What's the problem with "using namespace std;"?

I have heard using namespace std; is wrong, and that I should use std::cout and std::cin directly instead.

Why is this? Does it risk declaring variables that share the same name as something in the std namespace? Are there performance implications?

Consider 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 foo::Blah() and bar::Quux(), then the introduction of foo::Quux() would have been a non-event.

Stack Overflow Q#1452721, score: 2935