Namespace in C++

Namespace

A program includes many identifiers defined in different scopes. Sometimes an identifier of one scope will overlap (i.e. collide) with an identifier of the same name in a different scope, potentially creating a problem. Identifier overlapping also occurs frequently in third-party libraries that happen to use the same names for global identifiers (such as functions).

To solve this problem, we use the concept of the namespace. Each namespace defines a scope where identifiers are placed. The general form of the namespace is:

namespace namespace_name 

{ 

Namespace body (variables, functions, classes, etc.)

}

To use a namespace member, either the member’s name must be qualified with the namespace name and a binary scope resolution operator (::), as in

namespace_name :: member

or a using statement must occur before the name is used. The general form in this case is

using namespace namespace_name;

In this case, all the members of the namespace namespace-name can be used directly by their names. We can also use only the specified members by using “using the statement” as follows

using namespace_name::  member;

One example of the namespace is the C++ standard library (std). All classes, functions, and templates are declared within the namespace name std.

Example:

#include<iostream>

#include<conio>

namespace TestSpace

{ 

int m;

void display(int n)

{  

std :: cout< 

}

}

void main()

{ 

TestSpace :: m = 8; 

TestSpace :: display(TestSpace :: m);  

}

The same program can be written as follows:

#include<iostream>

#include<conio>

using namespace std;

namespace TestSpace {

int m; 

void display(int n)

{  

Cout<<n; 

}

}

using namespace TestSpace;

void main()

{ 

m = 8; 

display(m);

 }

We can again write the same program as follows:

#include<iostream>

#include<conio>

using std :: cout;

namespace TestSpace

{ 

int m; 

void display(int n)

{  

cout< 

}

}

using TestSpace :: m;

using TestSpace :: display;

 void main()

{ 

m = 8; 

display(m);

}

 Note: Like in class, we can also declare a function inside the namespace and define it outside. For example,

namespace TestSpace

{ 

int m; 

void display(int);

}

void TestSpace :: display(int n)

{ 

cout<<"You entered "<<n;

}

Nesting Namespaces A namespace can be nested within another namespace as follows: namespace Outer

{

 ……

……

namespace Inner

{

…… 

 …… 

} 

……

……

}

Example:

 #include<iostream>

#include<conio>

 using namespace std;

namespace TestSpace

{

 int m = 100;  

void display()

{  

Cout<<m; 

} 

namespace InnerSpace

{  

int m = 200;  

void display()

{   

c o u t << m ;

   }

  }

}

 void main()

{ 

TestSpace :: InnerSpace :: display(); 

}

Output:

200

Alternatively, we can write

using namespace TestSpace :: InnerSpace;   

display();

Or,

using TestSpace :: InnerSpace :: display;

display();

Unnamed Namespace An unnamed namespace is one that does not have a name. Unnamed namespace members occupy global scope and are accessible in all scopes following the declaration in the file. The members, in this case, can be accessed without using any qualification. A common use of unnamed namespace is to protect global data between files. 

Example:

#include<iostream>

#include<conio>

using namespace std;

namespace

{ 

int m; 

void display(int n)

{  

Cout<<"You entered "<<n; 

} 

} void main()

{

clrscr();  cout<<"m = "; 

cin>>m;   

display(m); 

getch();

}

Using Classes in the Namespace

We can also define classes inside the namespace. For example,

#include<iostream>

#include<conio>

namespace NS

{

class rectangle

{  

private:   

int length, breadth;  

public:   

rectangle(int l, int b)

{    

length = l;    

breadth = b;   

}    

 int findarea()

{   

 return 2 * length * breadth;   

}

 };

}

void main()

{

 clrscr(); 

using namespace std; 

NS :: rectangle r1(5, 3);

 Cout<<"Area = "<<r1.findarea(); 

getche();

}

Output:

Area = 30

Alternatively, we can write

using namespace NS;

rectangle r1(5, 3);

Or

using NS :: rectangle;

rectangle r1(5, 3);