Chapter 19:  Diagnostics
Chapter 19 deals with program diagnostics, such as exceptions
and assertions.  You know, all the things we wish weren't even
necessary at all.
Contents
#1
Adding data to exceptions
#2
Exception class hierarchy diagram
#3
Concept checkers --
new and improved!
Adding data to exceptions
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 hierarchy:
struct My_Exception : public std::runtime_error
{
public:
My_Exception (const string& whatarg)
: std::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
};
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Exception class hierarchy diagram
At one point we were going to make up a PDF of the exceptions
hierarchy, akin to the one done for the I/O class hierarchy.
Time was our enemy.  Since then we've moved to Doxygen, which has
the useful property of not sucking.  Specifically, when the source
code is changed, the diagrams are automatically brought up to date.
For the old way, we had to update the diagrams separately.
There are several links to the Doxygen-generated pages from
../documentation.html
here
.
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Concept checkers --
new and improved!
Better taste!  Less fat!  Literally!
In 1999, SGI added
concept checkers
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
vector
be "Assignable" (which means what you think
it means).  The checking was done during compilation, and none of
the code was executed at runtime.
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.
The primary author of the checking code, Jeremy Siek, had already
started work on a replacement implementation.  The new code has been
formally reviewed and accepted into
http://www.boost.org/libs/concept_check/concept_check.htm
the
Boost libraries
, and we are pleased to incorporate it into the
GNU C++ library.
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.
They are off by default for all versions of GCC from 3.0 to 3.4 (the
latest release at the time of writing).
They can be enabled at configure time with
../configopts.html
--enable-concept-checks
.
You can enable them on a per-translation-unit basis with
#define _GLIBCXX_CONCEPT_CHECKS
for GCC 3.4 and higher
(or with
#define _GLIBCPP_CONCEPT_CHECKS
for versions
3.1, 3.2 and 3.3).
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license.html
for copying conditions.
Comments and suggestions are welcome, and may be sent to
mailto:libstdc++@gcc.gnu.org
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