Covariant virtual function in C ++

Question

I tried the following program, but the compiler shows an error.

#include <iostream>
class Base
{
    public:
        virtual Base& fun() const 
        {
            std::cout<<"fun() in Base\n";
            return *this;
        }
};
class Derived : public Base
{
    public:
         Derived& fun() const
        {
            std::cout<<"fun() in Derived\n";
            return *this;
        }
};
int main()
{
    Base* p=new Derived();
    p->fun();
    delete p;
    return 0;
}

Compiler Errors:

[Error] invalid initialization of reference of type 'Base&' from expression of type 'const Base'
[Error] invalid initialization of reference of type 'Derived&' from expression of type 'const Derived'

Why am I getting these errors?
But when I write the following in both classes, the function program works fine.

return (Base&)*this;      // write this line in Base class function
return (Derived&)*this    // write this line in Derived class function

What is the meaning of the above statements?
Please help me.

Answer 1

Decision:

In your member functions fun(i.e. for Base::funand Derived::fun) you need to either reset the constfunction qualifier :

virtual Base& fun() 
{
  std::cout<<"fun() in Base\n";
  return *this;
}

or return a const reference for a class object Base.

virtual const Base& fun() const
{
  std::cout<<"fun() in Base\n";
  return *this;
}

Live Demo

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:

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Answer 2

const

virtual Base& fun() const 

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return *this;

... , Base&, const.

const , this const, const Base&.

Answer 3

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