Question

In other words if I have a class

class A
{
public:
   A() { .. }
   virtual void somemethod() { .. }
};

is it ok to write

class B : public A
{
public:
   B() { .. }
protected:
   virtual void somemethod() { .. }
};

or are there some drawbacks with this approach?

Answer 1

I would say this defeats the purpose of polymorphism, because when you write a function that accepts a polymorphic type, the derived type should work equally well with it:

void fun(A* a){
   a->somemethod();
}
...
A* a = new B();
fun(a); // Shouldn't this work?!
        // According to Liskov Principle, you are doing it wrong!
        // but really who cares, it depends on your justification
        // of a solution to the the problem at hand.

IMHO, it depends on the specific problem you are trying to solve because I don’t believe in “always” successful “best practice”.