c++ - Why is "using namespace std;" considered bad practice?

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I've been told by others that writing
using namespace std; in code is wrong, and that I should use
std::cout and std::cin directly
instead.

Why is using namespace
std; considered a bad practice? Is it inefficient or does it risk declaring
ambiguous variables (variables that share the same name as a function in
std namespace)? Does it impact
performance?

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class="normal">Answer

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