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<H1 CLASS="centered"><A NAME="top">Chapter 19: Diagnostics</A></H1>
<P>Chapter 19 deals with program diagnostics, such as exceptions
and assertions. You know, all the things we wish weren't even
necessary at all.
</P>
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<HR>
<H1>Contents</H1>
<UL>
<LI><A HREF="#1">Adding data to exceptions</A>
<LI><A HREF="#2">Exception class hierarchy diagram</A>
<LI><A HREF="#3">Concept checkers -- <STRONG>new and improved!</STRONG></A>
</UL>
<HR>
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<H2><A NAME="1">Adding data to exceptions</A></H2>
<P>The standard exception classes carry with them a single string as
data (usually describing what went wrong or where the 'throw' took
place). It's good to remember that you can add your own data to
these exceptions when extending the heirarchy:
</P>
<PRE>
using std::runtime_error;
struct My_Exception : public runtime_error
{
public:
My_Exception (const string& whatarg)
: runtime_error(whatarg), e(errno), id(GetDataBaseID()) { }
int errno_at_time_of_throw() const { return e; }
DBID id_of_thing_that_threw() const { return id; }
protected:
int e;
DBID id; // some user-defined type
};
</PRE>
<P>Return <A HREF="#top">to top of page</A> or
<A HREF="../faq/index.html">to the FAQ</A>.
</P>
<HR>
<H2><A NAME="2">Exception class hierarchy diagram</A></H2>
<P>The <A HREF="exceptions_hiearchy.pdf">diagram</A> is in PDF, or
at least it will be once it gets finished.
</P>
<P>Return <A HREF="#top">to top of page</A> or
<A HREF="../faq/index.html">to the FAQ</A>.
</P>
<HR>
<H2><A NAME="3">Concept checkers -- <STRONG>new and improved!</STRONG></A></H2>
<P>Better taste! Less fat! Literally!</P>
<P>In 1999, SGI added <EM>concept checkers</EM> to their implementation
of the STL: code which checked the template parameters of
instantiated pieces of the STL, in order to insure that the parameters
being used met the requirements of the standard. For example,
the Standard requires that types passed as template parameters to
<TT>vector</TT> be "Assignable" (which means what you think
it means). The checking was done during compilation, and none of
the code was executed at runtime.
</P>
<P>Unfortunately, the size of the compiler files grew significantly
as a result. The checking code itself was cumbersome. And bugs
were found in it on more than one occasion.
</P>
<P>The primary author of the checking code, Jeremy Siek, had already
started work on a replcement implementation. The new code has been
formally reviewed and accepted into
<A HREF="http://www.boost.org/libs/concept_check/concept_check.htm">the
Boost libraries</A>, and we are pleased to incorporate it into the
GNU C++ library.
</P>
<P>The new version imposes a much smaller space overhead on the generated
object file. The checks are also cleaner and easier to read and
understand.
</P>
<P>Right now they are off by default. More will be added once
GCC 3.0 is released and we have time to revisit this topic.
</P>
<P>Return <A HREF="#top">to top of page</A> or
<A HREF="../faq/index.html">to the FAQ</A>.
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Comments and suggestions are welcome, and may be sent to
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