Friend Functions

[ADTR2012]

Can someone explain to me what a friend function is used for? I am new to classes and I am confused of what a friend function is. So for example, what would this be for:

class Date {
public:
   friend bool equal(const Date&, const Date&);
private:
   int month;
   int day;
   int year;
};

Also I don't understand what equal is for. My professor showed us this:

if(equal(date1, date2)) { ...

If someone could explain this that would be awesome!

[MiiNiPaa]

basically non-member functions don't have access to privat members:

Date today;
today.month = 1;//EROR today.month is a private field. 

Friend functions have access to all fields in class.

bool equal(const Date& left, const Date& right);
{
    return ( (left.year == right.year) && (left.month == right.month) && (left.day == right.day) );
    //previous statement wont cause error on left.year and similar
    //because it is an friend function
}

In some cases you have to use friend functions (i.e. overloading operator<< for outputting your data).

equal() function returns true if left argument equals to right. So if(equal(date1, date2)) means: "if dates are equal..."

It is better to overload operator== but looks like you didn't get to operator overloading yet.

[ADTR2012]

Ok that makes more sense now. So would friend functions (in most cases) be a bad thing to use? Couldn't you usually just use an accessor instead of a friend function? I know there would be exceptions but I just mean in general.

[jlb]

So would friend functions (in most cases) be a bad thing to use?

You should try to limit the friend functions because they tend to break encapsulation.

Couldn't you usually just use an accessor instead of a friend function?

Usually yes. But sometimes you may not want to provide this access so some variable to any function except this special friend.

[ADTR2012]

Thanks for clarifying that! One more quick question, is a static const basically the same as const but is used in classes and is available to all methods of that class and also all objects? Basically like a global variable? Or am I misunderstanding?

[ResidentBiscuit]

Yea you pretty much have it.