Keywords: ISO C++, library

Namespaces
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## Namespaces

### Available Namespaces

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::tr1`.

- 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`.

A complete list of implementation namespaces (including namespace contents) is available in the generated source documentation.

### namespace std

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.)

### Using Namespace Composition

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)

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