2008-02-11 01:01:33 +01:00
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<?xml version='1.0'?>
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<!DOCTYPE part PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
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"http://www.oasis-open.org/docbook/xml/4.5/docbookx.dtd"
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[ ]>
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<part id="manual.diagnostics" xreflabel="Diagnostics">
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<?dbhtml filename="diagnostics.html"?>
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<partinfo>
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<keywordset>
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<keyword>
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ISO C++
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</keyword>
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<keyword>
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library
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</keyword>
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</keywordset>
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</partinfo>
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<title>Diagnostics</title>
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<chapter id="manual.diagnostics.exceptions" xreflabel="Exceptions">
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<title>Exceptions</title>
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<sect1 id="manual.diagnostics.exceptions.hierarchy" xreflabel="Exception Classes">
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<title>Exception Classes</title>
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<para>
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All exception objects are defined in one of the standard header
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files: <filename>exception</filename>,
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<filename>stdexcept</filename>, <filename>new</filename>, and
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<filename>typeinfo</filename>.
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</para>
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<para>
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The base exception object is <classname>exception</classname>,
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located in <filename>exception</filename>. This object has no
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<classname>string</classname> member.
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</para>
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<para>
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Derived from this are several classes that may have a
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2008-04-11 00:14:17 +02:00
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<classname>string</classname> member: a full hierarchy can be
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2008-02-11 01:01:33 +01:00
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found in the <ulink url="http://gcc.gnu.org/onlinedocs/libstdc++/latest-doxygen/a00233.html">source documentation</ulink>.
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</para>
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</sect1>
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<sect1 id="manual.diagnostics.exceptions.data" xreflabel="Adding Data to Exceptions">
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<title>Adding Data to Exceptions</title>
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<para>
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The standard exception classes carry with them a single string as
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data (usually describing what went wrong or where the 'throw' took
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place). It's good to remember that you can add your own data to
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these exceptions when extending the hierarchy:
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</para>
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<programlisting>
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struct My_Exception : public std::runtime_error
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{
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public:
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My_Exception (const string& whatarg)
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: std::runtime_error(whatarg), e(errno), id(GetDataBaseID()) { }
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int errno_at_time_of_throw() const { return e; }
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DBID id_of_thing_that_threw() const { return id; }
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protected:
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int e;
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DBID id; // some user-defined type
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};
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</programlisting>
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</sect1>
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<sect1 id="manual.diagnostics.exceptions.cancellation" xreflabel="Cancellation">
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<title>Cancellation</title>
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<para>
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</para>
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</sect1>
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</chapter>
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<chapter id="manual.diagnostics.concept_checking" xreflabel="Concept Checking">
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<title>Concept Checking</title>
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<para>
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In 1999, SGI added <quote>concept checkers</quote> to their
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implementation of the STL: code which checked the template
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parameters of instantiated pieces of the STL, in order to insure
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that the parameters being used met the requirements of the
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standard. For example, the Standard requires that types passed as
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template parameters to <classname>vector</classname> be
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"Assignable" (which means what you think it means). The
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checking was done during compilation, and none of the code was
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executed at runtime.
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</para>
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<para>
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Unfortunately, the size of the compiler files grew significantly
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as a result. The checking code itself was cumbersome. And bugs
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were found in it on more than one occasion.
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</para>
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<para>
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The primary author of the checking code, Jeremy Siek, had already
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started work on a replacement implementation. The new code has been
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formally reviewed and accepted into
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<ulink url="http://www.boost.org/libs/concept_check/concept_check.htm">the
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Boost libraries</ulink>, and we are pleased to incorporate it into the
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GNU C++ library.
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</para>
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<para>
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The new version imposes a much smaller space overhead on the generated
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object file. The checks are also cleaner and easier to read and
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understand.
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</para>
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<para>
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They are off by default for all versions of GCC.
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They can be enabled at configure time with
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<ulink url="../configopts.html"><literal>--enable-concept-checks</literal></ulink>.
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You can enable them on a per-translation-unit basis with
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<literal>-D_GLIBCXX_CONCEPT_CHECKS</literal>.
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</para>
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<para>
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Please note that the upcoming C++ standard has first-class
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support for template parameter constraints based on concepts in the core
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language. This will obviate the need for the library-simulated concept
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checking described above.
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</para>
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</chapter>
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</part>
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