First of all, let's make two distinctions:
1) There are using directives like using namespace std; and using declarations like using std::cout;
2) You can put using directives and declarations in either a header (.h) or an implementation file (.cpp)
Furthermore, using directives and declarations bring names into the namespace in which they're written, i.e.
namespace blah
{
using namespace std; // doesn't pollute the *global* namespace, only blah
}
Now, in terms of best practice, it's clear that putting using directives and declarations in a header file in the global namespace is a horrible no-no. People will hate you for it, because any file that includes that header will have its global namespace polluted.
Putting using directives and declarations in implementation files is somewhat more acceptable, although it may or may not make the code less clear. In general, you should prefer using declarations to using directives in such instances. My own preference is to always specify the namespace, unless it's annoyingly long (then I might be tempted).
In a header, if typing the namespace every single time is getting really tedious, you can always introduce a "local" namespace, e.g.
namespace MyLocalName
{
using namespace boost;
class X
{
// can use things from boost:: here without qualification
};
}
using MyLocalName::X; // brings X back into the global namespace
But never put using namespace boost; or the equivalent somewhere where it will drag all the stuff from Boost into the global namespace itself for other people.
