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C++/Can you override a non-virtual function

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Expert: - 3/9/2010


Question
Sir,

Can you override a non-virtual function in the derived classes in C++.

Thanks,
Prasanth.

Answer
Quick answer: no.

Longer answer: The whole point of declaring a member function virtual is to enable it to be overridden by sub-classes. Virtual functions and overriding of them is the way C++ supports the object oriented concept of polymorphism.

Note however that sub-classes need not also explicitly declare overridden member functions virtual - once they are declared virtual in a base class it sticks all the way down the class hierarchy. I tend to do so just to make it clear to readers of the code that the member function is (supposed to be) overriding base class behaviour. In other languages such as C# you mark member functions that are overriding base behaviour with a keyword such as 'override'.

Supporting polymorphic behaviour - and what is called dynamic binding - requires more resources than non-polymorphic statically bound member function calls. Hence, as is the C++ way, if you do not need it you do not pay for it.

For more information please start with the C++ FAQ I mention in my instructions to questioners at

   http://www.parashift.com/c++-faq-lite/

specifically section "[20] Inheritance - virtual functions" at

   http://www.parashift.com/c++-faq-lite/virtual-functions.html

and read at least 20.1 through to 20.5 and 20.7.

and then look at section "[23] Inheritance - what your mother never told you" at

   http://www.parashift.com/c++-faq-lite/strange-inheritance.html

and read at least items 23.8 and 23.9

You might like to look at this previous answer of mine as well:

   http://en.allexperts.com/q/C-1040/2009/10/prevent-method-superclass-overridden.h...

Hope when you have read all this i information that it clarifies the matter for you.

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Ralph McArdell

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I am a software developer with more than 15 years C++ experience and over 25 years experience developing a wide variety of applications for Windows NT/2000/XP, UNIX, Linux and other platforms. I can help with basic to advanced C++, C (although I do not write just-C much if at all these days so maybe ask in the C section about purely C matters), software development and many platform specific and system development problems.

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My career started in the mid 1980s working as a batch process operator for the now defunct Inner London Education Authority, working on Prime mini computers. I then moved into the role of Programmer / Analyst, also on the Primes, then into technical support and finally into the micro computing section, using a variety of 16 and 8 bit machines. Following the demise of the ILEA I worked for a small company, now gone, called Hodos. I worked on a part task train simulator using C and the Intel DVI (Digital Video Interactive) - the hardware based predecessor to Indeo. Other projects included a CGI based train simulator (different goals to the first), and various other projects in C and Visual Basic (er, version 1 that is). When Hodos went into receivership I went freelance and finally managed to start working in C++. I initially had contracts working on train simulators (surprise) and multimedia - I worked on many of the Dorling Kindersley CD-ROM titles and wrote the screensaver games for the Wallace and Gromit Cracking Animator CD. My more recent contracts have been more traditionally IT based, working predominately in C++ on MS Windows NT, 2000. XP, Linux and UN*X. These projects have had wide ranging additional skill sets including system analysis and design, databases and SQL in various guises, C#, client server and remoting, cross porting applications between platforms and various client development processes. I have an interest in the development of the C++ core language and libraries and try to keep up with at least some of the papers on the ISO C++ Standard Committee site at http://www.open-std.org/jtc1/sc22/wg21/.

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