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C++ Virtual Functions | Find output programs | Set 1

This section contains the C++ find output programs with their explanations on C++ Virtual Functions (set 1).
Submitted by Nidhi, on July 23, 2020

Program 1:

#include <iostream>
#include <string.h>
using namespace std;

int main()
{
    virtual float PI = 3.14F;
    virtual float R = 5.2F;
    float AREA = 0;

    AREA = PI * R * R;

    cout << AREA << endl;

    return 0;
}

Output:

main.cpp: In function ‘int main()’:
main.cpp:7:5: error: ‘virtual’ outside class declaration
     virtual float PI = 3.14F;
     ^~~~~~~
main.cpp:8:5: error: ‘virtual’ outside class declaration
     virtual float R = 5.2F;
     ^~~~~~~

Explanation:

It will generate an error because the virtual keyword cannot be used outside the class.

Program 2:

#include <iostream>
#include <string.h>
using namespace std;

class A {
    int val;

public:
    A()
        : val(10)
    {
    }

    void print()
    {
        cout << val << endl;
    }
};

class B {
    virtual A ob;

public:
    void fun()
    {
        ob.print();
    }
};

int main()
{
    B ob;

    ob.fun();
    return 0;
}

Output:

main.cpp:21:15: error: ‘ob’ declared as a ‘virtual’ field
     virtual A ob;
               ^~

Explanation:

It will generate an error. Here, We created a class A that contains a data member val and a member function print(). Then we created a class B, and we declared an object of class A as a virtual data member. We cannot use a virtual keyword with data members.

Program 3:

#include <iostream>
#include <string.h>
using namespace std;

class A {
public:
    virtual void fun()
    {
        cout << "A::fun() called" << endl;
    }
};

class B {
public:
    void fun()
    {
        cout << "B::fun() called" << endl;
    }
};

int main()
{
    A ob1, *p;
    B ob2;

    p = &ob1;
    p->fun();

    p = &ob2;
    p->fun();

    return 0;
}

Output:

main.cpp: In function ‘int main()’:
main.cpp:29:10: error: cannot convert ‘B*’ to ‘A*’ in assignment
     p = &ob2;
          ^~~

Explanation:

It will generate an error.

Here, we created two classes A and B, both contain member function fun(). Class A contain fun() function as a virtual.

The main cause of the error, here we did not inherit class A into class B that's why we cannot assign the address of B class object into a pointer of class A.

Program 4:

#include <iostream>
#include <string.h>
using namespace std;

class A {
public:
    virtual void fun()
    {
        cout << "A::fun() called" << endl;
    }
};

class B : public A {
public:
    void fun()
    {
        cout << "B::fun() called" << endl;
    }
};

int main()
{
    A ob1, *p;
    B ob2;

    p = &ob1;
    p->fun();

    p = &ob2;
    p->fun();

    return 0;
}

Output:

A::fun() called
B::fun() called

Explanation:

Here, we created two classes A and B. Both contain the function with the same name that is fun(). Class A inherited in class B.

Class A contains the virtual function. It will use dynamic binding for function calls.

In the main() function, we created object ob1 and a pointer p of class A. And, we created object ob2 of class B.

First, we assigned the address of ob1 to pointer p then it will call function fun() of class A, then we assigned the address of ob2 to the pointer p then it will call function fun() of class B.

Program 5:

#include <iostream>
#include <string.h>
using namespace std;

class A {
public:
    void fun()
    {
        cout << "A::fun() called" << endl;
    }
};

class B : public A {
public:
    void fun()
    {
        cout << "B::fun() called" << endl;
    }
};

int main()
{
    A ob1, *p;
    B ob2;

    p = &ob1;
    p->fun();

    p = &ob2;
    p->fun();

    return 0;
}

Output:

A::fun() called
A::fun() called

Explanation:

Here, we created two classes A and B. Both contain the function with the same name that is fun(). Class A inherited in class B.

In the main() function, we created object ob1 and a pointer p of class A. And, we created object ob2 of class B.

First, we assigned the address of ob1 to pointer p then it will call function fun() of class A, then we assigned the address of ob2 to the pointer p then it will again call function fun() of class A. Because, Here we did not create any virtual function then it will use static binding for function call.

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