When should a virtual method be pure?

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?

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

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