Why is "using namespace std;" considered bad practice?


Question

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?

1
2409
5/29/2019 1:27:55 PM

Accepted 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, 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.

2037
6/14/2016 4:37:43 PM

I agree with everything Greg wrote, but I'd like to add: It can even get worse than Greg said!

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 list, sort, string, iterator, etc.) which are very likely to appear in other code, too.

If you consider this unlikely: There was a question asked here on Stack Overflow where pretty much exactly this happened (wrong function called due to omitted std:: prefix) about half a year after I gave this answer. Here is another, more recent example of such a question. So this is a real problem.


Here's one more data point: Many, many years ago, I also used to find it annoying having to prefix everything from the standard library with std::. Then I worked in a project where it was decided at the start that both using directives and declarations are banned except for function scopes. Guess what? It took most of us very few weeks to get used to writing the prefix, and after a few more weeks most of us even agreed that it actually made the code more readable. There's a reason for that: Whether you like shorter or longer prose is subjective, but the prefixes objectively add clarity to the code. Not only the compiler, but you, too, find it easier to see which identifier is referred to.

In a decade, that project grew to have several million lines of code. Since these discussions come up again and again, I once was curious how often the (allowed) function-scope using actually was used in the project. I grep'd the sources for it and only found one or two dozen places where it was used. To me this indicates that, once tried, developers don't find std:: painful enough to employ using directives even once every 100 kLoC even where it was allowed to be used.


Bottom line: Explicitly prefixing everything doesn't do any harm, takes very little getting used to, and has objective advantages. In particular, it makes the code easier to interpret by the compiler and by human readers — and that should probably be the main goal when writing code.


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