C++ FAQ Celebrating Twenty-One Years of the C++ FAQ!!!
(Click here for a personal note from Marshall Cline.)
Section 21:
[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.)