Why Is "Using Namespace Std;" Considered Bad Practice

Why is using namespace std; considered bad practice?

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.

Why is using namespace std; considered bad practice?

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.

Is using namespace..like bad? [duplicate]

found this useful post elsewhere:

Namespaces separate and organize functionality. You can have a xander333::sort() function and it won't conflict with std::sort() or boost::sort() or any other sort(). Without namespaces there can be only one sort().

Now let's say you've put "using namespace std;" in all your source files and you've implemented a simple templated function called fill() in the global namespace of one of your files. This file also depends on a header from libFoo -- foo.hpp. Version 2.1 of libFoo comes out and all of a sudden your program no longer compiles. You version of fill() suddenly conflicts with another fill()! What happened?

It turns out that the folks implementing libFoo included in the new version of foo.hpp when they didn't before. Now you have all of the standard algorithms being included in your source file, and your using namespace std; has pulled them all into the global namespace. std::fill() now directly conflicts with your fill().

More insidious, you've gotten your code to compile by renaming your fill() to xander333_fill(), but something's not working right -- your report numbers are off. It turns out that your custom divides() function, which does fixed precision math, is no longer being called because the templated function from (also newly included by foo.hpp) makes for a better match because you're calling types did not exactly match the declared types.

Thread with relevant discussion is here:

http://www.cplusplus.com/forum/unices/27805/

not using namespace std; gives me an unexpected error (C++)

you need to use

std::to_string()

std:: means standard namespace

How to implement the header of a function such that the following code works?

I guess you meant the function declaration (not "header"), so that's what it should look like:

template<typename T>
T fct(const std::vector<T>& v);

PS:

1. You should not name that function "fct", but give it an appropriate name, like "max_element" or something (or use the function std::max_element provided by the standard lib)

2. Don't use using namespace std;
   see: why is 'using namespace std;' considered bad practice

3. If you're not bound to some older C++ standard (or compiler), you can omit the template parameter when calling your function: assert(fct(v1) == 6);

Ordering of using namespace std; and includes?

A perhaps interesting data point. When I compile the following:

using namespace std;
using namespace no_such_namespace;

with g++ 4.5.2, I get:

c.cpp:2:17: error: ‘no_such_namespace’ is not a namespace-name
c.cpp:2:34: error: expected namespace-name before ‘;’ token

To be clear, those two lines are the entire source file I compiled.

Neither std nor no_such_namespace has been defined as a namespace at that point, but g++ complains only about the second. I don't think there's anything special about the identifier std in the absence of a declaration of it. I think @James Kanze is right that this is a bug in g++.

EDIT: And it's been reported. (5 years ago!)

UPDATE: Now it's more than 8 years, and still hasn't been assigned to anyone, much less fixed. g++ 4.9.2 exhibits the problem. clang++ 3.5 doesn't, but it issues a warning for std and a fatal error for no_such_namespace:

c.cpp:1:17: warning: using directive refers to implicitly-defined namespace 'std'
using namespace std;
                ^
c.cpp:2:17: error: expected namespace name
using namespace no_such_namespace;
                ^
1 warning and 1 error generated.

UPDATE: As of 2021-09-24, the bug report is still open and the bug exists in g++ 11.2.0. A comment posted 2021-07-24 suggests that g++ should warn about this.

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