C++ Namespaces

namespace namespace_name {
   // code declarations
}

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name::code;  // code could be variable or function.

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#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;
}

						

Inside first_space
Inside second_space

						
The using directive:

#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;
int main ()
{
 
   // This calls function from first name space.
   func();
   
   return 0;
}

						

Inside first_space

						

using std::cout;

						

#include <iostream>
using std::cout;

int main ()
{
 
   cout << "std::endl is used with std!" << std::endl;
   
   return 0;
}

						

std::endl is used with std!

						
Discontiguous Namespaces:

namespace namespace_name {
   // code declarations
}

						
Nested Namespaces:

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

						

// to access members of namespace_name2
using namespace namespace_name1::namespace_name2;

// to access members of namespace:name1
using namespace namespace_name1;

						

#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 first name space.
   func();
   
   return 0;
}

						

Inside second_space