What's good to implement operator overloading with 'friend'?

meili100 · #1 07-May-2007, 18:03

We all know how to implement operator overloading as the following:

CPP / C++ / C Code:

class complex
{
public:
complex(double r, double i){
real = r, imag = i;}
complex operator +(const complex & c);
};

But sometimes I see people use friend methods:

CPP / C++ / C Code:

class complex
{
public:
complex(double r, double i){
real = r, imag = i;}
friend complex operator +(const complex &c1, const complex &c2);
};

Why do they do this way? What're the advantages?
Thank you.

davekw7x · #2 08-May-2007, 09:41

Quote:

Originally Posted by meili100

operator overloading as the following:

CPP / C++ / C Code:

class complex
{
public:
complex(double r, double i){
real = r, imag = i;}
complex operator +(const complex & c);
};

Usually the data members real and imag are private to the class.

The above function overloads the '+' operator for the complex class so that we can write z = x + y for complex objects. It operates on two objects of the class and the result is a member of the class. Since it is being defined as a member function of the class, it has access to all members (public and private) of the class. As you imply, this is pretty much standard stuff.

Quote:

Originally Posted by meili100

But sometimes I see ...

CPP / C++ / C Code:

class complex
{
public:
.

friend complex operator +(const complex &c1, const complex &c2);
};

Why do they do this way?

I don't have a clue.

Quote:

Originally Posted by meili100

What're the advantages?

You must ask the people who defined it this way. There is always a reason that people do things. (That doesn't mean that there's always a goodreason. However, just because I don't know why someone did something doesn't mean that he/she didn't have a good reason. So: ask them.)

Friend operators defined in a class will allow access to private members from other classes. Since this operates on two complex objects and gives a complex object, I'm not sure who could be a friend.

Now a good reason to define a function as a friend be if you wanted to overload the '<<' operator so that an ostream object could access the private members of the complex class in order to print them out.

CPP / C++ / C Code:

  friend std::ostream& operator <<(std::ostream &out, const complex& num);

Regards,

Dave

surya.k · #3 22-Jul-2007, 07:34

hai meili, they done this prg in this way becuase...........
while u pass any arguments that are not modified in the function(the data members of an object), it is always better to send them as const objects which enables faster compilation,so they done so....
n why they use friend fun is,u want to store the result of the two complex numbers in the third complex no., the calling of the function (overloaded operator) is interpreted by the compiler in the following way........
complex1+complex2; //in compiler-> operator +(complex1,complex2);
so in order to store the result in the third complex object it should return the reference to the resultant complex,n if u use class member fun, it is not possible to return the reference as the compiler under stands the call as follows
complex1+complex2
//in compiler it is treated as complex1.operator +(complex2);
so we need frend fun.....
thats wht dave explained u.....
regards
surya

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What's good to implement operator overloading with 'friend'?

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Re: What's good to implement operator overloading with 'friend'?