git://gcc.gnu.org / gcc.git / blame
| Commit | Line | Data |
|---|---|---|
| 46abada0 BK |
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 V1.73.2" /><meta name="keywords" content=" ISO C++ , library " /><link rel="start" href="../spine.html" title="The GNU C++ Library Documentation" /><link rel="up" href="using.html" title="Chapter 3. Using" /><link rel="prev" href="bk01pt01ch03s02.html" title="Headers" /><link rel="next" href="bk01pt01ch03s04.html" title="Macros" /></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="bk01pt01ch03s02.html">Prev</a> </td><th width="60%" align="center">Chapter 3. Using</th><td width="20%" align="right"> <a accesskey="n" href="bk01pt01ch03s04.html">Next</a></td></tr></table><hr /></div><div class="sect1" lang="en" xml:lang="en"><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" lang="en" xml:lang="en"><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 type="disc"><li><p>std</p><p>The ISO C++ standards specify that "all library entities are defined | |
| 4394b61e | 5 | within namespace std." This includes namespaces nested |
| 46abada0 BK |
6 | within <code class="code">namespace std</code>, such as <code class="code">namespace |
| 7 | std::tr1</code>. | |
| 8 | </p></li><li><p>abi</p><p>Specified by the C++ ABI. This ABI specifies a number of type and | |
| 9 | function APIs supplemental to those required by the ISO C++ Standard, | |
| 10 | but necessary for interoperability. | |
| 11 | </p></li><li><p>__gnu_</p><p>Indicating one of several GNU extensions. Choices | |
| 12 | include <code class="code">__gnu_cxx</code>, <code class="code">__gnu_debug</code>, <code class="code">__gnu_parallel</code>, | |
| 13 | and <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" lang="en" xml:lang="en"><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 type="disc"><li><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 | |
| 21 | std::string;</code>) This approach works well for individual source files, but | |
| 22 | should not be used in a global context, like header files. | |
| 23 | </p></li><li><p>use a <span class="emphasis"><em>fully | |
| 24 | qualified 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 | |
| 26 | used, and usually enhanced, by strategic use of typedefs. (In the | |
| 27 | cases where the qualified verbiage becomes unwieldy.) | |
| 28 | </p></li></ul></div></div><div class="sect2" lang="en" xml:lang="en"><div class="titlepage"><div><div><h3 class="title"><a id="manual.intro.using.namespaces.comp"></a>Using Namespace Composition</h3></div></div></div><p> | |
| 29 | Best practice in programming suggests sequestering new data or | |
| 30 | functionality in a sanely-named, unique namespace whenever | |
| 31 | possible. This is considered an advantage over dumping everything in | |
| 32 | the global namespace, as then name look-up can be explicitly enabled or | |
| 33 | disabled as above, symbols are consistently mangled without repetitive | |
| 34 | naming 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"> | |
| 42 | namespace 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 | <code class="email"><<a class="email" href="mailto:llewelly@dbritsch.dsl.xmission.com">llewelly@dbritsch.dsl.xmission.com</a>></code>, Karl Nelson <code class="email"><<a class="email" href="mailto:kenelson@ece.ucdavis.edu">kenelson@ece.ucdavis.edu</a>></code>) | |
| 61 | </p></div></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="bk01pt01ch03s02.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="bk01pt01ch03s04.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">Headers </td><td width="20%" align="center"><a accesskey="h" href="../spine.html">Home</a></td><td width="40%" align="right" valign="top"> Macros</td></tr></table></div></body></html> |