What requires me to declare “using namespace std;”?

Question

This question may be a duplicate, but I can\'t find a good answer. Short and simple, what requires me to declare

using namespace std;

in C+

Answer 1

The ability to refer to members in the std namespace without the need to refer to std::member explicitly. For example:

#include <iostream>
using namespace std;

...
cout << "Hi" << endl;

vs.

#include <iostream>

...
std::cout << "Hi" << std::endl;

Answer 2

It's used whenever you're using something that is declared within a namespace. The C++ standard library is declared within the namespace std. Therefore you have to do

using namespace std;

unless you want to specify the namespace when calling functions within another namespace, like so:

std::cout << "cout is declared within the namespace std";

You can read more about it at http://www.cplusplus.com/doc/tutorial/namespaces/.

Answer 3

All the files in the C++ standard library declare all of its entities within the std namespace.
e.g: To use cin,cout defined in iostream

Alternatives:

using std::cout;
using std::endl;
cout << "Hello" << endl;

std::cout << "Hello" << std::endl;

Answer 4

Since the C++ standard has been accepted, practically all of the standard library is inside the std namespace. So if you don't want to qualify all standard library calls with std::, you need to add the using directive.

However,

using namespace std;

is considered a bad practice because you are practically importing the whole standard namespace, thus opening up a lot of possibilities for name clashes. It is better to import only the stuff you are actually using in your code, like

using std::string;

Answer 5

Nothing requires you to do -- unless you are implementer of C++ Standard Library and you want to avoid code duplication when declaring header files in both "new" and "old" style:

// cstdio
namespace std
{
  // ...
  int printf(const char* ...);
  // ...
}

.

// stdio.h
#include <cstdio>
using namespace std;

Well, of course example is somewhat contrived (you could equally well use plain <stdio.h> and put it all in std in <cstdio>), but Bjarne Stroustrup shows this example in his The C++ Programming Language.

Answer 6

You should definitely not say:

using namespace std;

in your C++ headers, because that beats the whole point of using namespaces (doing that would constitute "namespace pollution"). Some useful resources on this topic are the following:

1) stackoverflow thread on Standard convention for using “std”

2) an article by Herb Sutter on Migrating to Namespaces

3) FAQ 27.5 from Marshall Cline's C++ Faq lite.