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Recently I attended an interview. The interviewer asked me to explain virtual function mechanism in C++. I explained using VPTR and VTABLE. I explained in detail how VPTR and VTABLE are used to achieve run time polymorphism.

While I was explaining how the compiler introduces hidden code to fetch VPTR from the class, get function address from VTABLE and call is resolved. But he was not satisfied with the answer. He asked me detail of hidden code? What exactly does a compiler do? If a compiler is doing everything for you then what is the use of developer?

I searched for details of the role of a compiler for virtual functions. Regarding the hidden code. But still not clear about the question.

Please, any help or pointers?

Molx
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vDeepak
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    Technically speaking, it's not required to use a vtable at all, sounds like the person interviewing was just boosting his/her own ego. – OMGtechy Jun 24 '15 at 18:34
  • I assume when the "use of developer" became a topic, the interview was drawing to a close? – Kerrek SB Jun 24 '15 at 18:34
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    possible duplicate of [How does virtual method invocation work in C++?](http://stackoverflow.com/questions/3804079/how-does-virtual-method-invocation-work-in-c) – NathanOliver Jun 24 '15 at 18:35
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    I don't understand why it matters how virtual methods are implemented by the compiler. IMHO, the only concept that is important is how the virtual methods are resolved in a high level understanding. – Thomas Matthews Jun 24 '15 at 19:06
  • Does it mean we, as developer need not to worry how compiler implements their virtual mechanism? My view is regarding "hidden" code. – vDeepak Jun 24 '15 at 19:07

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If a compiler is doing everything for you then what is the use of developer?

Developers are there to specify their intentions and compilers are there to transform the intentions to executables. As time passes, computers get faster and compilers smarter so there is no need to express developer's intentions in assembly code but in Erlang, F#, Prolog, whatever.

In other words, it's interesting to know details of the code produced by C++ compilers, but it is not the core of C++ development.

Finally, to answer the quoted question:

Compilers are not doing everything yet. Unfortunately.

dlask
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