Explain the advantages and disadvantages of friend functions?

Answer 1

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What is a Friend Function?

A friend function is a special function in c++ which inspite of not being member fuction
of a class has privalage to access private and protected data of a class.

A friend function is a non member function of a class, that is declared as a friend using
the keyword "friend" inside the class. By declaring a function as a friend, all the access
permissions are given to the function.

A friend function is used for accessing the non-public members of a class.
A class can allow non-member functions and other classes to access its own
private data, by making them friends. Thus, a friend function is an ordinary
function or a member of another class.





Need for Friend Function:

As discussed in the earlier sections on access specifiers, when a data
is declared as private inside a class, then it is not accessible from outside
the class. A function that is not a member or an external class will not
be able to access the private data. A programmer may have a situation where
he or she would need to access private data from non-member functions and
external classes. For handling such cases, the concept of Friend functions
is a useful tool.





How to define and use Friend Function in C++:

The friend function is written as any other normal function, except
the function declaration of these functions is preceded with the keyword
friend. The friend function must have the class to which it is declared as
friend passed to it in argument.






Some important points to note while using friend functions in C++:

* The keyword friend is placed only in the function declaration of the friend
function and not in the function definition.
.
* It is possible to declare a function as friend in any number of classes.
.
* When a class is declared as a friend, the friend class has access to the
private data of the class that made this a friend.
.
* A friend function, even though it is not a member function, would have the
rights to access the private members of the class.
.
* It is possible to declare the friend function as either private or public.
.
* The function can be invoked without the use of an object. The friend function
has its argument as objects, seen in example below.






properties of friend function:

1. if a function to be made friend of a class than it should be declared within body
of the class priciding with keyword friend.

2.freind function never breaks the security.

3.it should not be defined in name of class nor scope resolution operator is used in it's
defination even the keyword freind is also not used while defining friend function.

4.when friend function is called nither name of object nor dot operator is used. however
it may accept the object as argument who's value it want's to access.

5.it doen't matter in which section of the class we have declared a freind function.







Example to understand the friend function:


#include
class exforsys
{
private:
int a,b;
public:
void test()
{
a=100;
b=200;
}
friend int compute(exforsys e1)

//Friend Function Declaration with keyword friend and with the object of class exforsys to which it is friend passed
to it
};

int compute(exforsys e1)
{
//Friend Function Definition which has access to private data
return int(e1.a+e2.b)-5;
}

main()
{
exforsys e;
e.test();
cout<<"The result is:"<
//Calling of Friend Function with object as argument.
}

The output of the above program is

The result is:295

The function compute() is a non-member function of the class exforsys. In order
to make this function have access to the
private data a and b of class exforsys , it is created as a friend function
for the class exforsys. As a first step,
the function compute() is declared as friend in the class exforsys as:

friend int compute (exforsys e1)







disadvantage of friend functions is that they require an extra line
of code when you want dynamic binding. To get the effect of a virtual friend,
the friend function should call a hidden (usually protected:) virtual[20]
member function.