Pros and cons of using nested C++ classes and enumerations?

Question

What are the pros and cons of using nested public C++ classes and enumerations? For example, suppose you have a class called printer, and this class also store

Answer 1

There are no pros and cons per se of using nested public C++ classes. There are only facts. Those facts are mandated by the C++ standard. Whether a fact about nested public C++ classes is a pro or a con depends on the particular problem that you are trying to solve. The example you have given does not allow a judgement about whether nested classes are appropriate or not.

One fact about nested classes is, that they have privileged access to all members of the class that they belong to. This is a con, if the nested classes does not need such access. But if the nested class does not need such access, then it should not have been declared as a nested class. There are situations, when a class A wants to grant privileged access to certain other classes B. There are three solutions to this problem

1. Make B a friend of A
2. Make B a nested class of A
3. Make the methods and attributes, that B needs, public members of A.

In this situation, it's #3 that violates encapsulation, because A has control over his friends and over his nested classes, but not over classes that call his public methods or access his public attributes.

Another fact about nested classes is, that it is impossible to add another class A::C as a nested class of A unless you are owner of A. However, this is perfectly reasonable, because nested classes have privileged access. If it were possible to add A::C as a nested class of A, then A::C could trick A into granting access to privileged information; and that yould violate encapsulation. It's basically the same as with the friend declaration: the friend declaration does not grant you any special privileges, that your friend is hiding from others; it allows your friends to access information that you are hiding from your non-friends. In C++, calling someone a friend is an altruistic act, not an egoistic one. The same holds for allowing a class to be a nested class.

Som other facts about nested public classes:

• A's scope is candidate for symbol lookup of B: If you don't want this, make B a friend of A instead of a nested class. However, there are cases where you want exactly this kind of symbol lookup.
• A::B cannot be forward-declared: A and A::B are tightly coupled. Being able to use A::B without knowing A would only hide this fact.

To summarize this: if the tool does not fit your needs, don't blame the tool; blame yourself for using the tool; others might have different problems, for which the tool is perfect.