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jlechner |
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<meta name="AUTHOR" content="pme@gcc.gnu.org (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: Library Support</title>
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<link rel="Start" href="../documentation.html" type="text/html"
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title="GNU C++ Standard Library" />
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<link rel="Prev" href="../17_intro/howto.html" type="text/html"
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title="Library Introduction" />
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title="Diagnostics" />
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<link rel="Help" href="../faq/index.html" type="text/html" title="F.A.Q." />
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</head>
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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></li>
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<li><a href="#2">Implementation properties</a></li>
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<li><a href="#3">Start and Termination</a></li>
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<li><a href="#4">Verbose <code>terminate</code></a></li>
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<li><a href="#5">Dynamic memory management</a></li>
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<li><a href="#6">RTTI, the ABI, and demangling</a></li>
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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 <code>(void*)0</code>, 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 <code>__null</code>, 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://www.awprofessional.com/titles/0-201-92488-9/"><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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</p>
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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>
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<p>(Cribbed from the published version of
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<a href="http://www.awprofessional.com/titles/0-201-31015-5/">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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<code>foo(0);</code> instead of <code>foo(NULL);</code>, then you're back
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where you started.
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</p>
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<p><strong>Added Note:</strong> When we contacted Dr. Meyers to ask
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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://groups.google.com/groups?oi=djq&selm=an_644660779">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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<h3><code><limits></code></h3>
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<p>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>, documented
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<a href="http://gcc.gnu.org/onlinedocs/libstdc++/latest-doxygen/structstd_1_1numeric__limits.html">here</a>
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and defined as follows:
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</p>
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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>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 <code><cstdlib></code> (the old stdlib.h).
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You should note that the <code>abort()</code> 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 <code>atexit()</code> 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 <code>exit()</code> 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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</p>
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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>
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<li>Functions registered with <code>atexit()</code> 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>
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<li>The previous two actions are "interleaved," that is,
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given this pseudocode:
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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>
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then at a call of <code>exit()</code>, 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 propagate
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out of them, Bad Things happen.
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</li>
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</ol>
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<p>Note also that <code>atexit()</code> is only required to store 32
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functions, and the compiler/library might already be using some of
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those slots. If you think you may run out, we recommend using
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the xatexit/xexit combination from libiberty, which has no such limit.
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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">Verbose <code>terminate</code></a></h2>
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<p>If you are having difficulty with uncaught exceptions and want a
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little bit of help debugging the causes of the core dumps, you can
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make use of a GNU extension in GCC 3.1 and later:
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</p>
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<pre>
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#include <exception>
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int main()
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{
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std::set_terminate(__gnu_cxx::__verbose_terminate_handler);
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...
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throw <em>anything</em>;
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}</pre>
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<p>The <code> __verbose_terminate_handler </code> function obtains the name
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of the current exception, attempts to demangle it, and prints it to
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stderr. If the exception is derived from <code> std::exception </code>
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then the output from <code>what()</code> will be included.
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</p>
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<p>Any replacement termination function is required to kill the program
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without returning; this one calls abort.
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</p>
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<p>For example:
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</p>
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<pre>
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#include <exception>
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#include <stdexcept>
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struct argument_error : public std::runtime_error
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{
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argument_error(const std::string& s): std::runtime_error(s) { }
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};
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int main(int argc)
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{
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std::set_terminate(__gnu_cxx::__verbose_terminate_handler);
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if (argc > 5)
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throw argument_error("argc is greater than 5!");
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else
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throw argc;
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}
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</pre>
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<p>In GCC 3.1 and later, this gives
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</p>
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<pre>
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% ./a.out
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terminate called after throwing a `int'
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Aborted
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% ./a.out f f f f f f f f f f f
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terminate called after throwing an instance of `argument_error'
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what(): argc is greater than 5!
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Aborted
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%</pre>
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<p>The 'Aborted' line comes from the call to abort(), of course.
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</p>
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<p><strong>UPDATE:</strong> Starting with GCC 3.4, this is the default
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termination handler; nothing need be done to use it. To go back to
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the previous "silent death" method, simply include
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<code><exception></code> and <code><cstdlib></code>,
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and call
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</p>
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<pre>
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std::set_terminate(std::abort);</pre>
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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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<p>
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This function will attempt to write to stderr. If your application
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closes stderr or redirects it to an inappropriate location,
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<code>__verbose_terminate_handler</code> will behave in an
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unspecified manner.
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</p>
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<hr />
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<h2><a name="5">Dynamic memory management</a></h2>
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<p>There are six flavors each of <code>new</code> and
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<code>delete</code>, so make certain that you're using the right
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ones! Here are quickie descriptions of <code>new</code>:
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</p>
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<ul>
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<li>single object form, throwing a <code>bad_alloc</code> on errors;
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this is what most people are used to using</li>
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<li>single object "nothrow" form, returning NULL on errors</li>
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<li>array new, throwing <code>bad_alloc</code> on errors</li>
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<li>array nothrow new, returning NULL on errors</li>
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<li>placement new, which does nothing (like it's supposed to)</li>
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<li>placement array new, which also does nothing</li>
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</ul>
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<p>They are distinguished by the parameters that you pass to them, like
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|
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any other overloaded function. The six flavors of <code>delete</code>
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|
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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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| 315 |
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<p>Remember that it is perfectly okay to call <code>delete</code> on a
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|
|
NULL pointer! Nothing happens, by definition. That is not the
|
| 317 |
|
|
same thing as deleting a pointer twice.
|
| 318 |
|
|
</p>
|
| 319 |
|
|
<p>By default, if one of the "throwing <code>new</code>s" can't
|
| 320 |
|
|
allocate the memory requested, it tosses an instance of a
|
| 321 |
|
|
<code>bad_alloc</code> exception (or, technically, some class derived
|
| 322 |
|
|
from it). You can change this by writing your own function (called a
|
| 323 |
|
|
new-handler) and then registering it with <code>set_new_handler()</code>:
|
| 324 |
|
|
</p>
|
| 325 |
|
|
<pre>
|
| 326 |
|
|
typedef void (*PFV)(void);
|
| 327 |
|
|
|
| 328 |
|
|
static char* safety;
|
| 329 |
|
|
static PFV old_handler;
|
| 330 |
|
|
|
| 331 |
|
|
void my_new_handler ()
|
| 332 |
|
|
{
|
| 333 |
|
|
delete[] safety;
|
| 334 |
|
|
popup_window ("Dude, you are running low on heap memory. You
|
| 335 |
|
|
should, like, close some windows, or something.
|
| 336 |
|
|
The next time you run out, we're gonna burn!");
|
| 337 |
|
|
set_new_handler (old_handler);
|
| 338 |
|
|
return;
|
| 339 |
|
|
}
|
| 340 |
|
|
|
| 341 |
|
|
int main ()
|
| 342 |
|
|
{
|
| 343 |
|
|
safety = new char[500000];
|
| 344 |
|
|
old_handler = set_new_handler (&my_new_handler);
|
| 345 |
|
|
...
|
| 346 |
|
|
}
|
| 347 |
|
|
</pre>
|
| 348 |
|
|
<p><code>bad_alloc</code> is derived from the base <code>exception</code>
|
| 349 |
|
|
class defined in Chapter 19.
|
| 350 |
|
|
</p>
|
| 351 |
|
|
<p>Return <a href="#top">to top of page</a> or
|
| 352 |
|
|
<a href="../faq/index.html">to the FAQ</a>.
|
| 353 |
|
|
</p>
|
| 354 |
|
|
|
| 355 |
|
|
<hr />
|
| 356 |
|
|
<h2><a name="6">RTTI, the ABI, and demangling</a></h2>
|
| 357 |
|
|
<p>If you have read the <a href="../documentation.html#4">source
|
| 358 |
|
|
documentation</a> for <code> namespace abi </code> then you are aware
|
| 359 |
|
|
of the cross-vendor C++ ABI which we use. One of the exposed
|
| 360 |
|
|
functions is the one which we use for demangling in programs like
|
| 361 |
|
|
<code>c++filt</code>, and you can use it yourself as well.
|
| 362 |
|
|
</p>
|
| 363 |
|
|
<p>(The function itself might use different demanglers, but that's the
|
| 364 |
|
|
whole point of abstract interfaces. If we change the implementation,
|
| 365 |
|
|
you won't notice.)
|
| 366 |
|
|
</p>
|
| 367 |
|
|
<p>Probably the only times you'll be interested in demangling at runtime
|
| 368 |
|
|
are when you're seeing <code>typeid</code> strings in RTTI, or when
|
| 369 |
|
|
you're handling the runtime-support exception classes. For example:
|
| 370 |
|
|
</p>
|
| 371 |
|
|
<pre>
|
| 372 |
|
|
#include <exception>
|
| 373 |
|
|
#include <iostream>
|
| 374 |
|
|
#include <cxxabi.h>
|
| 375 |
|
|
|
| 376 |
|
|
struct empty { };
|
| 377 |
|
|
|
| 378 |
|
|
template <typename T, int N>
|
| 379 |
|
|
struct bar { };
|
| 380 |
|
|
|
| 381 |
|
|
|
| 382 |
|
|
int main()
|
| 383 |
|
|
{
|
| 384 |
|
|
int status;
|
| 385 |
|
|
char *realname;
|
| 386 |
|
|
|
| 387 |
|
|
// exception classes not in <stdexcept>, thrown by the implementation
|
| 388 |
|
|
// instead of the user
|
| 389 |
|
|
std::bad_exception e;
|
| 390 |
|
|
realname = abi::__cxa_demangle(e.what(), 0, 0, &status);
|
| 391 |
|
|
std::cout << e.what() << "\t=> " << realname << "\t: " << status << '\n';
|
| 392 |
|
|
free(realname);
|
| 393 |
|
|
|
| 394 |
|
|
|
| 395 |
|
|
// typeid
|
| 396 |
|
|
bar<empty,17> u;
|
| 397 |
|
|
const std::type_info &ti = typeid(u);
|
| 398 |
|
|
|
| 399 |
|
|
realname = abi::__cxa_demangle(ti.name(), 0, 0, &status);
|
| 400 |
|
|
std::cout << ti.name() << "\t=> " << realname << "\t: " << status << '\n';
|
| 401 |
|
|
free(realname);
|
| 402 |
|
|
|
| 403 |
|
|
return 0;
|
| 404 |
|
|
}</pre>
|
| 405 |
|
|
<p>With GCC 3.1 and later, this prints
|
| 406 |
|
|
</p>
|
| 407 |
|
|
<pre>
|
| 408 |
|
|
St13bad_exception => std::bad_exception : 0
|
| 409 |
|
|
3barI5emptyLi17EE => bar<empty, 17> : 0 </pre>
|
| 410 |
|
|
<p>The demangler interface is described in the source documentation
|
| 411 |
|
|
linked to above. It is actually written in C, so you don't need to
|
| 412 |
|
|
be writing C++ in order to demangle C++. (That also means we have to
|
| 413 |
|
|
use crummy memory management facilities, so don't forget to free()
|
| 414 |
|
|
the returned char array.)
|
| 415 |
|
|
</p>
|
| 416 |
|
|
<p>Return <a href="#top">to top of page</a> or
|
| 417 |
|
|
<a href="../faq/index.html">to the FAQ</a>.
|
| 418 |
|
|
</p>
|
| 419 |
|
|
|
| 420 |
|
|
|
| 421 |
|
|
<!-- ####################################################### -->
|
| 422 |
|
|
|
| 423 |
|
|
<hr />
|
| 424 |
|
|
<p class="fineprint"><em>
|
| 425 |
|
|
See <a href="../17_intro/license.html">license.html</a> for copying conditions.
|
| 426 |
|
|
Comments and suggestions are welcome, and may be sent to
|
| 427 |
|
|
<a href="mailto:libstdc++@gcc.gnu.org">the libstdc++ mailing list</a>.
|
| 428 |
|
|
</em></p>
|
| 429 |
|
|
|
| 430 |
|
|
|
| 431 |
|
|
</body>
|
| 432 |
|
|
</html>
|
