Namespace in C++ Programming Language

Namespace in C++
Presented By:-
Himanshu Choudhary
(15STUJPCS0004)

Contents
Introduction
Namespace fundamentals
Namespace features
 Nested namespace
 using namespace
 Global namespace
 Standard Library std namespace
 namespaces are open
Namespace v/s class

Introduction
If a program uses classes and functions written by
different programmers, it may be that the same
name is used for different things
Namespaces help us deal with this problem

Namespace Fundamentals
• A namespace is a declarative region that provides a scope to the
identifiers (the names of types, functions, variables, etc) inside it.
• It is used to organize code into logical groups and to prevent name
collisions that can occur especially when our code base includes
multiple libraries
• Namespace provides a class-like modularization without class-like
semantics
• Obliviates the use of File Level Scoping of C (file )static

Syntax
namespace Name_Space_Name
{
Some_Code
int x=4;
}
To print value of x in
main()
cout<<Name_Space_
Name::x;

#include <iostream>
using namespace std;
namespace MyNameSpace
{
int myData; // Variable in namespace
void myFunction()
{
cout << "MyNameSpace myFunction" << endl; }
// Function in namespace
class MyClass { int data; // Class in namespace
public:
MyClass(int d) : data(d) { }
void display() { cout << "MyClass data = " << data}
};
}
int main()
{
MyNameSpace::myData = 10;
// Variable name qualified by
namespace name
cout << "MyNameSpace::myData = " <<
MyNameSpace::myData << endl;
MyNameSpace::myFunction();
// Function name qualified by
namespace name
MyNameSpace::MyClass obj(25);
// Class name qualified by
namespace name
obj.display();
return 0;
}
Example

Namespace Features
• Nested namespace
• using namespace
• Global namespace
• Standard Library std namespace
• Namespaces are open

Nested Namespace
#include <iostream>
int data = 0; // Global
namespace name1
{
int data = 1; // In namespace name1
namespace name2
{
int data = 2; // In nested namespace
} name1::name2
}
int main() {
cout << data << endl; // 0
cout << name1::data << endl; // 1
cout << name1::name2::data; // 2
cout<< endl;
return 0;
}

9
“using namespace” keyword
A using declaration (using std::cout;) makes
only one name available from the namespace
A using directive makes all the names in the
namespace available
A using directive potentially introduces a name
If ns1 and ns2 both define my_function,
using namespace ns1;
using namespace ns2;
is OK, provided my_function is never used!

 Using using namespace we can avoid lengthy prexes
#include <iostream>
namespace name1 {
int v11 = 1;
int v12 = 2;
}
namespace name2 {
int v21 = 3;
int v22 = 4;
}
using namespace name1; // All symbols of namespace
name1 will be available
using name2::v21; // Only v21 symbol of namespace
name2 will be available
int main()
{
cout << v11 << endl; // name1::v11
cout << name1::v12 << endl; // name1::v12
cout << v21 << endl; // name2::v21
cout << name2::v21 << endl; // name2::v21
cout << v22 << endl; // Treated as undefined
return 0;
}
Using “using” keyword

#include <iostream>
int data = 0; // Global Data
namespace name1
{ int data = 1; // namespace Data }
int main()
{
using name1::data;
cout << data << endl; // 1 // name1::data -- Hides global data
cout << name1::data << endl; // 1
cout << ::data << endl; // 0 // ::data -- global data
return 0; }
Global Namespace
 Items in Global namespace
may be accessed by scope
resolution operator (::)

 Entire C++ Standard Library is put in its own namespace, called std
Standard library std Namespace
Without using using std With using using std
#include <iostream>
int main()
{
int num;
std::cout << "Enter a value: " ;
std::cin >> num;
std::cout << "value is: " ;
std::cout << num ;
return 0;
}
#include <iostream>
int main()
{
int num;
cout << "Enter a value: " ;
cin >> num;
cout << "value is: " ;
cout << num ;
return 0;
}

 namespace are open: New Declarations can be added
#include <iostream>
namespace cs
{ int x = 30; }
namespace cs
{ int y = 40; }
int main() {
using namespace cs;
x = y = 20;
cout << x << " " << y ;
return 0 ;
}
Output: 20 20
Namespaces are open

Namespace Class
Every namespace is not a class Every class defines a namespace
A namespace can be reopened and more
declaration can be added to it
A class cannot be reopened
No instance of a namespace can be created A class has multiple instances
using-declarations can be used to short-cut
namespace qualication
No using-like declaration for a class
A namespace may be unnamed An unnamed class is not allowed
Namespace v/s Class

Namespace in C++ Programming Language

Namespace in C++ Programming Language

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