There are three main namespaces.
std
The ISO C++ standards specify that "all library entities are defined
within namespace std." This includes namespaces nested
within namespace std, such as namespace
std::chrono.
abi
Specified by the C++ ABI. This ABI specifies a number of type and function APIs supplemental to those required by the ISO C++ Standard, but necessary for interoperability.
__gnu_
Indicating one of several GNU extensions. Choices
include __gnu_cxx, __gnu_debug, __gnu_parallel,
and __gnu_pbds.
The library uses a number of inline namespaces as implementation
details that are not intended for users to refer to directly, these include
std::__detail, std::__cxx11 and std::_V2.
A complete list of implementation namespaces (including namespace contents) is available in the generated source documentation.
One standard requirement is that the library components are defined
in namespace std::. Thus, in order to use these types or
functions, one must do one of two things:
put a kind of using-declaration in your source
(either using namespace std; or i.e. using
std::string;) This approach works well for individual source files, but
should not be used in a global context, like header files.
use a fully
qualified name for each library symbol
(i.e. std::string, std::cout) Always can be
used, and usually enhanced, by strategic use of typedefs. (In the
cases where the qualified verbiage becomes unwieldy.)
Best practice in programming suggests sequestering new data or functionality in a sanely-named, unique namespace whenever possible. This is considered an advantage over dumping everything in the global namespace, as then name look-up can be explicitly enabled or disabled as above, symbols are consistently mangled without repetitive naming prefixes or macros, etc.
For instance, consider a project that defines most of its classes in namespace gtk. It is possible to
adapt namespace gtk to namespace std by using a C++-feature called
namespace composition. This is what happens if
a using-declaration is put into a
namespace-definition: the imported symbol(s) gets imported into the
currently active namespace(s). For example:
namespace gtk
{
using std::string;
using std::tr1::array;
class Window { ... };
}
In this example, std::string gets imported into
namespace gtk. The result is that use of
std::string inside namespace gtk can just use string, without the explicit qualification.
As an added bonus,
std::string does not get imported into
the global namespace. Additionally, a more elaborate arrangement can be made for backwards compatibility and portability, whereby the
using-declarations can wrapped in macros that
are set based on autoconf-tests to either "" or i.e. using
std::string; (depending on whether the system has
libstdc++ in std:: or not). (ideas from
Llewelly and Karl Nelson)