📄 exceptionoverview.htm
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<title>Exceptions in the Database TemplateLibrary: An Overview</title>
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<p><font size="6" face="Bookman Old Style"><em><strong><u>dtl</u></strong></em></font></p>
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<h1>Exceptions in the Database Template Library: An Overview</h1>
<p>The Database Template Library (DTL) attempts to be robust in
the services it provides. To be robust, a piece of software
must maintain uncorrputed data structures and recover to resume
normal execution if possible in the face of errors.
Errors must be reported in a clear manner and allow the user
to track down exactly what went wrong in the running of the
program. In addition, the user should have some control over what kind of
behavior will take place if an error is encountered. DTL
addresses these issues by providing:</p>
<ul type="disc">
<li>Code written in an exception-safe manner to where
atomicity and consistency are maintained wherever
possible. <br>
<br>
</li>
<li>A clear and easy to understand exception hierarchy which
describes the various kinds of problems that can
occur during execution. This allows for
easier debugging as you know exactly what code threw and
get a clear idea of the problem. Exception classes in DTL inherit from
a base called RootException and describe specific common cases in DTL:
<p><a href="DBException.htm">DBException:</a> describes
problems that occur either at the API level or the ODBC/database
level dealing with reading or writing to the database,
either in IndexedDBView, DBView, or DBStmts (or their
iterators). </p>
<p><a href="ETIException.htm">ETIException:</a> gives an
error message about what went wrong with the
extended type information provided for SQL <---> C
type mappings, usually unrecognized types.</p>
<p><a href="ValidityException.htm">ValidityException:</a>
describes errors thrown from ValidatedObject's (specifically
for DBConnection and DBStmt objects), indicating the
object has hit an invalid state and cannot be recovered
without a call to the virtual method ValidatedObject::revalidate().</p>
<p><a href="VariantException.htm">VariantException:</a>
thrown for errors that occur with variant objects such as
variant_row, variant_fields, and IOHandlers (handlers
are implemented as variant objects). If we can't find a
member such as a function name or cast properly, then we
throw this exception.<br>
</p>
</li>
<li>All DTL iterator classes (DB_iterator and its subclasses)
support a notion of handlers. Each iterator maintains its
own state based on the operations it performs similar to
the C++ standard library's iostreams. They have good(),
bad(), and fail() methods plus the other methods which
are defined in std::ios_base.
DTL's notion of handlers is similar to C++'s concept of
new_handler that is invoked when the memory manager fails
to allocate memory on operator new(). DTL has handlers
known as IOHandlers, which are function objects that the
user can set using DB_iterator::set_io_handler()
and will be invoked when an exception is thrown by code
executed by that iterator. The handler decides what
actions to take based on the state of the iterator and
tells the caller whether to suppress the error
or to re-throw the exception. DTL also
provides two pre-defined handlers for common cases:
AlwaysThrowsHandler (which always just tells the code to
re-throw and does nothing else) and LoggingHandler (which
logs exceptions to a vector with useful information that
can aid the user in tracking down the problem, always
telling the caller to ignore the error). IOHandlers give the user
more control over what they want
to do with exceptions. <br>
<br>
</li>
<li>The notion of transactions over a range. In other words,
the ability to insert or update multiple rows and roll
the entire operation back if it is not completely
successful. This is a must for both exception safety
purposes and to provide proper support for the notion of
database transactions.</li>
</ul>
<p>More information and examples on the above items can be found in the "<a
href="ExceptionHandling.htm">Exception Safety in DTL</a>"
document. </p>
<hr>
<p><a href="index.htm"><img src="dtl_home.gif" alt="[DTL Home]"
width="54" height="54"></a> <br>
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