xml/manual/support.xml

<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&lt;typename T&gt;
     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.aristeia.com/books.html"><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 the
      <link xmlns:xlink="http://www.w3.org/1999/xlink"
      xlink:href="http://www.aristeia.com/books.html"><emphasis>Effective
      C++ CD</emphasis></link> example.
    </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 (&amp;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 &lt;exception&gt;

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 &lt;exception&gt;
#include &lt;stdexcept&gt;

struct argument_error : public std::runtime_error
{
  argument_error(const std::string&amp; s): std::runtime_error(s) { }
};

int main(int argc)
{
  std::set_terminate(__gnu_cxx::__verbose_terminate_handler);
  if (argc &gt; 5)
    throw argument_error("argc is greater than 5!");
  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>