Overview.

namespace  namespace_name 
{
    // code declarations i.e. variable  (int a;)
    method (void add();)
    classes ( class student{};)
}

• A namespace definition begins with the keyword namespace followed by the namespace name.
• To call the namespace-enabled version of either function or variable, prepend the namespace name as follows:namespace_name::code

Nested Namespaces.

• Namespaces can be nested where you can define one namespace inside another name space as follows:

namespace namespace_name1 
    {
          // code declarations
          namespace namespace_name2 
          {
             // code declarations
          }
    }

Example

#include <iostream>
using namespace std;
 
// first name space
namespace first_space
{
  void func()
  {
     cout << "Inside first_space" << endl;
  }
  // second name space
  namespace second_space
  {
     void func()
     {
        cout << "Inside second_space" << endl;
     }
  }
}
using namespace first_space::second_space;
int main ()
{
    // This calls function from second name space.
      func();
   
      return 0;
}

Let us see how namespace scope the entities including variable and functions

#include <iostream>
using namespace std;
// first name space
namespace first_space
{
  void func()
  {
     cout << "Inside first_space" << endl;
  }
}
// second name space
namespace second_space
{
  void func()
  {
     cout << "Inside second_space" << endl;
  }
}
 
int main ()
{
       // Calls function from first name space.
      first_space :: func();
    // Calls function from second name space.
      second_space :: func();
      return 0;
}