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270 lines
10 KiB
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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0//EN">
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<META HTTP-EQUIV="Content-Type" CONTENT="text/html; charset=iso-8859-1">
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<META NAME="AUTHOR" CONTENT="pme@sources.redhat.com (Phil Edwards)">
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<META NAME="KEYWORDS" CONTENT="HOWTO, libstdc++, GCC, g++, libg++, STL">
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<META NAME="DESCRIPTION" CONTENT="HOWTO for the libstdc++ chapter 18.">
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<META NAME="GENERATOR" CONTENT="vi and eight fingers">
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<TITLE>libstdc++-v3 HOWTO: Chapter 18</TITLE>
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<LINK REL=StyleSheet HREF="../lib3styles.css">
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<!-- $Id: howto.html,v 1.7 2000/12/03 23:47:47 jsm28 Exp $ -->
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<BODY>
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<H1 CLASS="centered"><A NAME="top">Chapter 18: Library Support</A></H1>
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<P>Chapter 18 deals with the functions called and objects created
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automatically during the course of a program's existence.
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</P>
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<P>While we can't reproduce the contents of the Standard here (you need to
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get your own copy from your nation's member body; see our homepage for
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help), we can mention a couple of changes in what kind of support a C++
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program gets from the Standard Library.
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</P>
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<!-- ####################################################### -->
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<HR>
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<H1>Contents</H1>
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<UL>
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<LI><A HREF="#1">Types</A>
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<LI><A HREF="#2">Implementation properties</A>
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<LI><A HREF="#3">Start and Termination</A>
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<LI><A HREF="#4">Dynamic memory management</A>
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</UL>
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<HR>
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<!-- ####################################################### -->
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<H2><A NAME="1">Types</A></H2>
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<P>All the types that you're used to in C are here in one form or
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another. The only change that might affect people is the type of
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NULL: while it is required to be a macro, the definition of that
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macro is <EM>not</EM> allowed to be <TT>(void*)0</TT>, which is
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often used in C.
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</P>
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<P>In g++, NULL is #define'd to be <TT>__null</TT>, a magic keyword
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extension of g++.
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</P>
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<P>The biggest problem of #defining NULL to be something like
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"0L" is that the compiler will view that as a long integer
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before it views it as a pointer, so overloading won't do what you
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expect. (This is why g++ has a magic extension, so that NULL is
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always a pointer.)
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</P>
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<P>In his book
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<A HREF="http://cseng.aw.com/bookdetail.qry?ISBN=0-201-92488-9&ptype=0"><EM>Effective C++</EM></A>,
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Scott Meyers points out that the best way to solve this problem is to
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not overload on pointer-vs-integer types to begin with. He also
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offers a way to make your own magic NULL that will match pointers
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before it matches integers:
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<PRE>
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const // this is a const object...
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class {
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public:
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template<class T> // convertible to any type
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operator T*() const // of null non-member
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{ return 0; } // pointer...
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template<class C, class T> // or any type of null
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operator T C::*() const // member pointer...
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{ return 0; }
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private:
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void operator&() const; // whose address can't be
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// taken (see Item 27)...
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} NULL; // and whose name is NULL
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</PRE>(Cribbed from the published version of
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<A HREF="http://www.awlonline.com/cseng/meyerscddemo/">the
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Effective C++ CD</A>, reproduced here with permission.)
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</P>
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<P>If you aren't using g++ (why?), but you do have a compiler which
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supports member function templates, then you can use this definition
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of NULL (be sure to #undef any existing versions). It only helps if
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you actually use NULL in function calls, though; if you make a call of
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<TT>foo(0);</TT> instead of <TT>foo(NULL);</TT>, then you're back
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where you started.
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</P>
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<P><B>Added Note:</B> When we contacted Dr. Meyers to ask permission to
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print this stuff, it prompted him to run this code through current
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compilers to see what the state of the art is with respect to member
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template functions. He posted
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<A HREF="http://www.deja.com/threadmsg_md.xp?AN=644660779.1&CONTEXT=964036823.871301239">an
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article to Usenet</A> after discovering that the code above is not
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valid! Even though it has no data members, it still needs a
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user-defined constructor (which means that the class needs a type name
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after all). The ctor can have an empty body; it just needs to be
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there. (Stupid requirement? We think so too, and this will probably
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be changed in the language itself.)
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</P>
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<P>Return <A HREF="#top">to top of page</A> or
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<A HREF="../faq/index.html">to the FAQ</A>.
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</P>
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<HR>
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<H2><A NAME="2">Implementation properties</A></H2>
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<P>
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<H3><CODE><limits></CODE></H3>
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This header mainly defines traits classes to give access to various
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implementation defined-aspects of the fundamental types. The
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traits classes -- fourteen in total -- are all specilizations of the
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template class <CODE>numeric_limits</CODE> defined as follows:
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<PRE>
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template<typename T> struct class {
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static const bool is_specialized;
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static T max() throw();
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static T min() throw();
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static const int digits;
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static const int digits10;
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static const bool is_signed;
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static const bool is_integer;
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static const bool is_exact;
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static const int radix;
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static T epsilon() throw();
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static T round_error() throw();
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static const int min_exponent;
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static const int min_exponent10;
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static const int max_exponent;
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static const int max_exponent10;
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static const bool has_infinity;
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static const bool has_quiet_NaN;
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static const bool has_signaling_NaN;
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static const float_denorm_style has_denorm;
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static const bool has_denorm_loss;
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static T infinity() throw();
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static T quiet_NaN() throw();
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static T denorm_min() throw();
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static const bool is_iec559;
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static const bool is_bounded;
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static const bool is_modulo;
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static const bool traps;
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static const bool tinyness_before;
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static const float_round_style round_style;
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};</PRE>
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</P>
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<P>Return <A HREF="#top">to top of page</A> or
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<A HREF="../faq/index.html">to the FAQ</A>.
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</P>
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<HR>
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<H2><A NAME="3">Start and Termination</A></H2>
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<P>Not many changes here to <TT><cstdlib></TT> (the old stdlib.h).
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You should note that the <TT>abort()</TT> function does not call
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the destructors of automatic nor static objects, so if you're depending
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on those to do cleanup, it isn't going to happen. (The functions
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registered with <TT>atexit()</TT> don't get called either, so you
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can forget about that possibility, too.)
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</P>
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<P>The good old <TT>exit()</TT> function can be a bit funky, too, until
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you look closer. Basically, three points to remember are:
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<OL>
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<LI>Static objects are destroyed in reverse order of their creation.
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<LI>Functions registered with <TT>atexit()</TT> are called in
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reverse order of registration, once per registration call.
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(This isn't actually new.)
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<LI>The previous two actions are "interleaved," that is,
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given this code:
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<PRE>
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extern "C or C++" void f1 (void);
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extern "C or C++" void f2 (void);
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static Thing obj1;
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atexit(f1);
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static Thing obj2;
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atexit(f2);
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</PRE>then at a call of <TT>exit()</TT>, f2 will be called, then
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obj2 will be destroyed, then f1 will be called, and finally obj1
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will be destroyed. If f1 or f2 allow an exception to propogate
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out of them, Bad Things happen.
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</OL>
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</P>
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<P>Return <A HREF="#top">to top of page</A> or
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<A HREF="../faq/index.html">to the FAQ</A>.
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</P>
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<HR>
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<H2><A NAME="4">Dynamic memory management</A></H2>
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<P>There are six flavors each of <TT>new</TT> and <TT>delete</TT>, so
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make certain that you're using the right ones! Here are quickie
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descriptions of <TT>new</TT>:
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<UL>
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<LI>single object form, throwing a <TT>bad_alloc</TT> on errors;
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this is what most people are used to using
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<LI>single object "nothrow" form, returning NULL on errors
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<LI>array new, throwing <TT>bad_alloc</TT> on errors
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<LI>array nothrow new, returning NULL on errors
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<LI>placement new, which does nothing (like it's supposed to)
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<LI>placement array new, which also does nothing
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</UL>
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They are distinguished by the parameters that you pass to them, like
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any other overloaded function. The six flavors of <TT>delete</TT>
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are distinguished the same way, but none of them are allowed to throw
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an exception under any circumstances anyhow. (They match up for
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completeness' sake.)
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</P>
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<P>Remember that it is perfectly okay to call <TT>delete</TT> on a
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NULL pointer! Nothing happens, by definition. That is not the
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same thing as deleting a pointer twice.
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</P>
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<P>By default, if one of the "throwing <TT>new</TT>s" can't
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allocate the memory requested, it tosses an instance of a
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<TT>bad_alloc</TT> exception (or, technically, some class derived
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from it). You can change this by writing your own function (called
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a new-handler) and then registering it with <TT>set_new_handler()</TT>:
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<PRE>
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typedef void (*PFV)(void);
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static char* safety;
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static PFV old_handler;
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void my_new_handler ()
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{
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delete[] safety;
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popup_window ("Dude, you are running low on heap memory. You
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should, like, close some windows, or something.
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The next time you run out, we're gonna burn!");
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set_new_handler (old_handler);
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return;
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}
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int main ()
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{
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safety = new char[500000];
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old_handler = set_new_handler (&my_new_handler);
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...
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}
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</PRE>
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</P>
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<P><TT>bad_alloc</TT> is derived from the base <TT>exception</TT>
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class defined in Chapter 19.
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</P>
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<P>Return <A HREF="#top">to top of page</A> or
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<A HREF="../faq/index.html">to the FAQ</A>.
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</P>
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<!-- ####################################################### -->
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<HR>
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<P CLASS="fineprint"><EM>
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Comments and suggestions are welcome, and may be sent to
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<A HREF="mailto:pme@sources.redhat.com">Phil Edwards</A> or
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<A HREF="mailto:gdr@gcc.gnu.org">Gabriel Dos Reis</A>.
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<BR> $Id: howto.html,v 1.7 2000/12/03 23:47:47 jsm28 Exp $
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</EM></P>
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