Namespace

Namespace

• Namespaces are used to group the entities like class, variables, objects, function under a name.

• The namespaces help to divide global scope into sub‒scopes, where each sub‒scope has its own name.

• Example:

#include <iostream>

using namespace std;

namespace ns1

{

     int a = 5;

}

namespace ns2

{

     char a[] = "Hello";

}

int main()

{

     cout<<ns1::a << endl;

     cout<<ns2::a << endl;

     return 0;

}

Output

5

Hello

Program Explanation:

In above program there are two different namespaces containing the variable a. The variable a in the first namespace ns1 is of type int but the variable a in the second namespace ns2 is of array of characters. The both the entities are treated separately. There is no re‒declaration error for variable a.

Keyword using

• The keyword using is used to introduce the namespace being used currently. For instance we can have

using namespace ns2

1.Std Namespaced

• All the files in the C++ standard library declare all of its entities within the std namespace. That is why in the C++ program the std namespace is declared as follows ‒

                             using namespace std;

•  This statement is used in the program which uses any entity declared in iostream.

2.Rules of Namespaces

• Following are the rules of namespaces –

1) The namespace must be defined using the keyword namespace.

2) The namespace definition must appear at the global scope, or it can be present inside another namespace(i.e. nested)

3) The definition of namespace must not be terminated with semicolon.

4) It is not allowed to create an instance of namespace.

5) There can be unnamed namespace as well.

6) The namespace definition is valid over multiple files. There is no need to redefine it across multiple files.

7) The namespace declaration do not have access specifiers (public or private)

Review Question

1. What is a namespace? How do you resolve the name conflicts using namespaces? Explain with an example.