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List:       boost
Subject:    [boost] [system] system_error and exception handling paradigm
From:       Oleg Abrosimov <beholder () gorodok ! net>
Date:       2006-06-29 17:12:06
Message-ID: 44A409E6.9000600 () gorodok ! net
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Beman Dawes wrote:
> The filesystem proposal accepted by the LWG for TR2 also includes a 
> <system_error> header with a bit of error reporting machinery. In 
> particular, class error_code to encapsulate error codes from the 
> operating system and class system_error to be thrown by exceptions.
> 
> See 
> http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2006/n1975.html#Diagnostics-library
> 
> The plan is to use these classes uniformly in TR2. Presumably we will 
> want Boost implementations, and use them in other Boost libraries 
> besides Boost.Filesystem.
> 
 > [snip]
> 
> And any other comments are welcome!
> 
> --Beman


Hello Beman!

The N1975 proposal has a section called "Important Design Decisions". It 
is a very good place to put a rational why you've chosen the error 
reporting strategy with system_error.

currently, it has only one sentence "Because filesystem operations often 
encounter unexpected runtime errors, the library by default reports 
runtime errors via C++ exceptions, and ensures enough information is 
provided for meaningful error messages, including internationalized 
error messages."

It is not enough to understand why you don't follow the way of other 
portable file system frameworks. For example, in java language there are
IOException and its descendants like FileNotFoundException. It can be 
said that it follows the usual OO-exception handling mantra - define a 
hierarchy of exception classes and use them to report error conditions 
from your functions. Furthermore, it can be said that it is a way to 
abstract low-level system errors and provide only high level logical 
errors (or exceptions).

It is clear from n1975 that you abandoned the OO approach and fall back 
to the C-style error reporting and handling. Yes there are exceptions, 
but no hierarchy. And exception handling is done by dispatching on a 
base of error codes but not on exception types. So, it is not really far 
from "C" I believe. One more point is that type safety is abandoned here 
too. I should add here that I agree with your decision, but I think that 
it shouldn't be made implicit, without a really good explanation and 
rational. The reason is simple: it changes the paradigm that is written 
in all OO and/or C++ books.

Best regards,
Oleg Abrosimov.


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