using namespace std; after #includes (was: C++ Homework. Please help :-))

using namespace std; after #includes (was: C++ Homework. Please help :-))

Post by blargg.ei » Sat, 28 Feb 2009 03:09:11


These are the two styles being compared:

#include <iostream>
using namespace std;

int main()
cout << "Hello!\n";
/* ... more uses of names in std ... */


#include <iostream>

int main()
std::cout << "Hello!\n";
/* ... more uses of names in std ... */

That is, a "using namespace std;" AFTER all #includes, and never in a
header file. What are the downsides of the second approach? The upside
is the lack of the repetitive std:: before every use of a standard name,
which can improve readability. There's no name clash cost imposed on
other clients because this is done in a source file.

One minor downside I can think of is that moving functions to the header
file for inlining will require addition of std:: prefixes. Also, not
being in the habit of regularly using std:: before all standard names
might cause one to accidentally use a member function of the same name
which is hiding the standard name at global scope.

[ See ]
[ comp.lang.c++.moderated. First time posters: Do this! ]

1. Is 'using namespace std;' valid without 'namespace std {};' first?

2. #include <iostream> & "using namespace std;"

Hi all,

I just started learning this spreme language of VC++ using
version 7.0.9037 on Visual Studio .NET (a bit buggy since
this is Beta 1). From my previous post I learnt that I
should use <iostream> instead of <iostream.h>.

Here is my VERY SIMPLE code;
#include <iostream>

using namespace std;

int main (void)
cout << "Hello World!";
return 0;

The above-mentioned code works without a hitch, but if I
take out "#include <iostream>" directive or "using
namespace std;", the error message says, "error
C2065: 'cout' : undeclared identifier". Why do I need
both? I thought "std" embodies standard input and output
libraries and therefore I don't need #include directive...

Why can I NOT find "std" header file under \Microsoft
Visual Studio.NET\Vc7\include\? All the other header files
are in this "include" folder. Is this "std" something very
special??? Where can I find it?

What is your opinion in using "using namespace std;"
extravagantly? I read some articles saying that we can use
it for a small project, but not for a big one since it
might mask global variables scope unnecessarily (or
unexpectedly), in other words, they suggest
use "std::cout". What do you think?

Thanks for your time,
Tetsuya Oguma, Sydney, Australia

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