What is the difference between these two usage of using keyword:
using boost::shared_ptr;
and
using namespace boost;
using boost::shared_ptr;
Includes only the shared_ptr from the boost namespace in your current namespace.
This means you can use the shared_ptr without qualifying it with namespace boost.
It is called a using declaration.
using namespace boost;
Includes all the symbols in the boost namespace in your current scope.
This means you can use all the symbols in the boostnamespace without qualifying them with namespace boost.
It is called as using directive.
Why should you always prefer using declaration over using directive?
It is always better to use the first(using declaration) and avoid the second(using directive) because the second causes namespace pollution by bringing in potentially huge numbers of names in to the current namespace, many of which are unnecessary. The presence of the unnecessary names greatly increases the possibility of unintended name conflicts.
To quote Herb Sutter on the usage of using directive:
I find it helpful to think of a using directive as a marauding army of crazed barbarians that sows indiscriminate destruction wherever it passes--something that by its mere presence can cause unintended conflicts, even when you think you're allied with it.