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2<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"><html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /><title>Namespaces</title><meta name="generator" content="DocBook XSL-NS Stylesheets V1.78.1" /><meta name="keywords" content="ISO C++, library" /><meta name="keywords" content="ISO C++, runtime, library" /><link rel="home" href="../index.html" title="The GNU C++ Library" /><link rel="up" href="using.html" title="Chapter��3.��Using" /><link rel="prev" href="using_dual_abi.html" title="Dual ABI" /><link rel="next" href="using_dynamic_or_shared.html" title="Linking" /></head><body><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="3" align="center">Namespaces</th></tr><tr><td width="20%" align="left"><a accesskey="p" href="using_dual_abi.html">Prev</a>��</td><th width="60%" align="center">Chapter��3.��Using</th><td width="20%" align="right">��<a accesskey="n" href="using_dynamic_or_shared.html">Next</a></td></tr></table><hr /></div><div class="section"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a id="manual.intro.using.namespaces"></a>Namespaces</h2></div></div></div><div class="section"><div class="titlepage"><div><div><h3 class="title"><a id="manual.intro.using.namespaces.all"></a>Available Namespaces</h3></div></div></div><p> There are three main namespaces.
3</p><div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem"><p>std</p><p>The ISO C++ standards specify that "all library entities are defined
4within namespace std." This includes namespaces nested
5within namespace <code class="code">std</code>, such as namespace
6<code class="code">std::chrono</code>.
7</p></li><li class="listitem"><p>abi</p><p>Specified by the C++ ABI. This ABI specifies a number of type and
8function APIs supplemental to those required by the ISO C++ Standard,
9but necessary for interoperability.
10</p></li><li class="listitem"><p>__gnu_</p><p>Indicating one of several GNU extensions. Choices
11include <code class="code">__gnu_cxx</code>, <code class="code">__gnu_debug</code>, <code class="code">__gnu_parallel</code>,
12and <code class="code">__gnu_pbds</code>.
13</p></li></ul></div><p> The library uses a number of inline namespaces as implementation
14details that are not intended for users to refer to directly, these include
15<code class="code">std::__detail</code>, <code class="code">std::__cxx11</code> and <code class="code">std::_V2</code>.
16</p><p> A complete list of implementation namespaces (including namespace contents) is available in the generated source <a class="link" href="http://gcc.gnu.org/onlinedocs/libstdc++/latest-doxygen/namespaces.html" target="_top">documentation</a>.
17</p></div><div class="section"><div class="titlepage"><div><div><h3 class="title"><a id="manual.intro.using.namespaces.std"></a>namespace std</h3></div></div></div><p>
18      One standard requirement is that the library components are defined
19      in <code class="code">namespace std::</code>. Thus, in order to use these types or
20      functions, one must do one of two things:
21</p><div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem"><p>put a kind of <span class="emphasis"><em>using-declaration</em></span> in your source
22(either <code class="code">using namespace std;</code> or i.e. <code class="code">using
23std::string;</code>) This approach works well for individual source files, but
24should not be used in a global context, like header files.
25	  </p></li><li class="listitem"><p>use a <span class="emphasis"><em>fully
26qualified name</em></span> for each library symbol
27(i.e. <code class="code">std::string</code>, <code class="code">std::cout</code>) Always can be
28used, and usually enhanced, by strategic use of typedefs. (In the
29cases where the qualified verbiage becomes unwieldy.)
30	  </p></li></ul></div></div><div class="section"><div class="titlepage"><div><div><h3 class="title"><a id="manual.intro.using.namespaces.comp"></a>Using Namespace Composition</h3></div></div></div><p>
31Best practice in programming suggests sequestering new data or
32functionality in a sanely-named, unique namespace whenever
33possible. This is considered an advantage over dumping everything in
34the global namespace, as then name look-up can be explicitly enabled or
35disabled as above, symbols are consistently mangled without repetitive
36naming prefixes or macros, etc.
37</p><p>For instance, consider a project that defines most of its classes in <code class="code">namespace gtk</code>. It is possible to
38	adapt <code class="code">namespace gtk</code> to <code class="code">namespace std</code> by using a C++-feature called
39	<span class="emphasis"><em>namespace composition</em></span>. This is what happens if
40	a <span class="emphasis"><em>using</em></span>-declaration is put into a
41	namespace-definition: the imported symbol(s) gets imported into the
42	currently active namespace(s). For example:
43</p><pre class="programlisting">
44namespace gtk
45{
46  using std::string;
47  using std::tr1::array;
48
49  class Window { ... };
50}
51</pre><p>
52	In this example, <code class="code">std::string</code> gets imported into
53	<code class="code">namespace gtk</code>.  The result is that use of
54	<code class="code">std::string</code> inside namespace gtk can just use <code class="code">string</code>, without the explicit qualification.
55	As an added bonus,
56	<code class="code">std::string</code> does not get imported into
57	the global namespace.  Additionally, a more elaborate arrangement can be made for backwards compatibility and portability, whereby the
58	<code class="code">using</code>-declarations can wrapped in macros that
59	are set based on autoconf-tests to either "" or i.e. <code class="code">using
60	  std::string;</code> (depending on whether the system has
61	libstdc++ in <code class="code">std::</code> or not).  (ideas from
62	Llewelly and Karl Nelson)
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