Are == and != mutually dependent?

Question

I'm learning about operator overloading in C++, and I see that `==` and `!=` are simply some special functions which can be customized for user-defined types. My concern is, though, why are there *two separate* definitions needed? I thought that if `a == b` is true, then `a != b` is automatically false, and vice versa, and there is no other possibility, because, by definition, `a != b` is `!(a == b)`. And I couldn't imagine any situation in which this wasn't true. But perhaps my imagination is limited or I am ignorant of something? I know that I can define one in terms of the other, but this is not what I'm asking about. I'm also not asking about the distinction between comparing objects by value or by identity. Or whether two objects could be equal and non-equal at the same time (this is definitely not an option! these things are mutually exclusive). What I'm asking about is this: Is there any situation possible in which asking questions about two objects being equal does make sense, but asking about them *not* being equal doesn't make sense? (either from the user's perspective, or the implementer's perspective) If there is no such possibility, then why on Earth does C++ have these two operators being defined as two distinct functions?

Answer 1

Depending on your class design you may want to either overload operator==() or operator!=() or both. It truly depends what the class is supposed to do.

First of all, let's make sure we are clear on what object are. Objects are instances of a class. And a class is a blueprint for an object.

That said, if you have a class Person

class Person

{

private:

string name;

unsigned int age;

public:

Person();

Person(string name, int age);

// Other methods go here

};

int main()

{

Person a("John", 29), b("Sue", 31);

return 0;

}

How do you compare Person a or Person b?

If you would try to do the following:

if (a == b)

{

// Do something

}

// OR

if (a != b)

{

// Do something

}

How does the complier knows how to compare these two Person objects? Answer: it doesn't. The programmer has to code it out like so.

class Person

{

private:

string name;

unsigned int age;

public:

Person();

Person(string name, int age);

// Other methods go here

bool operator==(const Person& that) { return this->age == that.age; }

bool operator!=(const Person& that) { return this->age != that.age; }

};

And yes, if you want to use either

if (a == b)

{

// Do something

}

// OR

if (a != b)

{

// Do something

}

Then one has to include both.

Now in most cases., either one or or the other comparison operator will suffice. In most cases, one can get away with just one (usually operator==()); but if one needs operator!=(), then one has to code it out. This goes for ALL of the comparison operators (like operator<() and operator>() ).