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<h1 class="centered"><a name="top">Chapter 18: Library Support</a></h1>
<p>Chapter 18 deals with the functions called and objects created
automatically during the course of a program's existence.
</p>
<p>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.
</p>
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<hr />
<h1>Contents</h1>
<ul>
<li><a href="#1">Types</a></li>
<li><a href="#2">Implementation properties</a></li>
<li><a href="#3">Start and Termination</a></li>
<li><a href="#4">Verbose <code>terminate</code></a></li>
<li><a href="#5">Dynamic memory management</a></li>
<li><a href="#6">RTTI, the ABI, and demangling</a></li>
</ul>
<hr />
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<h2><a name="1">Types</a></h2>
<p>All the types that you're used to in C are here in one form or
another. The only change that might affect people is the type of
NULL: while it is required to be a macro, the definition of that
macro is <em>not</em> allowed to be <code>(void*)0</code>, which is
often used in C.
</p>
<p>In g++, NULL is #define'd to be <code>__null</code>, a magic keyword
extension of g++.
</p>
<p>The biggest problem of #defining NULL to be something like
&quot;0L&quot; 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 g++ has a magic extension, so that NULL is
always a pointer.)
</p>
<p>In his book
<a href="http://www.awprofessional.com/titles/0-201-92488-9/"><em>Effective C++</em></a>,
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 NULL that will match pointers
before it matches integers:
</p>
<pre>
const // this is a const object...
class {
public:
template&lt;class T&gt; // convertible to any type
operator T*() const // of null non-member
{ return 0; } // pointer...
template&lt;class C, class T&gt; // or any type of null
operator T C::*() const // member pointer...
{ return 0; }
private:
void operator&amp;() const; // whose address can't be
// taken (see Item 27)...
} NULL; // and whose name is NULL
</pre>
<p>(Cribbed from the published version of
<a href="http://www.awprofessional.com/titles/0-201-31015-5/">the
Effective C++ CD</a>, reproduced here with permission.)
</p>
<p>If you aren't using g++ (why?), but you do have a compiler which
supports member function templates, then you can use this definition
of NULL (be sure to #undef any existing versions). It only helps if
you actually use NULL in function calls, though; if you make a call of
<code>foo(0);</code> instead of <code>foo(NULL);</code>, then you're back
where you started.
</p>
<p><strong>Added Note:</strong> When we contacted Dr. Meyers to ask
permission to
print this stuff, it prompted him to run this code through current
compilers to see what the state of the art is with respect to member
template functions. He posted
<a href="http://groups.google.com/groups?oi=djq&selm=an_644660779">an
article to Usenet</a> after discovering that the code above is not
valid! Even though it has no data members, it still needs a
user-defined constructor (which means that the class needs a type name
after all). The ctor can have an empty body; it just needs to be
there. (Stupid requirement? We think so too, and this will probably
be changed in the language itself.)
</p>
<p>Return <a href="#top">to top of page</a> or
<a href="../faq/index.html">to the FAQ</a>.
</p>
<hr />
<h2><a name="2">Implementation properties</a></h2>
<h3><code>&lt;limits&gt;</code></h3>
<p>This header mainly 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 <code>numeric_limits</code>, documented
<a href="http://gcc.gnu.org/onlinedocs/libstdc++/latest-doxygen/structstd_1_1numeric__limits.html">here</a>
and defined as follows:
</p>
<pre>
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;
};</pre>
<p>Return <a href="#top">to top of page</a> or
<a href="../faq/index.html">to the FAQ</a>.
</p>
<hr />
<h2><a name="3">Start and Termination</a></h2>
<p>Not many changes here to <code>&lt;cstdlib&gt;</code> (the old stdlib.h).
You should note that the <code>abort()</code> 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 <code>atexit()</code> don't get called either, so you
can forget about that possibility, too.)
</p>
<p>The good old <code>exit()</code> function can be a bit funky, too, until
you look closer. Basically, three points to remember are:
</p>
<ol>
<li>Static objects are destroyed in reverse order of their creation.
</li>
<li>Functions registered with <code>atexit()</code> are called in
reverse order of registration, once per registration call.
(This isn't actually new.)
</li>
<li>The previous two actions are &quot;interleaved,&quot; that is,
given this pseudocode:
<pre>
extern "C or C++" void f1 (void);
extern "C or C++" void f2 (void);
static Thing obj1;
atexit(f1);
static Thing obj2;
atexit(f2);
</pre>
then at a call of <code>exit()</code>, f2 will be called, then
obj2 will be destroyed, then f1 will be called, and finally obj1
will be destroyed. If f1 or f2 allow an exception to propagate
out of them, Bad Things happen.
</li>
</ol>
<p>Note also that <code>atexit()</code> 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 xatexit/xexit combination from libiberty, which has no such limit.
</p>
<p>Return <a href="#top">to top of page</a> or
<a href="../faq/index.html">to the FAQ</a>.
</p>
<hr />
<h2><a name="4">Verbose <code>terminate</code></a></h2>
<p>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 in GCC 3.1 and later:
</p>
<pre>
#include &lt;exception&gt;
int main()
{
std::set_terminate(__gnu_cxx::__verbose_terminate_handler);
...
throw <em>anything</em>;
}</pre>
<p>The <code> __verbose_terminate_handler </code> function obtains the name
of the current exception, attempts to demangle it, and prints it to
stderr. If the exception is derived from <code> std::exception </code>
then the output from <code>what()</code> will be included.
</p>
<p>Any replacement termination function is required to kill the program
without returning; this one calls abort.
</p>
<p>For example:
</p>
<pre>
#include &lt;exception&gt;
#include &lt;stdexcept&gt;
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 &gt; 5)
throw argument_error(&quot;argc is greater than 5!&quot;);
else
throw argc;
}
</pre>
<p>In GCC 3.1 and later, this gives
</p>
<pre>
% ./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
%</pre>
<p>The 'Aborted' line comes from the call to abort(), of course.
</p>
<p><strong>UPDATE:</strong> Starting with GCC 3.4, this is the default
termination handler; nothing need be done to use it. To go back to
the previous &quot;silent death&quot; method, simply include
<code>&lt;exception&gt;</code> and <code>&lt;cstdlib&gt;</code>,
and call
</p>
<pre>
std::set_terminate(std::abort);</pre>
<p>Return <a href="#top">to top of page</a> or
<a href="../faq/index.html">to the FAQ</a>.
</p>
<p>
This function will attempt to write to stderr. If your application
closes stderr or redirects it to an inappropriate location,
<code>__verbose_terminate_handler</code> will behave in an
unspecified manner.
</p>
<hr />
<h2><a name="5">Dynamic memory management</a></h2>
<p>There are six flavors each of <code>new</code> and
<code>delete</code>, so make certain that you're using the right
ones! Here are quickie descriptions of <code>new</code>:
</p>
<ul>
<li>single object form, throwing a <code>bad_alloc</code> on errors;
this is what most people are used to using</li>
<li>single object &quot;nothrow&quot; form, returning NULL on errors</li>
<li>array new, throwing <code>bad_alloc</code> on errors</li>
<li>array nothrow new, returning NULL on errors</li>
<li>placement new, which does nothing (like it's supposed to)</li>
<li>placement array new, which also does nothing</li>
</ul>
<p>They are distinguished by the parameters that you pass to them, like
any other overloaded function. The six flavors of <code>delete</code>
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.)
</p>
<p>Remember that it is perfectly okay to call <code>delete</code> on a
NULL pointer! Nothing happens, by definition. That is not the
same thing as deleting a pointer twice.
</p>
<p>By default, if one of the &quot;throwing <code>new</code>s&quot; can't
allocate the memory requested, it tosses an instance of a
<code>bad_alloc</code> 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 <code>set_new_handler()</code>:
</p>
<pre>
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);
...
}
</pre>
<p><code>bad_alloc</code> is derived from the base <code>exception</code>
class defined in Chapter 19.
</p>
<p>Return <a href="#top">to top of page</a> or
<a href="../faq/index.html">to the FAQ</a>.
</p>
<hr />
<h2><a name="6">RTTI, the ABI, and demangling</a></h2>
<p>If you have read the <a href="../documentation.html#4">source
documentation</a> for <code> namespace abi </code> then you are aware
of the cross-vendor C++ ABI which we use. One of the exposed
functions is the one which we use for demangling in programs like
<code>c++filt</code>, and you can use it yourself as well.
</p>
<p>(The function itself might use different demanglers, but that's the
whole point of abstract interfaces. If we change the implementation,
you won't notice.)
</p>
<p>Probably the only times you'll be interested in demangling at runtime
are when you're seeing <code>typeid</code> strings in RTTI, or when
you're handling the runtime-support exception classes. For example:
</p>
<pre>
#include &lt;exception&gt;
#include &lt;iostream&gt;
#include &lt;cxxabi.h&gt;
struct empty { };
template &lt;typename T, int N&gt;
struct bar { };
int main()
{
int status;
char *realname;
// exception classes not in &lt;stdexcept&gt;, thrown by the implementation
// instead of the user
std::bad_exception e;
realname = abi::__cxa_demangle(e.what(), 0, 0, &amp;status);
std::cout &lt;&lt; e.what() &lt;&lt; "\t=&gt; " &lt;&lt; realname &lt;&lt; "\t: " &lt;&lt; status &lt;&lt; '\n';
free(realname);
// typeid
bar&lt;empty,17&gt; u;
const std::type_info &amp;ti = typeid(u);
realname = abi::__cxa_demangle(ti.name(), 0, 0, &amp;status);
std::cout &lt;&lt; ti.name() &lt;&lt; "\t=&gt; " &lt;&lt; realname &lt;&lt; "\t: " &lt;&lt; status &lt;&lt; '\n';
free(realname);
return 0;
}</pre>
<p>With GCC 3.1 and later, this prints
</p>
<pre>
St13bad_exception =&gt; std::bad_exception : 0
3barI5emptyLi17EE =&gt; bar&lt;empty, 17&gt; : 0 </pre>
<p>The demangler interface is described in the source documentation
linked to above. It is actually written in C, so you don't need to
be writing C++ in order to demangle C++. (That also means we have to
use crummy memory management facilities, so don't forget to free()
the returned char array.)
</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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