I've just came across using-directive in a context, which is new for me - e.g "moving" a member of parent class from public to private. I've tested the example below and two question arose: Is the code at (1) portable? According to standard it should be deprecated. Does it make sense at all to hide public members in subclasses? (I think in Java the access could be made only less strict in subclasses.) Code format bug class A{ public: A():_i(1975){} int _i; }; class B : public A { public: B():_i(333){} int i()const{ return _i;} private: int _i; // (1)depricated? no warning in VS2008? }; class C : public A { public: C(){} //_i initialized by A int i()const{ return _i+1;} private: using A::_i; //(2) should be the right way }; int main() { B b; int i= b.i(); //value of B::_i (333) int a_i = static_cast<A*>(&b)->_i; //value of A::_i (1975) C c; int ca_i = c.i(); //value of A::_i via getter (1975+1) //however, A::_i is still accessible a_i = static_cast<A*>(&c)->_i; //value of A::_i (1975) return 0; }

Reputation: 7448

C++: Hiding members in subclasses. Does it make sense?

I've just came across using-directive in a context, which is new for me - e.g "moving" a member of parent class from public to private. I've tested the example below and two question arose:

Is the code at (1) portable? According to standard it should be deprecated.
Does it make sense at all to hide public members in subclasses? (I think in Java the access could be made only less strict in subclasses.)

Code format bug

class A{
public:
  A():_i(1975){}
  int _i;
};

class B : public A
{
public:
  B():_i(333){}
  int i()const{ return _i;}
private:
  int _i; // (1)depricated? no warning in VS2008?
};

class C : public A
{
public:
  C(){} //_i initialized by A
  int i()const{ return _i+1;}
private:
  using A::_i; //(2) should be the right way
};

int main()
{
  B b;
  int i= b.i(); //value of B::_i (333)
  int a_i = static_cast<A*>(&b)->_i; //value of A::_i (1975)

  C c;
  int ca_i = c.i(); //value of A::_i via getter (1975+1)
  //however, A::_i is still accessible  
  a_i = static_cast<A*>(&c)->_i; //value of A::_i (1975)

  return 0;
}

Upvotes: 1

Answers (3)

Sven Marnach

Reputation: 602155

ad (1): I don't see any reason why this should be deprecated. I think it's perfectly legal C++. (The class B adds a second copy of _i which shadows the _i in A, but you can still access the latter by b.A::_i.)

ad (2): That does not seem useful at all to me, since you can still access _i in c by using c.A::_i, so it isn't hidden at all.

Upvotes: 1