1<?xml version="1.0" encoding="UTF-8" standalone="no"?> 2<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"> 3<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 Stylesheets V1.75.2" /><meta name="keywords" content=" ISO C++ , library " /><link rel="home" href="../spine.html" title="The GNU C++ Library Documentation" /><link rel="up" href="using.html" title="Chapter 3. Using" /><link rel="prev" href="using_macros.html" title="Macros" /><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_macros.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="sect1" title="Namespaces"><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="sect2" title="Available Namespaces"><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. 4</p><div class="itemizedlist"><ul class="itemizedlist" type="disc"><li class="listitem"><p>std</p><p>The ISO C++ standards specify that "all library entities are defined 5within namespace std." This includes namespaces nested 6within <code class="code">namespace std</code>, such as <code class="code">namespace 7std::tr1</code>. 8</p></li><li class="listitem"><p>abi</p><p>Specified by the C++ ABI. This ABI specifies a number of type and 9function APIs supplemental to those required by the ISO C++ Standard, 10but necessary for interoperability. 11</p></li><li class="listitem"><p>__gnu_</p><p>Indicating one of several GNU extensions. Choices 12include <code class="code">__gnu_cxx</code>, <code class="code">__gnu_debug</code>, <code class="code">__gnu_parallel</code>, 13and <code class="code">__gnu_pbds</code>. 14</p></li></ul></div><p> A complete list of implementation namespaces (including namespace contents) is available in the generated source <a class="ulink" href="http://gcc.gnu.org/onlinedocs/libstdc++/latest-doxygen/namespaces.html" target="_top">documentation</a>. 15</p></div><div class="sect2" title="namespace std"><div class="titlepage"><div><div><h3 class="title"><a id="manual.intro.using.namespaces.std"></a>namespace std</h3></div></div></div><p> 16 One standard requirement is that the library components are defined 17 in <code class="code">namespace std::</code>. Thus, in order to use these types or 18 functions, one must do one of two things: 19</p><div class="itemizedlist"><ul class="itemizedlist" type="disc"><li class="listitem"><p>put a kind of <span class="emphasis"><em>using-declaration</em></span> in your source 20(either <code class="code">using namespace std;</code> or i.e. <code class="code">using 21std::string;</code>) This approach works well for individual source files, but 22should not be used in a global context, like header files. 23 </p></li><li class="listitem"><p>use a <span class="emphasis"><em>fully 24qualified name</em></span> for each library symbol 25(i.e. <code class="code">std::string</code>, <code class="code">std::cout</code>) Always can be 26used, and usually enhanced, by strategic use of typedefs. (In the 27cases where the qualified verbiage becomes unwieldy.) 28 </p></li></ul></div></div><div class="sect2" title="Using Namespace Composition"><div class="titlepage"><div><div><h3 class="title"><a id="manual.intro.using.namespaces.comp"></a>Using Namespace Composition</h3></div></div></div><p> 29Best practice in programming suggests sequestering new data or 30functionality in a sanely-named, unique namespace whenever 31possible. This is considered an advantage over dumping everything in 32the global namespace, as then name look-up can be explicitly enabled or 33disabled as above, symbols are consistently mangled without repetitive 34naming prefixes or macros, etc. 35</p><p>For instance, consider a project that defines most of its classes in <code class="code">namespace gtk</code>. It is possible to 36 adapt <code class="code">namespace gtk</code> to <code class="code">namespace std</code> by using a C++-feature called 37 <span class="emphasis"><em>namespace composition</em></span>. This is what happens if 38 a <span class="emphasis"><em>using</em></span>-declaration is put into a 39 namespace-definition: the imported symbol(s) gets imported into the 40 currently active namespace(s). For example: 41</p><pre class="programlisting"> 42namespace gtk 43{ 44 using std::string; 45 using std::tr1::array; 46 47 class Window { ... }; 48} 49</pre><p> 50 In this example, <code class="code">std::string</code> gets imported into 51 <code class="code">namespace gtk</code>. The result is that use of 52 <code class="code">std::string</code> inside namespace gtk can just use <code class="code">string</code>, without the explicit qualification. 53 As an added bonus, 54 <code class="code">std::string</code> does not get imported into 55 the global namespace. Additionally, a more elaborate arrangement can be made for backwards compatibility and portability, whereby the 56 <code class="code">using</code>-declarations can wrapped in macros that 57 are set based on autoconf-tests to either "" or i.e. <code class="code">using 58 std::string;</code> (depending on whether the system has 59 libstdc++ in <code class="code">std::</code> or not). (ideas from 60 Llewelly and Karl Nelson) 61</p></div></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="using_macros.html">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="using.html">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="using_dynamic_or_shared.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">Macros </td><td width="20%" align="center"><a accesskey="h" href="../spine.html">Home</a></td><td width="40%" align="right" valign="top"> Linking</td></tr></table></div></body></html> 62