Why is using namespace std considered bad practice

Question

Others have warned me that utilizing the namespace std; in code is incorrect, and that I should instead use std::cout and std::cin directly.

Why is it considered poor practice to use the namespace std;?

Is it inefficient, or does it risk declaring ambiguous variables (variables with the same name as a std namespace function)?

Does it have an effect on performance?

Answer 1

This has nothing to do with performance. 

However, imagine this: you're utilizing two libraries, Foo and Bar:

using namespace foo; 

 using namespace bar;

Everything is good, and you can easily use Blah() from Foo and Quux() from Bar. 

However, one day you upgrade to Foo 2.0, which now includes a feature named Quux (). 

You've created a conflict: 

Quux() is imported into your global namespace by both Foo 2.0 and Bar. 

Fixing this will take some time, especially if the function parameters are same.

The introduction of foo::Quux() would have been a non-event if you had used foo::Blah() and bar::Quux().