How often is non-public C++ inheritance used in practice? [duplicate]

Question

Possible Duplicate:
When should I use C++ private inheritance?

I wanted to make this community-wiki but don't see the button... can someone add it?

I can't think of any case I've derived from a class in a non-public way, and I can't recall off-hand seeing code which does this.

I'd like to hear real-world examples and patterns where it is useful.

Answer 1

Your mileage may vary...

The hard-core answer would be that non-public inheritance is useless.

Personally, I use it in either of two cases:

• I would like to trigger the Empty Base Optimization if possible (usually, in template code with predicates passed as parameters)
• I would like to override a virtual function in the class

In either cases, I thus use private inheritance because the inheritance itself is an implementation detail.

I have seen people using private inheritance more liberally, and near systematically, instead of composition when writing wrappers or extending behaviors. C++ does not provide an "easy" delegate syntax, so doing so allow you to write using Base::method; to immediately provide the method instead of writing a proper forwarding call (and all its overloads). I would argue it is bad form, although it does save time.

Answer 2

If you chose inheritance for developing a wrapper, private inheritance is the way to go. You no longer need or want access to your base class' methods and members from outside your wrapper class.

class B;
class A
{
public:
   A();
   void foo(B b);
};

class BWrap;
class AWrap : private A
{
public:
   AWrap();
   void foo(BWrap b);
};

//no longer want A::foo to be accessible by mistake here, so make it private