Question

I recently came to know that in C++ pure virtual functions can optionally have a body.

What are the real-world use cases for such functions?

Answer 1

The classic is a pure virtual destructor:

class abstract {
  public: 
    virtual ~abstract() = 0;
};

abstract::~abstract() {}

You make it pure because there’s nothing else to make so, and you want the class to be abstract, but you have to provide an implementation nevertheless, because the derived classes’ destructors call yours explicitly. Yeah, I know, a pretty silly textbook example, but as such it’s a classic. It must have been in the first edition of The C++ Programming Language.

Anyway, I can’t remember ever really needing the ability to implement a pure virtual function. To me it seems the only reason this feature is there is because it would have had to be explicitly disallowed and Stroustrup didn’t see a reason for that.

If you ever feel you need this feature, you’re probably on the wrong track with your design.