Namespace Definitions

The Scope Operator(`::`)

The name of a user-declared namespace member is automatically prefixed by the name of its namespace followed by the scope operator (::). A namespace member name is qualified by its namespace name.
From the perspective of the code outside the namespace, the declaration of a namespace member is hidden. Unless we specify to the compiler in which namespace to search for a declaration, the compiler simply searches the current scope, and any scopes in which the current scope is nested, to find a declaration for the name.
Note that the scope operator can also be used to refer to members of the global namespace. Because the global namespace does not have a name, the notation ::member_name; refers to a member of the global namespace.

The declaration of a member of a nested namespace is hidden within the nested namespace. The name of such a member is automatically prefixed with the name of the outermost namespace and also with the name of the nested namespace.
when a name is used in a namespace definition, the enclosing namespaces are searched for a declaration.
An entity declared in an enclosing namespace is hidden by an entity of the same name declared in a nested namespace.

It is also possible to define any namespace member outside its namespace definition. In such a case, the name of the namespace member must be qualified by the names of its enclosing namespaces.
The definition of qualified function can use the namespace member names in their short form in any declaration or expression within the parameter list or the function body.
Only namespaces enclosing the member declaration can contain its definition.
Note that a namespace member can be defined outside its namespace definition only if the member was previously declared within the namespace definition.

One can use an unnamed namespace to declare an entity local to a file. An unnamed namespace definition begins with the keyword namespace.
Unnamed namespaces are not like other namespaces; the definition of an unnamed namespace is local to a particular file and never spans multiple text files.
It is not necessary to use the scope operator to refer to members of unnamed namespaces.
The names of the members of an unnamed namespace are visible only within a specific file and are invisible to the other files that make up the program.
A member of an unnamed namespace has properties that are similar to those of a global entity declared static.
The use of global static declarations will be replaced with the use of unnamed namespace members.