Under what circumstances is it advantageous to give an implementation of a pure virtual function?

Question

In C++, it is legal to give an implementation of a pure virtual function:

class C
{
public:
  virtual int f() = 0;
};

int C::f() 
{
  return 0;
}
         


        

Answer 1

Because it's regarded as ill formed to write:

class Funct {
public:
  virtual int doit(int x) = 0;
  virtual ~Funct() = 0 {};
};

The destructor will still be called if you derive from this class. Declaring all methods pure virtual is just for clarity. You might as well write it like this:

class Funct {
public:
  virtual int doit(int x) = 0;
  virtual ~Funct() {};
};

The class will still be abstract since at least one method is pure virtual. The destructor is also still inline.