What is nested class and what is its use in c plus plus?

Answer 1

A nested structure is simply one structure inside another. The inner structure is local to the enclosing structure.

struct A { struct B {}; };

Here, we can instantiate an instance of A as normal.

A a;

But to instantiate B we must qualify the type because it is local to A:

A::B b;

If B were only required by A, then we can prevent users from instantiating instances of B simply by declaring it private:

struct A { private: struct B {}; };

Answer 2

A nested class is a class that is declared local to another class, the enclosing class.

Example:

class A // enclosing class

{

public:

class B {}; // public nested class

};

To access the nested class from outside of the A class, you would use the scope resolution operator, A::B.

A typical scenario where you would use a nested class is where the nested class is only of relevance to its enclosing class, and you do not wish instances of that class to be created from outside of the enclosing class. This is achieved by declaring the nested class private to the enclosing class:

class A // enclosing class

{

private:

class B {}; // private nested class

};

Since the enclosing class is also a namespace, you may also nest classes within explicit namespaces:

namespace foo

{

class A {}; // nested class

};

This allows two or more namespaces to use the same class name for entirely different purposes. For instance, the class name "cuboid" could mean the 3-dimensional shape or it could mean a 3-dimensional array. If you ever needed to use both definitions within the same program, then you can either rename one of them to remove the ambiguity, or you can simply enclose them within separate namespaces, such as shape::cube and multi_dimensional::cube.

If you wanted to use both definitions within the same program, then you must enclose at least one of these definitions in a separate namespace, such as shape::cube. Thus shape::cube is treated as being a separate entity to the cube type in the global namespace. Ideally, both should be in separate namespaces, such as multi-dimensional::cube, thus

Answer 3

A nested structure in any language is a structure within a structure.

class A {

class B {}; // nested structure

};