C++ namespaces - "using" or explicitly stated? [duplicate]

Question

Possible Duplicates:
Why is ‘using namespace std;’ considered a bad practice in C++?
Using std Namespace

Is it just a matter of preference? Or is there a valid reason for preferring

using namespace std;
#include <string>
myString string;

or

#include <string>
myString std::string;

I suppose that explicitly stating the namespace each time, while a drag to type, avoids any possibility of name clashes (or would the compiler warn of ambiguity?)

Question: is there a convincing argument one way or another?

Answer 1

Personally I prefer the using declaration rather than the using directive.

For example:

#include<string>
using std::string;

string x="abc";

Using the using directive brings the entire namespace into scope which might cause name collision problems later.

For more information read this(Strongly recommended).

Answer 2

usings are fine in cpp files. You would prefer the second syntax in headers so you don't get them propagated throughout your project.