mirror of git://gcc.gnu.org/git/gcc.git
442 lines
14 KiB
XML
442 lines
14 KiB
XML
<chapter xmlns="http://docbook.org/ns/docbook" version="5.0"
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xml:id="std.support" xreflabel="Support">
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<?dbhtml filename="support.html"?>
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<info><title>
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Support
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<indexterm><primary>Support</primary></indexterm>
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</title>
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<keywordset>
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<keyword>ISO C++</keyword>
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<keyword>library</keyword>
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</keywordset>
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</info>
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<para>
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This part 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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</para>
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<para>
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While we can't reproduce the contents of the Standard here (you
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need to get your own copy from your nation's member body; see our
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homepage for help), we can mention a couple of changes in what
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kind of support a C++ program gets from the Standard Library.
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</para>
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<section xml:id="std.support.types" xreflabel="Types"><info><title>Types</title></info>
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<?dbhtml filename="fundamental_types.html"?>
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<section xml:id="std.support.types.fundamental" xreflabel="Fundamental Types"><info><title>Fundamental Types</title></info>
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<para>
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C++ has the following builtin types:
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</para>
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<itemizedlist>
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<listitem><para>
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char
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</para></listitem>
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<listitem><para>
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signed char
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</para></listitem>
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<listitem><para>
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unsigned char
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</para></listitem>
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<listitem><para>
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signed short
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</para></listitem>
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<listitem><para>
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signed int
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</para></listitem>
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<listitem><para>
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signed long
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</para></listitem>
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<listitem><para>
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unsigned short
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</para></listitem>
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<listitem><para>
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unsigned int
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</para></listitem>
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<listitem><para>
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unsigned long
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</para></listitem>
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<listitem><para>
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bool
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</para></listitem>
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<listitem><para>
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wchar_t
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</para></listitem>
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<listitem><para>
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float
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</para></listitem>
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<listitem><para>
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double
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</para></listitem>
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<listitem><para>
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long double
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</para></listitem>
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</itemizedlist>
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<para>
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These fundamental types are always available, without having to
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include a header file. These types are exactly the same in
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either C++ or in C.
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</para>
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<para>
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Specializing parts of the library on these types is prohibited:
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instead, use a POD.
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</para>
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</section>
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<section xml:id="std.support.types.numeric_limits" xreflabel="Numeric Properties"><info><title>Numeric Properties</title></info>
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<para>
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The header <filename class="headerfile">limits</filename> defines
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traits classes to give access to various implementation
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defined-aspects of the fundamental types. The traits classes --
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fourteen in total -- are all specializations of the template class
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<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>
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and defined as follows:
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</para>
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<programlisting>
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template<typename T>
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struct class
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{
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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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};
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</programlisting>
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</section>
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<section xml:id="std.support.types.null" xreflabel="NULL"><info><title>NULL</title></info>
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<para>
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The only change that might affect people is the type of
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<constant>NULL</constant>: while it is required to be a macro,
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the definition of that macro is <emphasis>not</emphasis> allowed
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to be <constant>(void*)0</constant>, which is often used in C.
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</para>
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<para>
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For <command>g++</command>, <constant>NULL</constant> is
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<code>#define</code>'d to be
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<constant>__null</constant>, a magic keyword extension of
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<command>g++</command>.
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</para>
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<para>
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The biggest problem of #defining <constant>NULL</constant> to be
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something like <quote>0L</quote> is that the compiler will view
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that as a long integer before it views it as a pointer, so
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overloading won't do what you expect. (This is why
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<command>g++</command> has a magic extension, so that
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<constant>NULL</constant> is always a pointer.)
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</para>
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<para>In his book <link xmlns:xlink="http://www.w3.org/1999/xlink"
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xlink:href="http://www.aristeia.com/books.html"><emphasis>Effective
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C++</emphasis></link>, Scott Meyers points out that the best way
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to solve this problem is to not overload on pointer-vs-integer
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types to begin with. He also offers a way to make your own magic
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<constant>NULL</constant> that will match pointers before it
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matches integers.
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</para>
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<para>See the
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<link xmlns:xlink="http://www.w3.org/1999/xlink"
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xlink:href="http://www.aristeia.com/books.html"><emphasis>Effective
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C++ CD</emphasis></link> example.
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</para>
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</section>
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</section>
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<section xml:id="std.support.memory" xreflabel="Dynamic Memory"><info><title>Dynamic Memory</title></info>
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<?dbhtml filename="dynamic_memory.html"?>
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<para>
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There are six flavors each of <function>new</function> and
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<function>delete</function>, so make certain that you're using the right
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ones. Here are quickie descriptions of <function>new</function>:
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</para>
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<itemizedlist>
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<listitem><para>
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single object form, throwing a
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<classname>bad_alloc</classname> on errors; this is what most
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people are used to using
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</para></listitem>
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<listitem><para>
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Single object "nothrow" form, returning NULL on errors
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</para></listitem>
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<listitem><para>
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Array <function>new</function>, throwing
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<classname>bad_alloc</classname> on errors
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</para></listitem>
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<listitem><para>
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Array nothrow <function>new</function>, returning
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<constant>NULL</constant> on errors
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</para></listitem>
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<listitem><para>
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Placement <function>new</function>, which does nothing (like
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it's supposed to)
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</para></listitem>
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<listitem><para>
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Placement array <function>new</function>, which also does
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nothing
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</para></listitem>
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</itemizedlist>
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<para>
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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 <function>delete</function>
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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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</para>
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<para>
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Remember that it is perfectly okay to call <function>delete</function> 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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</para>
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<para>
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By default, if one of the <quote>throwing <function>new</function>s</quote> can't
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allocate the memory requested, it tosses an instance of a
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<classname>bad_alloc</classname> exception (or, technically, some class derived
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from it). You can change this by writing your own function (called a
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new-handler) and then registering it with <function>set_new_handler()</function>:
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</para>
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<programlisting>
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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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</programlisting>
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<para>
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<classname>bad_alloc</classname> is derived from the base <classname>exception</classname>
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class defined in Sect1 19.
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</para>
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</section>
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<section xml:id="std.support.termination" xreflabel="Termination"><info><title>Termination</title></info>
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<?dbhtml filename="termination.html"?>
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<section xml:id="support.termination.handlers" xreflabel="Termination Handlers"><info><title>Termination Handlers</title></info>
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<para>
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Not many changes here to <filename class="headerfile">cstdlib</filename>. You should note that the
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<function>abort()</function> function does not call the
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destructors of automatic nor static objects, so if you're
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depending on those to do cleanup, it isn't going to happen.
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(The functions registered with <function>atexit()</function>
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don't get called either, so you can forget about that
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possibility, too.)
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</para>
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<para>
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The good old <function>exit()</function> function can be a bit
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funky, too, until you look closer. Basically, three points to
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remember are:
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</para>
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<orderedlist inheritnum="ignore" continuation="restarts">
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<listitem>
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<para>
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Static objects are destroyed in reverse order of their creation.
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</para>
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</listitem>
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<listitem>
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<para>
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Functions registered with <function>atexit()</function> 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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</para>
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</listitem>
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<listitem>
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<para>
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The previous two actions are <quote>interleaved,</quote> that is,
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given this pseudocode:
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</para>
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<programlisting>
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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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</programlisting>
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<para>
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then at a call of <function>exit()</function>,
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<varname>f2</varname> will be called, then
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<varname>obj2</varname> will be destroyed, then
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<varname>f1</varname> will be called, and finally
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<varname>obj1</varname> will be destroyed. If
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<varname>f1</varname> or <varname>f2</varname> allow an
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exception to propagate out of them, Bad Things happen.
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</para>
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</listitem>
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</orderedlist>
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<para>
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Note also that <function>atexit()</function> 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 <function>xatexit</function>/<function>xexit</function> combination from <literal>libiberty</literal>, which has no such limit.
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</para>
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</section>
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<section xml:id="support.termination.verbose" xreflabel="Verbose Terminate Handler"><info><title>Verbose Terminate Handler</title></info>
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<?dbhtml filename="verbose_termination.html"?>
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<para>
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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, the verbose terminate handler.
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</para>
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<programlisting>
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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 <replaceable>anything</replaceable>;
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}
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</programlisting>
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<para>
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The <function>__verbose_terminate_handler</function> function
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obtains the name of the current exception, attempts to demangle
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it, and prints it to stderr. If the exception is derived from
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<classname>exception</classname> then the output from
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<function>what()</function> will be included.
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</para>
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<para>
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Any replacement termination function is required to kill the
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program without returning; this one calls abort.
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</para>
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<para>
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For example:
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</para>
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<programlisting>
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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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</programlisting>
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<para>
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With the verbose terminate handler active, this gives:
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</para>
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<screen>
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<computeroutput>
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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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</computeroutput>
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</screen>
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<para>
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The 'Aborted' line comes from the call to
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<function>abort()</function>, of course.
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</para>
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<para>
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This is the default termination handler; nothing need be done to
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use it. To go back to the previous <quote>silent death</quote>
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method, simply include <filename>exception</filename> and
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<filename>cstdlib</filename>, and call
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</para>
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<programlisting>
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std::set_terminate(std::abort);
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</programlisting>
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<para>
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After this, all calls to <function>terminate</function> will use
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<function>abort</function> as the terminate handler.
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</para>
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<para>
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Note: the verbose terminate handler will attempt to write to
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stderr. If your application closes stderr or redirects it to an
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inappropriate location,
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<function>__verbose_terminate_handler</function> will behave in
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an unspecified manner.
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</para>
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</section>
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</section>
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</chapter>
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