Abstract class and friend function

Virtual functions

• A virtual function is a member function in the base class that we expect to redefine in derived classes. It is declared using the virtual keyword. A virtual function is used in the base class to ensure that the function is overridden. This especially applies to cases where a pointer of base class points to a derived class object.
• C++ determines which function is invoked at the runtime based on the type of object pointed by the base class pointer when the function is made virtual.
• Virtual function mainly used to achieve run time polymorphism.

class Base {
public:
    virtual void print()
    {
        cout << "Base Function" << endl;
    }
};
class Derived : public Base {
public:
    void print()
    {
        cout << "Derived Function" << endl;
    }
};
int main()
{
    Derived derived1;
    // pointer of Base type that points to derived1
    Base* base1 = &derived1;
    // calls member function of Derived class
    base1->print();
    return 0;
}
// Output :
// Derived Function

Pure virtual functions

• A pure virtual function is a virtual function in C++ for which we need not write any function definition and only have to declare it. It is declared by assigning 0 in the declaration.
• A pure virtual function (or abstract function) in C++ is a virtual function for which we can implement, But we must override that function in the derived class; otherwise, the derived class will also become an abstract class.

class A {
public:
    virtual void s() = 0;
    // Pure Virtual Function
};

Abstract class

• Abstract classes can’t be instantiated, i.e., we cannot create an object of this class. However, we can derive a class from it and instantiate the object of the derived class. An Abstract class has at least one pure virtual function.
• If we do not override the pure virtual function in the derived class, then the derived class also becomes an abstract class.
• We cannot create objects of an abstract class. However, we can derive classes from them and use their data members and member functions (except pure virtual functions).

Properties of the abstract classes:

• It can have normal functions and variables along with pure virtual functions.
• Prominently used for upcasting(converting a derived-class reference or pointer to a base-class. In other words, upcasting allows us to treat a derived type as a base type), so its derived classes can use its interface.
• If an abstract class has a derived class, they must implement all pure virtual functions, or they will become abstract.

class Base {
public:
    virtual void s() = 0; // Pure Virtual Function
};

class Derived : public Base {
public:
    void s(){
        cout << "Virtual Function in Derived_class";
    }
};

int main()
{
    Base* b;
    Derived d_obj;
    b = &d_obj;
    b->s();
}
// Output
// Virtual Function in Derived_class

Friend function

• A friend function is a function that is not a member of a class but has access to its private and protected members. It is declared using the keyword friend.
• The function can be defined anywhere in the program like a normal C++ function. The function definition does not use either the keyword friend or scope resolution operator.

class class_name {
    friend data_type function_name(argument);
    // syntax of friend function.
};

• Even though the prototypes for friend functions appear in the class definition, friends are not member functions.

class Rectangle {
private:
    int length;

public:
    Rectangle() {
        length = 10;
    }
    friend int printLength(Rectangle); // friend function
};

int printLength(Rectangle b)
{
    b.length += 10;
    return b.length;
}

int main()
{
    Rectangle b;
    cout << "Length of Rectangle: " << printLength(b) << endl;
    return 0;
}
// Output :
// Length of Rectangle : 20

Characteristics of friend function:

• A friend function can be declared in the private or public section of the class.
• It can be called a normal function without using the object.
• A friend function is not in the scope of the class, of which it is a friend.
• A friend function is not invoked using the class object as it is not in the class’s scope.
• A friend function cannot access the private and protected data members of the class directly. It needs to make use of a class object and then access the members using the dot operator.
• A friend function can be a global function or a member of another class.

Interview Questions

• Interview Questions

Resources

• Abstract class and friend function