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How to compare two objects (the calling object and the parameter) in a class?

Asked 2010-02-03T19:19:44.370
10

I am writing a "Date" class for an assignment and I am having trouble doing one the of the functions.

This is the header file for the class.

class Date
{
public:
Date();                                  // Constructor without parameters
Date(int m, int d, int y); // Constructor with parameters.

// accessors
int GetMonth();               // returns the size of the diamond
int GetDay();
int GetYear();

// mutators
bool Set(int m, int d, int y);
bool SetFormat(char f);

// standard input and output routines
void Input();             
void Show();              
void Increment(int numDays = 1);                 
int Compare(const Date& d);     

private:
int month,                    // month variables
    day,                 // day variable
    year;               // year variable
char format;
};

The member function I am trying to make is the int Compare(const Date& d) function. I need this function to compare two Date objects (the calling object and the parameter), and should return: -1 if the calling object comes first chronologically, 0 if the objects are the same date, and 1 if the parameter object comes first chronologically.

I have tried doing a simple if statement with the == operator but I get errors. 

  if (d1 == d2)
     cout << "The dates are the same";
     return (0);

After the objects are created, the function should be called like this d1.Compare(d2)

Thank you in advance!

c++ oop class compare
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1 Answer

8

Here's how I might implement your Compare function, although the format takes a moment to get used to:

int Date::Compare(const Date& d) const {
  return
    (year < d.year)   ? -1 :
    (year > d.year)   ?  1 :
    (month < d.month) ? -1 :
    (month > d.month) ?  1 :
    (day < d.day)     ? -1 :
    (day > d.day)     ?  1 :
                         0;
}

Or perhaps:

template<typename T>
int Compare(T a, T b) {
    if (a < b) return -1;
    if (b < a) return 1;
    return 0;
}

int Date::Compare(const Date& d) const {
    int a = Compare(year, d.year);
    if (a == 0) a = Compare(month, d.month);
    if (a == 0) a = Compare(day, d.day);
    return a;
}

I wouldn't use operator== in Compare, although the answers telling you how to implement operator== are fine if you want that as well. The reason is that operator== is clearly going to have to look at the same fields compare does, and if it returns false then Compare will do very similar work again. Efficiency probably isn't an issue, but it duplicates the logic.

And for what it's worth, idiomatic C++ is to implement operator< and possibly also a consistent operator== and operator>, rather than an all-in-one Compare function. The operators are what the standard algorithms use for searching and sorting, and everything else follows. Java chose to do things differently.

answered 2010-02-03T20:02:56.580

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