Why shouldn't I put “using namespace std” in a header?

Question

Someone once hinted that doing this in a header file is not advised:

using namespace std;

Why is it not advised?

Could it cause linker errors like this: (linewrapped for convenience)

error LNK2005: "public: __thiscall std::basic_string<char,struct 
std::char_traits<char>,class std::allocator<char> >::
~basic_string<char,struct std::char_traits<char>,class std::allocator<char> > 
(void)" (??1?$basic_string@DU?$char_traits@D@std@@V?$allocator@D@2@@std@@QAE@XZ) 
already defined in tools.lib(Exception.obj) 

Answer 1

Because it forces anyone who uses your header file to bring the std namespace into global scope. This could be a problem if they have a class that has the same name as one of the standard library classes.

Answer 2

If the file gets included elsewhere the compilation unit will implicitely get the using directive. This can lead to confusing errors when names overlap.