This is not related to performance at all. But consider this: you are using two libraries called Foo and Bar:
using namespace foo; using namespace bar;
Everything works fine, and you can call
Blah() from Foo and
Quux() from Bar without problems. But one day you upgrade to a new version of Foo 2.0, which now offers a function called
Quux(). Now you've got a conflict: Both Foo 2.0 and Bar import
Quux() into your global namespace. This is going to take some effort to fix, especially if the function parameters happen to match.
If you had used
bar::Quux(), then the introduction of
foo::Quux() would have been a non-event.
Would like to add: It can even get worse!
Library Foo 2.0 could introduce a function,
Quux(), that is an unambiguously better match for some of your calls to
Quux() than the
bar::Quux() your code called for years. Then your code still compiles, but it silently calls the wrong function and does god-knows-what. That's about as bad as things can get.
Keep in mind that the
std namespace has tons of identifiers, many of which are very common ones (think
iterator, etc.) which are very likely to appear in other code, too.
Source: google "use namespace std or not" > StackoverFlow