Namespace in C++

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• Namespace is a container for identifiers and functions.

• It puts the names of its members in a distinct space so that they don't conflict with the names in other namespaces or global namespace.

• C++ provides namespace keyword to declare namespace.

  Syntax:

  namespace identifier
   {
        //declarations
   }
                              

Rules to Create Namespace

1. The namespace definition must be done at global scope, or nested inside another namespace.

2. Namespace definition doesn't terminates with a semicolon.

3. You can use an alias name for your namespace name, for ease of use.

   Example :

   namespace MySpace{
   int a,b;
   void display();
   class MyClass{
       };
   }
   namespace ms = MySpace;
   // ms is alias for MySpace

4. You cannot create instance of namespace.

How to Use Namespace ?

There are three ways to use a namespace in C++ program,

1. Scope Resolution
2. The using directive
3. The using declaration

1. Using Scope Resolution ( : : )

Any member declared in a namespace can be explicitly specified using the namespace's name using scope resolution :: operator .

for e.g. [std::cout, std::cin]

Example:

#include<iostream>
namespace MySpace{
 void show(){
 std::cout<<"Hello!"<< std::endl;
 std::cout<<"Access Namespace";
 std::cout<<" with SRO.";
 cout<< std::endl;	
 }
}
int main() {
MySpace::show();
return 0;
}

OUTPUT:

Hello!
Access namespacce with SRO.

2. Using Directive

• The using directive allows all the names in a namespace to be used without the namespace-name as an explicit qualifier.

• using keyword allows you to import an entire namespace into your program with a global scope.

• A using directive can be placed at the top of a .cpp file (at file scope), or inside a class or function definition.

for e.g. [using namespace std;]

Example:

#include<iostream>
using namespace std;
namespace MySpace{
 void show(){
 cout<<"Hello!"<< endl;
 cout<<"Access Namespace";
 cout<<" using directive.";
 cout<< endl;	
 }
}
int main() {
using namespace MySpace;
show();
return 0;
}

OUTPUT:

Hello!
Access namespacce using directive.

3. Using Declaration

• The using directive imports all the names in namespace.

• if you are just using one or two identifiers, then consider a using declaration to only bring those identifiers into scope and not all the identifiers in the namespace.

• A using directive can be placed at the top of a .cpp file (at file scope), or inside a class or function definition.

for e.g. [using std::cout;]

Example:

#include<iostream>
using std::cout;
namespace MySpace{
 void show(){
/*here cout is using direct,
but to use endl, usin SRO
*/
 cout<<"Hello!"<< std::endl;
 cout<<"Access Namespace";
 cout<<" using declaration.";
 cout<< std::endl;	
 }
}
int main() {
using namespace MySpace;
show();
return 0;
}

OUTPUT:

Hello!
Access namespacce using declaration.

Nested Namespace:

Namespace can be nested where you can define one namespace inside another namespace.

namespace outer_namespace{
  //members declaration
    namespace nested_namespace{
     //member declaration
    }
}

You can access member of nested namespace by using scope resolution operator as follows:

using namespace outer_namespace::nested_namespace;

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Anonymous Namespace:

You can create an explicit namespace without providing any name to it.

namespace
 {
   int MyFunc(){}    
 }

This is called an unnamed or anonymous namespace.

Next topic is Aliasing or reference variable

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