Data members of a class are usually a part of its implementation rather than its interface. They may be replaced when the internal implementation of the class is to be changed, and hence, they should be hidden from other classes. If derived classes need to access these data members, they should use accessor methods (“getters”) rather than directly access data members of another object. As a result, no modification will be required for derived classes when a change is made in the base class:

 class Date { private:int d,m,y //how a date is represented is an implementation detailpublic:int Day() const {return d; } };class DateTime : public Date {int h,m,s};

Now lets assume that class Date is used mostly on display devices, so it has to supply some method of converting its d,m,y integers into a displayable string. In order to enhance performance, a design modification is made: instead of the three integers, a single string will now hold the date representation. Had class DateTime relied on the internal implementation of Date, it would have had to be modified as well. But since it can access Dates data members only through access methods, all that is required is a small change in Date::Day() method. Please note that accessor methods are inlined anyway, so their use should not incur any overhead.