what does "using namespace std" mean?

[sharadbags]

In many programs
using VC++ compilers
i have seen this thing

[Artem]

In many programs
using VC++ compilers
i have seen this thing

There are a lot of libraries in c++. In some there may be functions with the same name. To distinguish them namespaces are used.
for example, you create a class library:

namespace myn
{
 class cin
 {
  puclic:
    int c(void);
 }
}

Now you can call your function and be sure that compiler understand that it is your class cin and not a standard input stream:

#include <iostream>
...
int kk = myn::cin.c(); // your function
char c = std:: cin.getc(); // standard cin has 'std' namespace
...

You can use namespaces as written in prev. example, or you add line

using namespace std;

and write everywhere without 'std::'. Like here:

#include <iostream>
using namespace std;
...
char c = cin.getc();
cin >> c;

for more info, look msdn ;-)