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howto.html: Write #5, char_traits.

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2002-10-10  Phil Edwards  <pme@gcc.gnu.org>

	* docs/html/21_strings/howto.html:  Write #5, char_traits.
	* docs/html/17_intro/porting.texi:  Expand on os_include_dir.
	* docs/html/17_intro/porting.html:  Regenerate.

From-SVN: r58031
This commit is contained in:
Phil Edwards 2002-10-10 22:00:29 +00:00
parent 9785f1d9c3
commit 64e8f36132
4 changed files with 84 additions and 15 deletions
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libstdc++-v3/ChangeLog
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@ -1,3 +1,9 @@
2002-10-10 Phil Edwards <pme@gcc.gnu.org>
* docs/html/21_strings/howto.html: Write #5, char_traits.
* docs/html/17_intro/porting.texi: Expand on os_include_dir.
* docs/html/17_intro/porting.html: Regenerate.
2002-10-09 Benjamin Kosnik <bkoz@redhat.com>
* src/locale.cc: Fix comments, move ctors together.

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libstdc++-v3/docs/html/17_intro/porting.html
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@ -68,12 +68,13 @@ directory. The important information is that there needs to be a
directory under <code>config/os</code> to store the files for your operating
system.
<p>You'll have to change the <code>configure.target</code> file to ensure that
your new directory is activated. Look for the switch statement that
sets <code>os_include_dir</code>, and add a pattern to handle your operating
system. The switch statement switches on only the OS portion of the
standard target triplet; e.g., the <code>solaris2.8</code> in
<code>sparc-sun-solaris2.8</code>.
<p>You might have to change the <code>configure.target</code> file to ensure that
your new directory is activated. Look for the switch statement that sets
<code>os_include_dir</code>, and add a pattern to handle your operating system
if the default will not suffice. The switch statement switches on only
the OS portion of the standard target triplet; e.g., the <code>solaris2.8</code>
in <code>sparc-sun-solaris2.8</code>. If the new directory is named after the
OS portion of the triplet (the default), then nothing needs to be changed.
<p>The first file to create in this directory, should be called
<code>os_defines.h</code>. This file contains basic macro definitions

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libstdc++-v3/docs/html/17_intro/porting.texi
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@ -102,12 +102,13 @@ directory. The important information is that there needs to be a
directory under @file{config/os} to store the files for your operating
system.
You'll have to change the @file{configure.target} file to ensure that
your new directory is activated. Look for the switch statement that
sets @code{os_include_dir}, and add a pattern to handle your operating
system. The switch statement switches on only the OS portion of the
standard target triplet; e.g., the @code{solaris2.8} in
@code{sparc-sun-solaris2.8}.
You might have to change the @file{configure.target} file to ensure that
your new directory is activated. Look for the switch statement that sets
@code{os_include_dir}, and add a pattern to handle your operating system
if the default will not suffice. The switch statement switches on only
the OS portion of the standard target triplet; e.g., the @code{solaris2.8}
in @code{sparc-sun-solaris2.8}. If the new directory is named after the
OS portion of the triplet (the default), then nothing needs to be changed.
The first file to create in this directory, should be called
@file{os_defines.h}. This file contains basic macro definitions

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libstdc++-v3/docs/html/21_strings/howto.html
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@ -340,15 +340,76 @@
<hr />
<h2><a name="5">Making strings of arbitrary character types</a></h2>
<p>how to work with char_traits -- in the archives, just need to
go through and pull the examples together
<p>The <code>std::basic_string</code> is tantalizingly general, in that
it is parameterized on the type of the characters which it holds.
In theory, you could whip up a Unicode character class and instantiate
<code>std::basic_string&lt;my_unicode_char&gt;</code>, or assuming
that integers are wider than characters on your platform, maybe just
declare variables of type <code>std::basic_string&lt;int&gt;</code>.
</p>
<p>That's the theory. Remember however that basic_string has additional
type parameters, which take default arguments based on the character
type (called CharT here):
<pre>
template &lt;typename CharT,
typename Traits = char_traits&lt;CharT&gt;,
typename Alloc = allocator&lt;CharT&gt; &gt;
class basic_string { .... };</pre>
Now, <code>allocator&lt;CharT&gt;</code> will probably Do The Right
Thing by default, unless you need to do something very strange with
memory allocation in your characters.
</p>
<p>But <code>char_traits</code> takes more work. The char_traits
template is <em>declared</em> but not <em>defined</em>.
That means there is only
<pre>
template &lt;typename CharT&gt;
struct char_traits
{
static void foo (type1 x, type2 y);
...
};</pre>
and functions such as char_traits&lt;CharT&gt;::foo() are not
actually defined anywhere for the general case. The C++ standard
permits this, because writing such a definition to fit all possible
CharT's cannot be done. (For a time, in earlier versions of GCC,
there was a mostly-correct implementation that let programmers be
lazy. :-) But it broke under many situations, so it was removed.
You are no longer allowed to be lazy and non-portable.)
</p>
<p>The C++ standard also requires that char_traits be specialized for
instantiations of <code>char</code> and <code>wchar_t</code>, and it
is these template specializations that permit entities like
<code>basic_string&lt;char,char_traits&lt;char&gt;&gt;</code> to work.
</p>
<p>If you want to use character types other than char and wchar_t,
such as <code>unsigned char</code> and <code>int</code>, you will
need to write specializations for them at the present time. If you
want to use your own special character class, then you have
<a href="http://gcc.gnu.org/ml/libstdc++/2002-08/msg00163.html">a lot
of work to do</a>, especially if you with to use i18n features
(facets require traits information but don't have a traits argument).
</p>
<p>One example of how to specialize char_traits is given
<a href="http://gcc.gnu.org/ml/libstdc++/2002-08/msg00260.html">in
this message</a>. We agree that the way it's used with basic_string
(scroll down to main()) doesn't look nice, but that's because
<a href="http://gcc.gnu.org/ml/libstdc++/2002-08/msg00236.html">the
nice-looking first attempt</a> turned out to
<a href="http://gcc.gnu.org/ml/libstdc++/2002-08/msg00242.html">not
be conforming C++</a>, due to the rule that CharT must be a POD.
(See how tricky this is?)
</p>
<p>Other approaches were suggested in that same thread, such as providing
more specializations and/or some helper types in the library to assist
users writing such code. So far nobody has had the time...
<a href="../17_intro/contribute.html">do you?</a>
</p>
<p>Return <a href="#top">to top of page</a> or
<a href="../faq/index.html">to the FAQ</a>.
</p>
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