friend function(c++)

Access privileges in C++.
You have access privileges in C++ such as public, protected
and private that helps in encapsulation of data at various
level.

Private
If data are declared as private in a class then it is accessible by the
member functions of the class where they are declared. The private
member functions can be accessed only by the members of the class.
By default, any member of the class is considered as private by the C+
+ compiler, if no specifier is declared for the member.
Public
The member functions with public access specifier can be accessed
outside of the class. This kind of members is accessed by creating
instance of the cass.

Protected
Protected members are accessible by the class itself and it's sub-
classes. The members with protected specifier act exactly like
private as long as they are referenced within the class or from the
instance of the class. This specifier specially used when you need
to use inheritance facility of C++. The protected members
become private of a child class in case of private inheritance,
public in case of public inheritance, and stay protected in case of
protected inheritance.

Access specifier
Visible to own
class members
Visible to
objects of
same/other
class
public Yes Yes
private Yes No
protected Yes No

When we want our private data to be shared by a non
member function
Then:
Basically, we declare something as a friend, you give it access
to your private data members.
Single functions or entire classes may be declared as friends
of a class.

• A Friend function is a non-member function of the class that
has been granted access to all private members of the class.
• We simply declare the function within the class by a prefixing
its declaration with keyword friend.
• Function definition must not use keyword friend.
• Definition of friend function is specified outside the class body
and is not treated as a part of the class.
• The major difference b/w member function and friend
function is that the member function is accessed through the
object while friend function requires object to be passed as
parameter.

Syntax:
class ABC
{
………….
public:
friend void xyz(object of class);
};

Friend function characterstics
It is not in scope of class.
It cannot be called using object of that class.
It can be invoked like a normal function.
It should use a dot operator for accessing members.
It can be public or private.
It has objects as arguments.
Perhaps the most common use of friend functions is
overloading << and >> for I/O.

Example
class demo
{
int x;
public:
demo(int a)
{
x=a;
}
friend void display(demo);
};

void display(demo d1)
{
cout<<d1.x;
}
void main()
{
demo d2(5);
display(d2);
}

 class sample
{
int a;
int b;
public:
void setval(){ a=25,b=40}
friend float mean(sample s);
};
float mean(sample s)
{ return (s.a+s.b)/2.0;
}
void main()
{ sample X;
cout<<mean(X);
}

Friend class
In previous section of class we declared only one function as a
friend of another class.but it is possible that all member of the
one class can be friend of another class.this is friend class

Friends (a few gory details)
Friendship is not inherited, transitive, or reciprocal.
Derived classes don’t receive the privileges of friendship (more
on this when we get to inheritance in a few classes).
The privileges of friendship aren’t transitive. If class A declares
class B as a friend, and class B declares class C as a friend, class
C doesn’t necessarily have any special access rights to class A.
If class A declares class B as a friend (so class B can see class A’s
private members), class A is not automatically a friend of class
B (so class A cannot necessarily see the private data members of
class B).
Friendship is not inherited, transitive, or reciprocal.

Example
class demo
{
private:
int x,y;
public:
demo(int a,int b)
{
x=a;
y=b;
}
friend class frnd;
};

class frnd
{
public:
void display(demo d1)
{
cout<<“x is=”d1.x;
cout<<“y is=”d1.y;
}
};

void main()
{
demo d2(10,40);
frnd f1;
f1.display(d2);
getch();
}

This pointer
Every object in C++ has access to its own address
through an important pointer called this pointer. The
this pointer is an implicit parameter to all member
functions. Therefore, inside a member function, this may
be used to refer to the invoking object.
Friend functions do not have a this pointer, because
friends are not members of a class. Only member
functions have a this pointer.

friend function(c++)

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