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<chapter xmlns="http://docbook.org/ns/docbook" version="5.0"
xml:id="std.support" xreflabel="Support">
<?dbhtml filename="support.html"?>
<info><title>
Support
<indexterm><primary>Support</primary></indexterm>
</title>
<keywordset>
<keyword>
ISO C++
</keyword>
<keyword>
library
</keyword>
</keywordset>
</info>
<para>
This part deals with the functions called and objects created
automatically during the course of a program's existence.
</para>
<para>
While we can't reproduce the contents of the Standard here (you
need to get your own copy from your nation's member body; see our
homepage for help), we can mention a couple of changes in what
kind of support a C++ program gets from the Standard Library.
</para>
<section xml:id="std.support.types" xreflabel="Types"><info><title>Types</title></info>
<?dbhtml filename="fundamental_types.html"?>
<section xml:id="std.support.types.fundamental" xreflabel="Fundamental Types"><info><title>Fundamental Types</title></info>
<para>
C++ has the following builtin types:
</para>
<itemizedlist>
<listitem><para>
char
</para></listitem>
<listitem><para>
signed char
</para></listitem>
<listitem><para>
unsigned char
</para></listitem>
<listitem><para>
signed short
</para></listitem>
<listitem><para>
signed int
</para></listitem>
<listitem><para>
signed long
</para></listitem>
<listitem><para>
unsigned short
</para></listitem>
<listitem><para>
unsigned int
</para></listitem>
<listitem><para>
unsigned long
</para></listitem>
<listitem><para>
bool
</para></listitem>
<listitem><para>
wchar_t
</para></listitem>
<listitem><para>
float
</para></listitem>
<listitem><para>
double
</para></listitem>
<listitem><para>
long double
</para></listitem>
</itemizedlist>
<para>
These fundamental types are always available, without having to
include a header file. These types are exactly the same in
either C++ or in C.
</para>
<para>
Specializing parts of the library on these types is prohibited:
instead, use a POD.
</para>
</section>
<section xml:id="std.support.types.numeric_limits" xreflabel="Numeric Properties"><info><title>Numeric Properties</title></info>
<para>
The header <filename class="headerfile">limits</filename> defines
traits classes to give access to various implementation
defined-aspects of the fundamental types. The traits classes --
fourteen in total -- are all specializations of the template class
<classname>numeric_limits</classname>, documented <link xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="http://gcc.gnu.org/onlinedocs/libstdc++/latest-doxygen/a00593.html">here</link>
and defined as follows:
</para>
<programlisting>
template<typename T>
struct class
{
static const bool is_specialized;
static T max() throw();
static T min() throw();
static const int digits;
static const int digits10;
static const bool is_signed;
static const bool is_integer;
static const bool is_exact;
static const int radix;
static T epsilon() throw();
static T round_error() throw();
static const int min_exponent;
static const int min_exponent10;
static const int max_exponent;
static const int max_exponent10;
static const bool has_infinity;
static const bool has_quiet_NaN;
static const bool has_signaling_NaN;
static const float_denorm_style has_denorm;
static const bool has_denorm_loss;
static T infinity() throw();
static T quiet_NaN() throw();
static T denorm_min() throw();
static const bool is_iec559;
static const bool is_bounded;
static const bool is_modulo;
static const bool traps;
static const bool tinyness_before;
static const float_round_style round_style;
};
</programlisting>
</section>
<section xml:id="std.support.types.null" xreflabel="NULL"><info><title>NULL</title></info>
<para>
The only change that might affect people is the type of
<constant>NULL</constant>: while it is required to be a macro,
the definition of that macro is <emphasis>not</emphasis> allowed
to be <constant>(void*)0</constant>, which is often used in C.
</para>
<para>
For <command>g++</command>, <constant>NULL</constant> is
<code>#define</code>'d to be
<constant>__null</constant>, a magic keyword extension of
<command>g++</command>.
</para>
<para>
The biggest problem of #defining <constant>NULL</constant> to be
something like <quote>0L</quote> is that the compiler will view
that as a long integer before it views it as a pointer, so
overloading won't do what you expect. (This is why
<command>g++</command> has a magic extension, so that
<constant>NULL</constant> is always a pointer.)
</para>
<para>In his book <link xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="http://www.awprofessional.com/titles/0-201-92488-9/"><emphasis>Effective
C++</emphasis></link>, Scott Meyers points out that the best way
to solve this problem is to not overload on pointer-vs-integer
types to begin with. He also offers a way to make your own magic
<constant>NULL</constant> that will match pointers before it
matches integers.
</para>
<para>See
<link xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="http://www.awprofessional.com/titles/0-201-31015-5/">the
Effective C++ CD example</link>
</para>
</section>
</section>
<section xml:id="std.support.memory" xreflabel="Dynamic Memory"><info><title>Dynamic Memory</title></info>
<?dbhtml filename="dynamic_memory.html"?>
<para>
There are six flavors each of <function>new</function> and
<function>delete</function>, so make certain that you're using the right
ones. Here are quickie descriptions of <function>new</function>:
</para>
<itemizedlist>
<listitem><para>
single object form, throwing a
<classname>bad_alloc</classname> on errors; this is what most
people are used to using
</para></listitem>
<listitem><para>
Single object "nothrow" form, returning NULL on errors
</para></listitem>
<listitem><para>
Array <function>new</function>, throwing
<classname>bad_alloc</classname> on errors
</para></listitem>
<listitem><para>
Array nothrow <function>new</function>, returning
<constant>NULL</constant> on errors
</para></listitem>
<listitem><para>
Placement <function>new</function>, which does nothing (like
it's supposed to)
</para></listitem>
<listitem><para>
Placement array <function>new</function>, which also does
nothing
</para></listitem>
</itemizedlist>
<para>
They are distinguished by the parameters that you pass to them, like
any other overloaded function. The six flavors of <function>delete</function>
are distinguished the same way, but none of them are allowed to throw
an exception under any circumstances anyhow. (They match up for
completeness' sake.)
</para>
<para>
Remember that it is perfectly okay to call <function>delete</function> on a
NULL pointer! Nothing happens, by definition. That is not the
same thing as deleting a pointer twice.
</para>
<para>
By default, if one of the <quote>throwing <function>new</function>s</quote> can't
allocate the memory requested, it tosses an instance of a
<classname>bad_alloc</classname> exception (or, technically, some class derived
from it). You can change this by writing your own function (called a
new-handler) and then registering it with <function>set_new_handler()</function>:
</para>
<programlisting>
typedef void (*PFV)(void);
static char* safety;
static PFV old_handler;
void my_new_handler ()
{
delete[] safety;
popup_window ("Dude, you are running low on heap memory. You
should, like, close some windows, or something.
The next time you run out, we're gonna burn!");
set_new_handler (old_handler);
return;
}
int main ()
{
safety = new char[500000];
old_handler = set_new_handler (&my_new_handler);
...
}
</programlisting>
<para>
<classname>bad_alloc</classname> is derived from the base <classname>exception</classname>
class defined in Sect1 19.
</para>
</section>
<section xml:id="std.support.termination" xreflabel="Termination"><info><title>Termination</title></info>
<?dbhtml filename="termination.html"?>
<section xml:id="support.termination.handlers" xreflabel="Termination Handlers"><info><title>Termination Handlers</title></info>
<para>
Not many changes here to <filename class="headerfile">cstdlib</filename>. You should note that the
<function>abort()</function> function does not call the
destructors of automatic nor static objects, so if you're
depending on those to do cleanup, it isn't going to happen.
(The functions registered with <function>atexit()</function>
don't get called either, so you can forget about that
possibility, too.)
</para>
<para>
The good old <function>exit()</function> function can be a bit
funky, too, until you look closer. Basically, three points to
remember are:
</para>
<orderedlist inheritnum="ignore" continuation="restarts">
<listitem>
<para>
Static objects are destroyed in reverse order of their creation.
</para>
</listitem>
<listitem>
<para>
Functions registered with <function>atexit()</function> are called in
reverse order of registration, once per registration call.
(This isn't actually new.)
</para>
</listitem>
<listitem>
<para>
The previous two actions are <quote>interleaved,</quote> that is,
given this pseudocode:
</para>
<programlisting>
extern "C or C++" void f1 (void);
extern "C or C++" void f2 (void);
static Thing obj1;
atexit(f1);
static Thing obj2;
atexit(f2);
</programlisting>
<para>
then at a call of <function>exit()</function>,
<varname>f2</varname> will be called, then
<varname>obj2</varname> will be destroyed, then
<varname>f1</varname> will be called, and finally
<varname>obj1</varname> will be destroyed. If
<varname>f1</varname> or <varname>f2</varname> allow an
exception to propagate out of them, Bad Things happen.
</para>
</listitem>
</orderedlist>
<para>
Note also that <function>atexit()</function> is only required to store 32
functions, and the compiler/library might already be using some of
those slots. If you think you may run out, we recommend using
the <function>xatexit</function>/<function>xexit</function> combination from <literal>libiberty</literal>, which has no such limit.
</para>
</section>
<section xml:id="support.termination.verbose" xreflabel="Verbose Terminate Handler"><info><title>Verbose Terminate Handler</title></info>
<?dbhtml filename="verbose_termination.html"?>
<para>
If you are having difficulty with uncaught exceptions and want a
little bit of help debugging the causes of the core dumps, you can
make use of a GNU extension, the verbose terminate handler.
</para>
<programlisting>
#include <exception>
int main()
{
std::set_terminate(__gnu_cxx::__verbose_terminate_handler);
...
throw <replaceable>anything</replaceable>;
}
</programlisting>
<para>
The <function>__verbose_terminate_handler</function> function
obtains the name of the current exception, attempts to demangle
it, and prints it to stderr. If the exception is derived from
<classname>exception</classname> then the output from
<function>what()</function> will be included.
</para>
<para>
Any replacement termination function is required to kill the
program without returning; this one calls abort.
</para>
<para>
For example:
</para>
<programlisting>
#include <exception>
#include <stdexcept>
struct argument_error : public std::runtime_error
{
argument_error(const std::string& s): std::runtime_error(s) { }
};
int main(int argc)
{
std::set_terminate(__gnu_cxx::__verbose_terminate_handler);
if (argc > 5)
throw argument_error(<quote>argc is greater than 5!</quote>);
else
throw argc;
}
</programlisting>
<para>
With the verbose terminate handler active, this gives:
</para>
<screen>
<computeroutput>
% ./a.out
terminate called after throwing a `int'
Aborted
% ./a.out f f f f f f f f f f f
terminate called after throwing an instance of `argument_error'
what(): argc is greater than 5!
Aborted
</computeroutput>
</screen>
<para>
The 'Aborted' line comes from the call to
<function>abort()</function>, of course.
</para>
<para>
This is the default termination handler; nothing need be done to
use it. To go back to the previous <quote>silent death</quote>
method, simply include <filename>exception</filename> and
<filename>cstdlib</filename>, and call
</para>
<programlisting>
std::set_terminate(std::abort);
</programlisting>
<para>
After this, all calls to <function>terminate</function> will use
<function>abort</function> as the terminate handler.
</para>
<para>
Note: the verbose terminate handler will attempt to write to
stderr. If your application closes stderr or redirects it to an
inappropriate location,
<function>__verbose_terminate_handler</function> will behave in
an unspecified manner.
</para>
</section>
</section>
</chapter>
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