Rule Definition
The 'using namespace' directive pulls an entire namespace into not only your header, but also any client code that uses your header. This can provoke ambiguities in client code, or even select the wrong function during overload resolution. A namespace 'using' declaration is not as polluting, but can have strange side effects since it only captures function overload sets at the time it appears. Using 'using' is a very valuable way to increase code brevity in implementation files (.cpp), but should be avoided in headers files.

Remediation
In header files, always explicitly qualify names from other namespaces.

Violation Code Sample

// In header
#include <string>
#include <vector>
using namespace std;

vector<string> split(string const &s);

Fixed Code Sample

// In header
#include <string>
#include <vector>
using namespace std;

std::vector<std::string> split(std::string const &s);

C++ coding standard, Sutter/Alexandrescu, Item 59 (http://www.gotw.ca/publications/c++cs.htm)