Namespace

Namespaces organize variable, function and class names.

#include <iostream>
using namespace std; 

namespace ns1
{
  int i = 1; 
}

int main() 
{
  cout << " I " << ns1::i << endl;
  return 1;
}

We can write 'cout' instead of 'std::cout' because of the 'using' statement.

For reasons of backward compatibility the following line

#include <iostream.h>

is replaced by these:

#include <iostream>
using namespace std;