summaryrefslogtreecommitdiff
path: root/libstdc++-v3/doc/html/manual/using_namespaces.html
blob: ffd6c6f637606e6c1a88cbf8419ee441b47e5f2d (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
<?xml version="1.0" encoding="UTF-8" standalone="no"?>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.1//EN" "http://www.w3.org/TR/xhtml11/DTD/xhtml11.dtd">
<html xmlns="http://www.w3.org/1999/xhtml"><head><title>Namespaces</title><meta name="generator" content="DocBook XSL-NS Stylesheets V1.76.1"/><meta name="keywords" content="&#10;      ISO C++&#10;    , &#10;      library&#10;    "/><link rel="home" href="../spine.html" title="The GNU C++ Library"/><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 align="left"><a accesskey="p" href="using_macros.html">Prev</a> </td><th width="60%" align="center">Chapter 3. Using</th><td align="right"> <a accesskey="n" href="using_dynamic_or_shared.html">Next</a></td></tr></table><hr/></div><div class="section" title="Namespaces"><div class="titlepage"><div><div><h2 class="title"><a id="manual.intro.using.namespaces"/>Namespaces</h2></div></div></div><div class="section" title="Available Namespaces"><div class="titlepage"><div><div><h3 class="title"><a id="manual.intro.using.namespaces.all"/>Available Namespaces</h3></div></div></div><p> There are three main namespaces.
</p><div class="itemizedlist"><ul class="itemizedlist"><li class="listitem"><p>std</p><p>The ISO C++ standards specify that "all library entities are defined
within namespace std." This includes namespaces nested
within <code class="code">namespace std</code>, such as <code class="code">namespace
std::tr1</code>.
</p></li><li class="listitem"><p>abi</p><p>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.
</p></li><li class="listitem"><p>__gnu_</p><p>Indicating one of several GNU extensions. Choices
include <code class="code">__gnu_cxx</code>, <code class="code">__gnu_debug</code>, <code class="code">__gnu_parallel</code>,
and <code class="code">__gnu_pbds</code>.
</p></li></ul></div><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">documentation</a>.
</p></div><div class="section" title="namespace std"><div class="titlepage"><div><div><h3 class="title"><a id="manual.intro.using.namespaces.std"/>namespace std</h3></div></div></div><p>
      One standard requirement is that the library components are defined
      in <code class="code">namespace std::</code>. Thus, in order to use these types or
      functions, one must do one of two things:
</p><div class="itemizedlist"><ul class="itemizedlist"><li class="listitem"><p>put a kind of <span class="emphasis"><em>using-declaration</em></span> in your source
(either <code class="code">using namespace std;</code> or i.e. <code class="code">using
std::string;</code>) This approach works well for individual source files, but
should not be used in a global context, like header files.
	  </p></li><li class="listitem"><p>use a <span class="emphasis"><em>fully
qualified name</em></span> for each library symbol
(i.e. <code class="code">std::string</code>, <code class="code">std::cout</code>) Always can be
used, and usually enhanced, by strategic use of typedefs. (In the
cases where the qualified verbiage becomes unwieldy.)
	  </p></li></ul></div></div><div class="section" title="Using Namespace Composition"><div class="titlepage"><div><div><h3 class="title"><a id="manual.intro.using.namespaces.comp"/>Using Namespace Composition</h3></div></div></div><p>
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.
</p><p>For instance, consider a project that defines most of its classes in <code class="code">namespace gtk</code>. It is possible to
	adapt <code class="code">namespace gtk</code> to <code class="code">namespace std</code> by using a C++-feature called
	<span class="emphasis"><em>namespace composition</em></span>. This is what happens if
	a <span class="emphasis"><em>using</em></span>-declaration is put into a
	namespace-definition: the imported symbol(s) gets imported into the
	currently active namespace(s). For example:
</p><pre class="programlisting">
namespace gtk
{
  using std::string;
  using std::tr1::array;

  class Window { ... };
}
</pre><p>
	In this example, <code class="code">std::string</code> gets imported into
	<code class="code">namespace gtk</code>.  The result is that use of
	<code class="code">std::string</code> inside namespace gtk can just use <code class="code">string</code>, without the explicit qualification.
	As an added bonus,
	<code class="code">std::string</code> does not get imported into
	the global namespace.  Additionally, a more elaborate arrangement can be made for backwards compatibility and portability, whereby the
	<code class="code">using</code>-declarations can wrapped in macros that
	are set based on autoconf-tests to either "" or i.e. <code class="code">using
	  std::string;</code> (depending on whether the system has
	libstdc++ in <code class="code">std::</code> or not).  (ideas from
	Llewelly and Karl Nelson)
</p></div></div><div class="navfooter"><hr/><table width="100%" summary="Navigation footer"><tr><td align="left"><a accesskey="p" href="using_macros.html">Prev</a> </td><td align="center"><a accesskey="u" href="using.html">Up</a></td><td align="right"> <a accesskey="n" href="using_dynamic_or_shared.html">Next</a></td></tr><tr><td align="left" valign="top">Macros </td><td align="center"><a accesskey="h" href="../spine.html">Home</a></td><td align="right" valign="top"> Linking</td></tr></table></div></body></html>