A nested class is a class which is declared in another enclosing class. A nested class is a member and as such has the same access rights as any other member. The members of an enclosing class have no special access to members of a nested class; the usual access rules shall be obeyed.

In C++, a nested class is a class declared inside another class. Nested classes have access to the members of the enclosing class, just like any other member of that class. They can be declared as private, public, or protected, depending on their intended usage. There are two types of nested classes in C++: static nested classes and inner classes.

Static nested classes are declared inside the enclosing class, but they are independent of any particular instance of the enclosing class. They can be accessed using the scope resolution operator (::) with the name of the enclosing class as the qualifier. Static nested classes are often used to group related functionality together, but they can also be used to hide implementation details from users of the enclosing class.

Inner classes, on the other hand, are associated with a particular instance of the enclosing class. They can access the members of the enclosing class, even if they are private, and they can be accessed using the dot operator (.) with an instance of the enclosing class as the qualifier. Inner classes are often used to implement data structures or algorithms that are closely tied to the enclosing class.

Nested classes can be useful for organizing complex code and reducing namespace pollution. They also provide a way to encapsulate implementation details and improve the modularity of a program. However, they can also increase the complexity of a program and make it harder to understand and maintain. As with any programming feature, it is important to use nested classes judiciously and only when they provide clear benefits to the design of a program.