C++ – Purpose and Usage of ‘using namespace’

c++language-designnamespacesusing-directives

There are convincing arguments against using namespace std, so why was it introduced into the language at all? Doesn't using namespace defeat the purpose of namespaces? Why would I ever want to write using namespace? Is there any problem I am not aware of that is solved elegantly by using namespace, maybe in the lines of the using std::swap idiom or something like that?

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For one thing, this is the way to use operator overloads in a namespace (e.g using namespace std::rel_ops; or using namespace boost::assign;)

Brevity is also a strong argument. Would you really enjoy typing and reading std::placeholders::_1 instead of _1? Also, when you write code in functional style, you'll be using a myriad of objects in std and boost namespace.

Another important usage (although normally one doesn't import whole namespaces) is to enable argument-dependent look-up:

template <class T>
void smart_swap(T& a, T& b)
{
    using std::swap;
    swap(a, b);
}

If swap is overloaded for some type of T in the same namespace as T, this will use that overload. If you explicitly called std::swap instead, that overload would not be considered. For other types this falls back to std::swap.

BTW, a using declaration/directive does not defeat the purpose of namespaces, since you can always fully qualify the name in case of ambiguity.

Related Solutions

C++: Issues with ‘using namespace std;’

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.

C++ – What Does ‘using namespace’ Do Exactly?

The directive using namespace X; makes names from namespace X visible inside the namespace containing the directive. That is, when looking up a name n in that scope, X::n can be found. However, it will only be looked for if the compiler needs to look for it.

In your example, this declaration:

std::string test = "Test";

inside the global namespace makes perfect sense as-is. The name test is simply introduced, as with any other declaration. No need to look it up anywhere.

This would be an entirely different kettle of fish:

namespace X
{
  struct C
  {
    static std::string test;
  };
}

using namespace X;
std::string C::test = "Test";

In this code, the compiler needs to know what C is to make sense of the definition of C::test. It therefore does a name lookup of C, which indeed finds X::C thanks to the using directive.

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Related Solutions

C++: Issues with ‘using namespace std;’

C++ – What Does ‘using namespace’ Do Exactly?

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