what is the need of declaring the base class public/private?

[johndoe444]

Hi,

class base {
   int i, j;
public:
   void set(int a, int b) { i=a; j=b; }
   void show() { cout << i << " " << j << "\n"; }
};
class derived : public base {
   int k;
public:
   derived(int x) { k=x; }
   void showk() { cout << k << "\n"; }
};

In the declaration above why is it necessary to declare base as public(or private) within the definition of class derived?

[NicAx64]

the default inheritance is private inheritance if you not explicitly mentioned.

well private inheritance is more likely an has-a relationship than the is-a relationship. and the bottom line is use private inheritance where you can't avoid and use composition where every elsewhere.

there are three object oriented concepts relate to this,
source : http://weblogs.foxite.com/andykramek/archive/2007/01/07/3096.aspx
quote--
· Decorator Modify behavior, without modifying an existing interface

· Adapter Modify interface without modifying behavior

· Wrapper Provide interface for, and services to, behavior that is defined elsewhere
quote--
and the C++ language bottom line is always use private inheritance
that you can't avoid it. ( source from C++ FAQ ).

[Bench]

I believe private inheritance more accurately represents "implemented in terms of"; which is slightly different to 'has a'; the main difference being that a privately inherited implementation is guaranteed to be initialised before a class' own private members. Something which is associated with a few interesting initialisation tricks.