Re: Composition versus Implementation Inheritance

Most modern sources on C++ design say that composition is usually
preferable to implementation inheritance because inheritance tends to
lead to problems down the road unless it follows a strict model
allowing substitution, where each derived class has an "is-a"
relationship with the base class.

I was wondering, however, if implementation inheritance (private
inheritance) might be a better idea in some cases. Consider the SGI C+
+ extension implementation of hash_set and hash_map. Both hash_set
and hash_map use composition to reuse code from an internal class.
They each have an internal hash_table class, which implements the
basic functions of a hash table. The hash_set and hash_map class use
delegation to access the functionality of the internal hash_table
class. Google's sparse hash map/hash set also uses the same design
scheme.

This is a nice clean design, except one thing about it really bothers
me. There's nothing preventing someone from instantiating an instance
of the internal hash_table class somewhere else. Unlike an abstract
base class, or a base class with a protected constructor, the internal
hash_table "base" class can be instantiated anywhere, even though this
would be useless.