Default behaviour implemented to pure virtual function

Question

I've read that Scott Meyers suggest default behaviour to virtual functions to be:

class base
{
.....
protected:
   void vfDefault();
public:
   virtual void vf() = 0;
};

when it is implemented in a derived class

class d1:public base
{
   virtual vf()
   {
       vfDefault();
       ....
   }
};

But it is possible also to implement the pure virtual function and use it as default behaviour:

class base
{
...
public:
   virtual void vf() = 0;
}
void base::vf()
{
 .....
};

when it is implemented in a derived class

class d1:public base
{
   virtual vf()
   {
       base::vf();
       ....
   }
};

is there any disadvatage of using a pure virtual function inmplementation for default behaviour?

Answer 1

Note that vfDefault() and vf() have different access specifiers. Everybody can call base::vf(), including directly calling base implementation. But only children of the base can call vfDefault(). So if you implement default behaviour as a separate protected function you can be sure that external code can't call it directly.