C++ FAQ Celebrating Twenty-One Years of the C++ FAQ!!!
(Click here for a personal note from Marshall Cline.)
Section 21:
21.1 Should I hide member functions that were public in my base class?
21.2 Errors trying to convert Derived** → Base**?
21.3 Is a parking-lot-of-Car a kind-of parking-lot-of-Vehicle?
21.4 Is an array of Derived a kind-of array of Base?
21.5 Does array-of-Derived is-not-a-kind-of array-of-Base mean arrays are bad?
21.6 Is a Circle a kind-of an Ellipse?
21.7 Are there other options to the "Circle is/isnot kind-of Ellipse" dilemma?
21.8 Why does this Circle-kind-of-Ellipse problem seem confusing?
21.9 Perhaps Ellipse should inherit from Circle then?
21.10 But my problem doesn't have anything to do with circles and ellipses, so what good is that silly example to me?
21.11 How could "it depend"??!? Aren't terms like "Circle" and "Ellipse" defined mathematically?
21.12 Is SortedList a kind-of List?
[21.3] Is a parking-lot-of-Car a kind-of parking-lot-of-Vehicle?

Nope.

I know it sounds strange, but it's true. You can think of this as a direct consequence of the previous FAQ, or you can reason it this way: if the kind-of relationship were valid, then someone could point a parking-lot-of-Vehicle pointer at a parking-lot-of-Car, which would allow someone to add any kind of Vehicle to a parking-lot-of-Car (assuming parking-lot-of-Vehicle has a member function like add(Vehicle&)). In other words, you could park a Bicycle, SpaceShuttle, or even a NuclearSubmarine in a parking-lot-of-Car. Certainly it would be surprising if someone accessed what they thought was a Car from the parking-lot-of-Car, only to find that it is actually a NuclearSubmarine. Gee, I wonder what the openGasCap() method would do??

Perhaps this will help: a container of Thing is not a kind-of container of Anything even if a Thing is a kind-of an Anything. Swallow hard; it's true.

You don't have to like it. But you do have to accept it.

One last example which we use in our OO/C++ training courses: "A Bag-of-Apple is not a kind-of Bag-of-Fruit." If a Bag-of-Apple could be passed as a Bag-of-Fruit, someone could put a Banana into the Bag, even though it is supposed to only contain Apples!

(Note: this FAQ has to do with public inheritance; private and protected inheritance are different.)