2000-04-21 22:33:34 +02:00
|
|
|
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0//EN">
|
|
|
|
<HTML>
|
|
|
|
<HEAD>
|
|
|
|
<META HTTP-EQUIV="Content-Type" CONTENT="text/html; charset=iso-8859-1">
|
2000-07-11 23:45:08 +02:00
|
|
|
<META NAME="AUTHOR" CONTENT="pme@sources.redhat.com (Phil Edwards)">
|
configopts.html, [...]: Remove many EGCS references...
* docs/configopts.html, docs/install.html, docs/17_intro/BADNAMES,
docs/17_intro/howto.html, docs/18_support/howto.html,
docs/19_diagnostics/howto.html, docs/20_util/howto.html,
docs/21_strings/howto.html, docs/22_locale/howto.html,
docs/23_containers/howto.html, docs/24_iterators/howto.html,
docs/25_algorithms/howto.html, docs/26_numerics/howto.html,
docs/27_io/howto.html, docs/ext/howto.html, docs/faq/index.html:
Remove many EGCS references; use current absolute URLs on
gcc.gnu.org or sources.redhat.com for messages in list archives.
* docs/faq/index.txt: Regenerate.
From-SVN: r36988
2000-10-21 02:51:50 +02:00
|
|
|
<META NAME="KEYWORDS" CONTENT="HOWTO, libstdc++, GCC, g++, libg++, STL">
|
2000-04-21 22:33:34 +02:00
|
|
|
<META NAME="DESCRIPTION" CONTENT="HOWTO for the libstdc++ chapter 18.">
|
|
|
|
<META NAME="GENERATOR" CONTENT="vi and eight fingers">
|
|
|
|
<TITLE>libstdc++-v3 HOWTO: Chapter 18</TITLE>
|
|
|
|
<LINK REL=StyleSheet HREF="../lib3styles.css">
|
2000-12-10 05:04:56 +01:00
|
|
|
<!-- $Id: howto.html,v 1.7 2000/12/03 23:47:47 jsm28 Exp $ -->
|
2000-04-21 22:33:34 +02:00
|
|
|
</HEAD>
|
|
|
|
<BODY>
|
|
|
|
|
|
|
|
<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>
|
|
|
|
|
|
|
|
|
|
|
|
<!-- ####################################################### -->
|
|
|
|
<HR>
|
|
|
|
<H1>Contents</H1>
|
|
|
|
<UL>
|
|
|
|
<LI><A HREF="#1">Types</A>
|
|
|
|
<LI><A HREF="#2">Implementation properties</A>
|
|
|
|
<LI><A HREF="#3">Start and Termination</A>
|
|
|
|
<LI><A HREF="#4">Dynamic memory management</A>
|
|
|
|
</UL>
|
|
|
|
|
|
|
|
<HR>
|
|
|
|
|
|
|
|
<!-- ####################################################### -->
|
|
|
|
|
|
|
|
<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 <TT>(void*)0</TT>, which is
|
|
|
|
often used in C.
|
|
|
|
</P>
|
2000-07-07 23:13:28 +02:00
|
|
|
<P>In g++, NULL is #define'd to be <TT>__null</TT>, a magic keyword
|
|
|
|
extension of g++.
|
|
|
|
</P>
|
|
|
|
<P>The biggest problem of #defining NULL to be something like
|
|
|
|
"0L" 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://cseng.aw.com/bookdetail.qry?ISBN=0-201-92488-9&ptype=0"><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:
|
|
|
|
<PRE>
|
|
|
|
const // this is a const object...
|
|
|
|
class {
|
|
|
|
public:
|
|
|
|
template<class T> // convertible to any type
|
|
|
|
operator T*() const // of null non-member
|
|
|
|
{ return 0; } // pointer...
|
|
|
|
|
|
|
|
template<class C, class T> // or any type of null
|
|
|
|
operator T C::*() const // member pointer...
|
|
|
|
{ return 0; }
|
|
|
|
|
|
|
|
private:
|
|
|
|
void operator&() const; // whose address can't be
|
|
|
|
// taken (see Item 27)...
|
|
|
|
|
|
|
|
} NULL; // and whose name is NULL
|
|
|
|
</PRE>(Cribbed from the published version of
|
|
|
|
<A HREF="http://www.awlonline.com/cseng/meyerscddemo/">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
|
|
|
|
<TT>foo(0);</TT> instead of <TT>foo(NULL);</TT>, then you're back
|
|
|
|
where you started.
|
2000-04-21 22:33:34 +02:00
|
|
|
</P>
|
2000-07-19 22:20:51 +02:00
|
|
|
<P><B>Added Note:</B> 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://www.deja.com/threadmsg_md.xp?AN=644660779.1&CONTEXT=964036823.871301239">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>
|
2000-04-21 22:33:34 +02:00
|
|
|
<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>
|
|
|
|
<P>
|
|
|
|
<H3><CODE><limits></CODE></H3>
|
|
|
|
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 specilizations of the
|
|
|
|
template class <CODE>numeric_limits</CODE> defined as follows:
|
|
|
|
<PRE>
|
|
|
|
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;
|
|
|
|
};</PRE>
|
|
|
|
</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="3">Start and Termination</A></H2>
|
|
|
|
<P>Not many changes here to <TT><cstdlib></TT> (the old stdlib.h).
|
|
|
|
You should note that the <TT>abort()</TT> 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 <TT>atexit()</TT> don't get called either, so you
|
|
|
|
can forget about that possibility, too.)
|
|
|
|
</P>
|
|
|
|
<P>The good old <TT>exit()</TT> function can be a bit funky, too, until
|
|
|
|
you look closer. Basically, three points to remember are:
|
|
|
|
<OL>
|
|
|
|
<LI>Static objects are destroyed in reverse order of their creation.
|
|
|
|
<LI>Functions registered with <TT>atexit()</TT> are called in
|
|
|
|
reverse order of registration, once per registration call.
|
|
|
|
(This isn't actually new.)
|
|
|
|
<LI>The previous two actions are "interleaved," that is,
|
|
|
|
given this code:
|
|
|
|
<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 <TT>exit()</TT>, 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 propogate
|
|
|
|
out of them, Bad Things happen.
|
|
|
|
</OL>
|
|
|
|
</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">Dynamic memory management</A></H2>
|
|
|
|
<P>There are six flavors each of <TT>new</TT> and <TT>delete</TT>, so
|
|
|
|
make certain that you're using the right ones! Here are quickie
|
|
|
|
descriptions of <TT>new</TT>:
|
|
|
|
<UL>
|
|
|
|
<LI>single object form, throwing a <TT>bad_alloc</TT> on errors;
|
|
|
|
this is what most people are used to using
|
|
|
|
<LI>single object "nothrow" form, returning NULL on errors
|
|
|
|
<LI>array new, throwing <TT>bad_alloc</TT> on errors
|
|
|
|
<LI>array nothrow new, returning NULL on errors
|
|
|
|
<LI>placement new, which does nothing (like it's supposed to)
|
|
|
|
<LI>placement array new, which also does nothing
|
|
|
|
</UL>
|
|
|
|
They are distinguished by the parameters that you pass to them, like
|
|
|
|
any other overloaded function. The six flavors of <TT>delete</TT>
|
|
|
|
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 <TT>delete</TT> 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 "throwing <TT>new</TT>s" can't
|
|
|
|
allocate the memory requested, it tosses an instance of a
|
|
|
|
<TT>bad_alloc</TT> 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 <TT>set_new_handler()</TT>:
|
|
|
|
<PRE>
|
|
|
|
typedef void (*PFV)(void);
|
|
|
|
|
|
|
|
static char* safety;
|
|
|
|
static PFV old_handler;
|
|
|
|
|
|
|
|
void my_new_handler ()
|
|
|
|
{
|
2000-09-19 23:44:30 +02:00
|
|
|
delete[] safety;
|
2000-04-21 22:33:34 +02:00
|
|
|
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);
|
|
|
|
...
|
|
|
|
}
|
|
|
|
</PRE>
|
|
|
|
</P>
|
|
|
|
<P><TT>bad_alloc</TT> is derived from the base <TT>exception</TT>
|
|
|
|
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>
|
|
|
|
<P CLASS="fineprint"><EM>
|
|
|
|
Comments and suggestions are welcome, and may be sent to
|
2000-07-11 23:45:08 +02:00
|
|
|
<A HREF="mailto:pme@sources.redhat.com">Phil Edwards</A> or
|
configopts.html, [...]: Remove many EGCS references...
* docs/configopts.html, docs/install.html, docs/17_intro/BADNAMES,
docs/17_intro/howto.html, docs/18_support/howto.html,
docs/19_diagnostics/howto.html, docs/20_util/howto.html,
docs/21_strings/howto.html, docs/22_locale/howto.html,
docs/23_containers/howto.html, docs/24_iterators/howto.html,
docs/25_algorithms/howto.html, docs/26_numerics/howto.html,
docs/27_io/howto.html, docs/ext/howto.html, docs/faq/index.html:
Remove many EGCS references; use current absolute URLs on
gcc.gnu.org or sources.redhat.com for messages in list archives.
* docs/faq/index.txt: Regenerate.
From-SVN: r36988
2000-10-21 02:51:50 +02:00
|
|
|
<A HREF="mailto:gdr@gcc.gnu.org">Gabriel Dos Reis</A>.
|
2000-12-10 05:04:56 +01:00
|
|
|
<BR> $Id: howto.html,v 1.7 2000/12/03 23:47:47 jsm28 Exp $
|
2000-04-21 22:33:34 +02:00
|
|
|
</EM></P>
|
|
|
|
|
|
|
|
|
|
|
|
</BODY>
|
|
|
|
</HTML>
|