)
    {
        s += 60;
        m--;
    }

    while (m < 0)
    {
        m += 60;
        h--;
    }

    while (h < 0)
    {
        h = h + 24;
    }

    seconds = s % 60;
    minutes = (m + s/60) % 60;
    hours = (h + m/60 + s/3600) % 24;
}

istream& operator >>(istream& ins, Time& t1)
{
    ins >> t1.hours;
    ins >> t1.minutes;
    ins >> t1.seconds;

    t1.normalize();

    return ins;
}
ostream& operator <<(ostream& out, Time& t1)
{
    if (t1.hours < 10)
        out << '0' << t1.hours << ":";
    else
        out << t1.hours << ":";
    if (t1.minutes < 10)
        out << '0' << t1.minutes << ":";
    else
        out << t1.minutes << ":";
    if (t1.seconds < 10)
        out << '0' << t1.seconds;
    else
        out << t1.seconds;

    return out;
}

int main()
{
    Time t1, t2, t3, t4;
    cin >> t1;
    cin >> t2;
    cin >> t3;

    cout << "Time1: " << t1 << endl;
    cout << "Time2: " << t2 << endl;
    cout << "Time3: " << t3 << endl;

    t4 = t1 + t2;
    cout << "Time4: " << t4 << endl;

    t1 = t3 - t4;
    cout << "Time1: " << t1 << endl;

    if (t1 < t3)
        cout << "Time1 < Time3" << endl;
    else
        cout << "Time3 >= Time1" << endl;

    Time t5 = t2 + Time(0,0,1);
    if (t5 < t2)
        cout << "Time5 < Time2" << endl;
    else
        cout << "Time5 >= Time2" << endl;

    cout << "Almost midnight: " << Time(0,0,0) - Time(0,0,1) << endl;

    return 0;
}

Time operator + (const Time& t1, const Time& t2)
{
    Time temp;
    temp.hours = t1.hours + t2.hours;
    temp.minutes = t1.minutes + t2.minutes;
    temp.seconds = t1.seconds + t2.seconds;

    return temp;
}

Time operator - (const Time& t1, const Time& t2)
{
    Time temp;

    temp.hours = t1.hours - t2.hours;
    temp.minutes = t1.minutes - t2.minutes;
    temp.seconds = t1.seconds - t2.seconds;

    temp.normalize();

    return temp;
}

bool operator < (const Time& t1, const Time& t2)
{
    if (t1.hours < t2.hours && t1.minutes < t2.minutes && t1.seconds < t2.seconds)
        return true;
    else
        return false;
}

Answer 1

The constructor is fine. What you're missing are suitable overloads for operator- and operator<<. Define them.

Answer 2

ostream& operator <<(ostream& out, Time& t1)
{
    if(t1.hours < 10)
        out << "0" << t1.hours << ":";
    else
        out << t1.hours << ":";
    if(t1.minutes < 10)
        out << "0" << t1.minutes << ":";
    else
        out << t1.minutes << ":";
    if(t1.seconds < 10)
        out << "0" << t1.seconds;
    else
        out << t1.seconds;
}

Time operator - (const Time& t1, const Time& t2)
{
    Time temp;

    temp.hours = t1.hours - t2.hours;
    temp.minutes = t1.minutes - t2.minutes;
    temp.seconds = t1.seconds - t2.seconds;

    temp.normalize();

    return temp;
}

These are the - operator function and ostream& operator function

Answer 3

You have to overload the operator minus operator- and the operator << operator<<

Do it like this for operator-:

Time& Time::operator-(Time const& time){
     hours = time.getHours();
     minutes = time.getMinutes();
     seconds = time.getSeconds();
    return *this;
}

Answer 4

In this case the compiler doesn't know how to show your class in the cout, and it doesn't know how you want to subtract the 2 classes, you need to tell to the compiler how it should work, overloading the operators.

You need to define the operators << for your class to be showed properly in the ostream. and you need to overload the - operator to make your subtraction.

How to overload << for you own class: How to properly overload the << operator for an ostream?

A good documentation about operator overloading: http://en.cppreference.com/w/cpp/language/operators

Answer 5

You did not overload your operator << correctly, as you forgot to return a value.

The following program compiles and runs without error. I removed the normalize function, but still has no issues:

#include <ostream>
#include <iostream>

struct Time
{
    int hours;
    int minutes;
    int seconds;
    Time(int h=0, int m=0, int s=0) : hours(h), minutes(m), seconds(s) {}
};

std::ostream& operator <<(std::ostream& out, const Time& t1)
{
    if (t1.hours < 10)
        out << "0" << t1.hours << ":";
    else
        out << t1.hours << ":";
    if (t1.minutes < 10)
        out << "0" << t1.minutes << ":";
    else
        out << t1.minutes << ":";
    if (t1.seconds < 10)
        out << "0" << t1.seconds;
    else
        out << t1.seconds;
    return out;
}


Time operator - (const Time& t1, const Time& t2)
{
    Time temp;
    temp.hours = t1.hours - t2.hours;
    temp.minutes = t1.minutes - t2.minutes;
    temp.seconds = t1.seconds - t2.seconds;
    return temp;
}

using namespace std;

int main()
{
    cout << "Almost midnight: " << Time(0, 0, 0) - Time(0, 0, 1) << endl;
}

Output:

Almost midnight: 00:00:0-1

Now, if the output is incorrect, that is a different issue than the one you posted. Your original issue dealt with a compiler error, and the code above does not have such an error. However, it does contain the fix of not returning a value.

In addition to this, note that the parameter to operator << is a const reference, not a reference. This will allow expressions that returns temporaries to be used within operator <<.

To show how const makes a difference: If we take your original code (where Time& is used instead of const Time& in the implementation of operator <<), but instead store the value of the subtraction of the Time objects to a non-temporary object, you will see that the code will compile successfully:

   Time tSub = Time(0,0,0) - Time(0,0,1); 
   cout << "Almost midnight: " << tSub << endl;

while this will not compile successfully:

   cout << "Almost midnight: " << Time(0,0,0) - Time(0,0,1) << endl;

So the issue is that operator << in your original code only works with non-const objects. By making the parameter const Time&, you alleviate these errors.