особенно грамотная часть - про инварианты и классы/интерфейсы.
"Бриан - голова"Bjarne Stroustrup: My rule of thumb is that you should have a real class with an interface and a hidden representation if and only if you can consider an invariant for the class.
Bill Venners: What do you mean by invariant?
Bjarne Stroustrup: What is it that makes the object a valid object? An invariant allows you to say when the object's representation when it's good and when it isn't. Take a vector as a very simple example. A vector knows that it has n elements. It has a pointer to n elements. The invariant is exactly that: the pointer points to something, and that something can hold n elements. If it holds n+1 or n-1 elements, that's a bug. If that pointer is zero, it's a bug, because it doesn't point to anything. That means it's a violation of an invariant. So you have to be able to state which objects make sense. Which are good and which are bad. And you can write the interfaces so that they maintain that invariant. That's one way of keeping track that your member functions are reasonable. It's also a way of keeping track of which operations need to be member functions. Operations that don't need to mess with the representation are better done outside the class. So that you get a clean, small interface that you can understand and maintain.
