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C++/What kind of variables and functions should be public, or private?


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What kind of variables and functions should be public, or private?


Public interface member functions should be public. That is, the members functions that user's of your class are supposed to use are public.

Internal implementation details, including data members, should be private, or if really necessary, protected.

Of course all rules have exceptions. I came across one recently with an event class template I had produced - the members of these types were public. However the event types were meant to provide something a like C# event member semantics for C++ classes so were a special case as they in effect extended the public interface of the classes concerned.

You might think that this is overkill for simple classes for such things as (for example) a point. You have its x and y coordinates as public data members. Then you provide various useful member functions and operators to manipulate point values. However the x and y coordinates are implementation details - you could just as well store polar r, theta values for the point as the Cartesian x, y values. So make them private and provide accessor functions to obtain the values - you could have 2 sets of such values: Those that operate with Cartesian x, y point values and those that use polar r, theta point values. Which you actually store is, as I said, a mere implementation detail.  


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Ralph McArdell


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.


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


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