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Introduction of Namespace In C++

Namespace
 
Namespace is the collection of or group of same type of classes. Meaning of this it is collection of same type of entities.
 
To give a name to collection of classes of same type is known as namespace. Namespaces permit to collection of entities like classes, objects and functions under a name.
 
This way the global scope can be divided in “sub-scopes”, each one with its own name.
 
The format of name space is :
 
namespace identifier
{
entities
}
 
Where identifier is any valid identifier and entities is the set of classes, objects and functions that are included within the namespace. For example:
 
{
int i, j ;
}
 
In this case, the variables i and j are regular variables declared within a namespace called myName.
 
In order to contact these variables from outside the myName namespace we have to use the scope operator:: . For example, to access the previous variables from outside it may be write as
 
general : : i
genaral : : j
 
The functionality of namespaces is mostly useful in the case that there is a possibility that a global object or function uses the same identifier as another one, causing redefinition errors.
 
For Example :
 
#include
 
using namespace std;
 
namespace first
 
{
int num = 5;
}
namespace  second
{
double num = 3.1416;
}
int main () {
cout << first::num << endl;
cout << second::num << endl;
return 0 ;
}
 
Using
 
The keyword using is used to introduce a name from a namespace into the current declarative region. Using is a keyword which tells which namespace is being used.
 
For example:
 
#include
 
using namespace std;
 
namespace firstvalue{
 
{
int i = 5 ;
int j =10 ;
}
 
namespace secondvalue
{
double i = 3.1416;
double j = 2.7183;
}
int main () {
using firstvalue::x;
using secondvalue::y;
cout << i << endl;
 cout << j << endl;
cout << firstvalue::j << endl;
cout << secondvalue::i << endl;
return 0 ;
}
 
Namespace std
 
As we know namespace is logical collection of same type of entities. In every program there is use of std namespace. These namespace contains the definition of cout, cin etc
 
Every one files in the C++ standard library announce every one of its entities within the std namespace.
 
This is reason why we have normally integrated the using namespace std; statement in all programs that used any entity defined in iostream. std stands for standard.

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