Why is "virtual" keyword used there?

Neela Krishna Teja Tadikonda 0
Viewed 84

While Simulating final Class in C++, why is "virtual" keyword used? C++ program without any compilation error to demonstrate that instances of the Final class can be created

#include <iostream>
using namespace std;

class Final;

class MakeFinal {
private:
    MakeFinal() { cout << "MakeFinal constructor" << endl; }
    friend class Final;
};

class Final : virtual MakeFinal {
public:
    Final() { cout << "Final constructor" << endl; }
};

int main(int argc, char* argv[])
{
    Final f;
    return 0;
}
c++
1 Answers

With virtual inheritance the most derived class must call the constructor of the virtual base class (in contrast to non-virtual inheritance where each class constructs its direct bases). Because MakeFinals constructor is private and only Final is a friend, no other classes can access MakeFinals constructor. Consequently you cannot derive from Final. More accurately you can derive:

struct OK : Final {};

but you cannot create instances, because constructing a OK would require to call the private MakeFinal constructor:

OK not_ok; // error: use of deleted function 'OK::OK()'

Without virtual inheritance this wouldn't work as intended, because then OKs constructor would call Finals constructor which in turn calls MakeFinals constructor. All would be fine. Only with virtual inheritance the most derived class must call the virtual base class constructor.

Note that since C++11, this "simulation" is not required. There is final to explicitly make a class final.

[...] without any compilation error to demonstrate that instances of the Final class can be created

Not sure if this is a misunderstanding. Of course you can create instances of a final class. When a class is final you just cannot inherit from it any further. (As explained above in your "simulation" one can inherit from Final, its just pretty useless because no instances can be constructed of a class inheriting from Final).

463035818_is_not_a_number0
Related