Friend Function in C++: Syntax & How to Use It

Updated on September 11, 2024

Article Outline

What is the Friend Function in C++?Use of Friend Class in C++Friend Functions in C++ Types Friend Function in C++ with Example To Wind Up:FAQs

In the dynamic realm of programming, C++ stands tall as a versatile language, continually sought after by professionals ranging from software developers to game designers and backend engineers. Holding a reliable reputation, C++ boasts a rich feature set, including the intriguing concept of friend functions. In C++, a friend function is a unique entity defined outside the class’s boundaries yet bestowed with the remarkable authority to access both protected and private members of the class it befriends. Despite not being a member function itself, the prototype of a friend function finds its home within the class declaration, showcasing the language’s elegance and flexibility.

This capability isn’t limited solely to classes; it expands its scope to encompass class templates, function templates, and even standard functions and member functions, affirming its status as a potent asset in the toolkit of C++ developers. The TIOBE index of 2022 underscores C++’s prominence as the fourth most widely used language worldwide, underscoring its enduring significance and broad acceptance in the constantly evolving realm of technology.

What is the Friend Function in C++?

In C++, the friend function is declared outside the class’s scope, granting it access to both protected and private members of the class. While not a member function, the prototypes for friend functions are specified within the class declaration. This encompasses class templates, classes, function templates, functions, and member functions, allowing the friend function to access all members of the class.

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Use of Friend Class in C++

The friend class offers various advantages and applications. Some key scenarios include:

Accessing private and protected members from other classes, as you’re likely aware.
Designating all functions of a class as friend functions.
Facilitating extension of storage and access to its components while preserving encapsulation.
Permitting classes to exchange information about private members.
Commonly employed when multiple classes have interconnected data members.

Friend Functions in C++ Types

In C++, friend functions come in the following variations:

Function with no arguments and no return value.
Function with no arguments but with a return value.
Function with arguments but no return value.
Function with arguments and a return value.

The syntax for implementing a friend class is as follows:

class ClassB;

class ClassA {

// ClassB is a friend class of ClassA

friend class ClassB;

… .. …

}

class ClassB {

… .. …

}

As evident from the syntax, simply prepend the keyword ‘friend’ before a class to designate it as a friend class. Employing this syntax will establish ClassB as a friend class of ClassA. As ClassB assumes the role of a friend class, it gains access to all public, private, and protected members of ClassA. Conversely, the reverse is not applicable. This is because C++ permits only the granting of the friend relationship, not its reciprocation. Consequently, ClassA won’t be able to access the private members of ClassB.

Friend Function in C++ with Example

Let’s see the simple friend function in c++ example used to print the length of a box.

#include <iostream>
using namespace std;
class Box
{
private:
int length;
public:
Box(): length(0) { }
friend int printLength(Box); //friend function
};
int printLength(Box b)
{
b.length += 10;
return b.length;
}
int main()
{
Box b;
cout<<“Length of box: “<< printLength(b)<<endl;
return 0;
}

Output:

Length of box: 10

Let’s see a simple example when the function is friendly to two classes.

#include <iostream>
using namespace std;
class B; // forward declaration.
class A
{
int x;
public:
void setdata(int i)
{
x=i;
}
friend void min(A,B); // friend function.
};
class B
{
int y;
public:
void setdata(int i)
{
y=i;
}
friend void min(A,B); // friend function
};
void min(A a,B b)
{
if(a.x<=b.y)
std::cout << a.x << std::endl;
else
std::cout << b.y << std::endl;
}
int main()
{
A a;
B b;
a.setdata(10);
b.setdata(20);
min(a,b);
return 0;
}

Output:

In the above example, min() function is friendly to two classes, i.e., the min() function can access the private members of both classes A and B.

To Wind Up:

The friend function is one of the powerful tools in C++ that violates the encapsulation barrier and provides access by non-member functions to private and protected members of the class. Such a facility provides ease with great versatility for relating to complex relationships among classes. As you further reach into the realm of software development, consider expanding your skills with an Advanced Certification Program in Data Science & Analytics. Empower yourself with critical skills and knowledge to enable you to be successful in this rapidly changing domain and prepare for many highly-rewarding careers within the context of data-driven innovation. Take advantage of the opportunity for increasing your skill level in data science and analytics today!

FAQs

What is a friend function in C++?

A friend function, although not a member of a class, can access the private and protected members of that class. These functions are not regarded as class members; rather, they are regular external functions granted special access privileges.

What is the inline function and friend function in C++?

In C++, "friend" indicates that a non-member function can access the private members of the class. On the other hand, "inline" signifies that there is no actual function call; instead, the body of the function is duplicated at every call site, typically in assembly code.

What are friend function types?

Friend function in C++ is the creative function that breaks the data hiding in an object-oriented programming language. The data hiding prevents the access of private members externally from the class. The protected members are inaccessible from the outside and can only be accessed by the derived classes.

What are friend function types?

In C++, a friend function serves as a versatile tool that circumvents the data hiding inherent in object-oriented programming languages. Data hiding typically bars external access to private members of a class. Similarly, protected members remain inaccessible from outside the class and can solely be accessed by derived classes.

What are the properties of the friend function?

Since it's not within the scope of the class, it can't be called using the object. Instead, it's invoked just like a regular function without needing an object. Direct access to member names isn't possible; it requires using the object name followed by the dot membership operator and the member name.

Updated on September 11, 2024

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