using namespace std

[Arthur Lucas]

In the C programing, I always used library, but now in C++ I found this "using namespace std", and I don't know what's this.

Can you help me?

[Moschops]

http://www.cplusplus.com/doc/tutorial/namespaces/

[Galik]

I also recommend reading this:
http://www.parashift.com/c++-faq-lite/coding-standards.html#faq-27.5

using namespace std can be okay in trivial example code, but it really should be avoided in proper code. In particular it should never appear in the global scope of a header file as that 'contaminates' the global namespace of any code that includes it. It can be the cause of some rather subtle and hard to find errors.

[Arthur Lucas]

Thanks.