When should a virtual method be pure?

Question

I have found some code that I am working on, and was wondering what the best design implementation is.

If a base class defines a method as virtual, but implements an empty body as well, thus not requiring the derived classes to implement a body, should it not be made pure instead?

virtual void AMethod1() {}                 // 1
virtual void AMethod2() {assert(false);}   // 2
virtual void AMethod3() = 0;               // 3

1. Current code.
2. Idea1: Alerts user that this derived object has not implemented this method body.
3. Idea2: Forces derived classes to implement a body, empty or not.

What do you, the trusted amazing SO people, think?

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Edit1: After posting (and reading answers), I realize that assert is bad!

virtual void AMethod3() = {throw (ENotImplemented)};               // 4

Answer 1

It depends a bit on how "pure" your coding style is. Some people believe that you should always define an interface with pure virtual functions only and derive all concrete classes from that.

Others are more pragmatic and belive that if there is a good default implementation, you can add that to the base class (option 1).

The second option seems to be the least useful, as it delays detection until runtime. Most programmers would rather prefer a compilation error from option 3.

As usual, C++ supports several paradigms and you can choose the one you prefer.

Answer 2

You should use option 3 if the derived class must implement this method. Use option 1 if implementing in the derived class is optional. Avoid option 2 altogether.