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C++ Accidental Access to Private Object Variable

Posted on 2013-01-11
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Last Modified: 2013-01-11
Given a class with a private variable and a public getter method, if a programmer were to return a reference to the private variable, then users of that method could have direct modifiable access to that private variable (i.e., modify the variable value without using an object method).

I don't like this. (Tough luck on me, I guess. But why would the C++ standard allow this?)

Is there a way to keep private variables truly private so that the above cannot happen?
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Question by:phoffric
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Accepted Solution

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evilrix earned 800 total points
ID: 38769270
>> why would the C++ standard allow this?
Simple answer: because it does - it is up to you as the coder to ensure your code is safe. C++, unlike most other languages, doesn't hold your hand.

>> Is there a way to keep private variables truly private so that the above cannot happen?
Yes, return a const reference/pointer... although there is nothing to prevent the user from casting const away.
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Author Closing Comment

by:phoffric
ID: 38769279
Darn, I am being considered for a lead position at a new company, so I started think about how someone can mess things up and thought about these kinds of problems. I knew about your suggestions, but I guess it will be the code reviews that will have to save us from this type of coding from getting into production. Without the code reviews to catch this, no doubt initial systems testing will pass fine. But someday, someone would start modifying these private object variables using their own "local" variables.

I heard that C can shoot you in the foot; and that C++ tries to improve on that; but when the coding goes too far awry, C++ can blow off your face.
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Expert Comment

by:evilrix
ID: 38769290
C++, like C, is a very low-level programming language. It gives you the tools but doesn't really stop you abusing them. Just like a hammer, when used incorrectly, can cause havoc so can C/C++. If you are the team lead you should ensure you have a coding standard document and ensure all your team adhere to it.  If you don't have one then this is a good place to start.
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Author Comment

by:phoffric
ID: 38769304
Thanks! I have worked with government coding standards; yet I have not seen the protection required to avoid the above potential maintance issue. There were some useful things like:
 if ( x == 0 ) {...} // not allowed
 if ( 0 == x ) {...} // allowed
to prevent accidental use of = instead of ==.
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