FRIEND FUNCTIONS
1) A non member function cannot have an access to the private data of
a class. However there could be situation where we would like two
classes to share a particular function.
2) In such situation, C++ allows the common function to be made
friendly with both the classes, thereby allowing the function to have
access to the private data of these classes such a function need not
be a member of any of these classes.
3) The syntax for friend function is
class ABC
{
=======
public:
=======
friend void function1(void);
}
The function is declared with friend keyword. But while defining
friend function. It does not use either keyword friend or :: operator.
A friend function, although not a member function, has full access
right to the private member of the class.
4) A friend, function has following characteristics.
i) It is not in the scope of the class to which it has been
declared as friend.
ii) A friend function cannot be called using the object of that
class. If can be invoked like a normal function without help of
any object.
iii) It cannot access the member variables directly & has to use
an object name dot membership operator with member name.
iv) It can be declared either in the public or the private part of a
class without affecting its meaning.
v) Usually, it has the object as arguments.
5) Ex-
# include
class sample
{
int a, b;
public:
void setvalue()
{
a = 25; b = 40;
}
friend float mean (sample s);
};
float mean(sample s)
{
return float(s.a + s.b)/ 2.0;
}
main ( )
{
sample x ;
x.setvalue();
cout << " Mean Value = ";
mean(x);
}
output -
Mean value = 32.5
6) Member function of one class can be friend functions of another
class. In such cases they are defined using the scope resolution
operator.
EX - class x
{ -------
int fun(); // member function of x
};
class y
{---------
----------
friend int x : : fun()
}; ---------
7) We can declare all the member functions of one class as the friend
functions of another class by declaring that class as a friend class.
Ex - class z
{
--------
--------
friend class x ;
};
8) A friend function work as a bridge between the classes. Ex
#include
class ABC ; //forward declaration
class xyz
{
int x;
public:
void setvalue(int i) { x = i; }
friend void max (xyz, ABC);
};
class ABC
{
int a;
public a:
void setvalue(int i) { a =i;}
friend void max(xyz, ABC);
};
void max(xyz m, ABC n) // definition of friend.
{
if(m. x > = n. a)
cout << m .x;
else
cout << n.a;
}
main ( )
{
ABC K;
K. setvalue (20);
XYZ l;
l
setvalue (40);
max (l , k);
}
output is - 40
9) A friend function can be called by reference. In this local copies of
the object are not made. Instead, a pointer to the address of the
object is passed and the called function directly works on the actual
object used in the call.
Ex-following program shows how to use a common friend function to
exchange private value of two classes.
# include
class c2;
class c1
{
int value1;
public:
void indata (int a) {value1 = a;}
void display (void) {cout << value1;}
friend void exchange (c1 &, c2 &);
};
class c2
{
int value2;
public:
void indata (int a) {value 2 = a;}
void display (void) {cout << value2;}
friend void exchange (C1 &, C2 &)
};
void exchange (cl &x, c2, &y)
{
int temp = x.value1;
x.value 1= y.value2;
y.value 2 = temp
}
main()
{
c1 co1;
c2 co2;
co1. indata(10);
co2. indata(20)
cout << "Values before exchange";
co1. display()
co2.display();
exchange(co1, co2);
cout << "values after exchange";
co1.display();
co2.display();
}
output of the program
values before exchange
10
20
values after exchange
20
10
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