Is inheritance from a base class with no virtual methods bad practice?

Question

I read an answer some time back to a question regarding dynamic_cast. The dynamic_cast failed to work because the base class had no virtual methods. One of the answers said that deriving from classes with no virtual methods generally means a bad design. Is this correct? Even without taking advantage of polymorphism, I still can't see the wrongness in doing this.

Answer 1

It depends what we're talking about:

• for Traits classes (no data) it's fine (std::unary_function comes to mind)
• for private inheritance (used instead of composition to benefit from Empty Base Optimization) it's fine too

The problem comes when you starts treating such a Derived object polymorphically wrt this Base class. If you ever attain such a position, then it's definite code smell.

Note: Even when noted as fine above, you are still providing the ability to use the class polymorphically, and you thus expose yourself to subtle bugs.