using-directive

using-directive allows referencing identifiers present in a namespace without qualification.

We can use using namespace std statement to directly refer the names defined in std namespace.

#include <iostream>
 
using namespace std;
 
int b{1};
 
int main()
{
  cout << ::b;
  return 0;
}

However, we should avoid using it as it defeats the purpose of having namespaces in c++.

References

1. https://www.learncpp.com/cpp-tutorial/using-declarations-and-using-directives/