Question

How does the Virtual Differs in C++ and c#? basically i wanted to know is there any difference in the way Vtable is represented in C# and c++? Also is there any difference in the way the compiler creates the V Table in C# and c++?

Basically the interviewer wanted to know how does the compiler behaviour differrs for virtual functions in case of C# and c++.

I have answered the question mentioning there might be no difference in VTABLE expect that there is no Virtual Destructor in c#.

Answer 1

(1) In C# one can choose to override a virtual function from base class. Both base and derived methods can have same syntax, but derived method can participate in polymorphism only if it uses the keyword override before it. See this answer for more details.

In C++, if provided the same syntax method in derived class as base class virtual method; then it will be automatically be overridden. No choice.

(2) To be short, in C# we have abstract methods and in C++ pure virtual methods. Both may not be exactly same, but are equivalent (as pure virtual can have function body).

(3) In C++ we have choice to call virtual method without letting it participating in polymorphism. e.g. calling with an object, calling inside constructor.

(4) In C++, if the base virtual method is public and derived method is private than there is no difference for virtual mechanism, when the method is called with base handle (pointer or reference). Which means that their access specifier can be different.

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Now coming to your 2nd question of how vtables are implemented. This is not really an objective question. Because in C++ itself different vendors implement the vtable in different ways (it’s not in standard as pointed by @littleadve). So for interview sake you can answer that, vtables are platform dependent.