Item 39: Use Private Inheritance Judiciously

Private inheritance means “is-implemented-in-terms-of” (never “has-a” nor “is-a”).

Some rare use cases for private inheritance:

• Access to protected members
• Prevent derived class from redefining a private virtual function
  • For example, you inherit this private virtual function from private inheritance of another class and you don’t want your derived class to redefine it
  • Modern C++ has final keyword for just that
• EBO (empty base optimization)
  • Note that C++ forbids zero-size freestanding objects
  • But EBO might not work under multiple inheritance