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Is there any reason not to make a member function virtual?

Asked 2008-11-15T03:57:11.887
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Is there any real reason not to make a member function virtual in C++? Of course, there's always the performance argument, but that doesn't seem to stick in most situations since the overhead of virtual functions is fairly low.

On the other hand, I've been bitten a couple of times with forgetting to make a function virtual that should be virtual. And that seems to be a bigger argument than the performance one. So is there any reason not to make member functions virtual by default?

c++ oop virtual-functions
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Stroustrup, the designer of the language, says:

Because many classes are not designed to be used as base classes. For example, see class complex.

Also, objects of a class with a virtual function require space needed by the virtual function call mechanism - typically one word per object. This overhead can be significant, and can get in the way of layout compatibility with data from other languages (e.g. C and Fortran).

See The Design and Evolution of C++ for more design rationale.

answered 2008-11-15T04:29:56.643

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