Re: [boost] Boost and exceptions

Subject: Re: [boost] Boost and exceptions
From: Steven Watanabe (watanabesj_at_[hidden])
Date: 2012-06-23 20:09:58

AMDG

On 06/23/2012 04:19 PM, Robert Ramey wrote:
> or if you want to add your own sauce
> 
> catch(std::exception &e){
>     std::exception_ptr e = std::current_exception();
>     someclass se(e, extra stuff);  // home brew or from some library
>     std::throw(se);
> }
> 
> or if you want to catch anyone's stuff
> 
> catch(...){
>     std::exception_ptr e = std::current_exception();
>     my_unknown_exception me(e); // though its not clear what the upper level 
> could do with this
> }
> 
> this would confine the usage of boost exception to areas where the user
> can decide whether or not he want's to use it.  It looks to me that the 
> standard
> was conceived to support just this usage.

I disagree.  std::exception_ptr was primarily
designed to allow exceptions to be passed
between threads.  The interface doesn't allow
you to do much besides rethrowing it unchanged.

> It doesn't require anything 
> special at the
> throw point.  The standard facility of using one's own exception class is 
> sufficient.
> 

Doing this means that the caller has
to catch someclass or my_unknown_exception
instead of boost::archive_exception.
It's true that using exception_ptr preserves
all the information from the original exception,
but it doesn't do it in a very convenient or
accessible way.

Even worse, if you use this in pre-existing code,
you've just introduced a breaking change, because
the type of the exception that comes out
is different.

>> If the point of this catch is to translate the type of the exception
>> object, it has to look like:
>>
>> <snip>
>>
>> And of course this nonsense has to go around every call to Boost
>> Serialization.
>>
>> WHY?
> 
> lol - well you've got a point here.  But I not convinced a huge problem in 
> practice.
> I would expect most of these would be placed at the much higer levels.
> 

I don't see why that would be the case.
The point of using Boost.Exception is
to be able to add context information
to any exception as it goes by, not
to do any real error handling.

>>> How is this different from using std::exception ?
> 
>> You can't add your own stuff to a live std::exception.
> 
> I believe equivalent functionality is available. I believe that boost 
> exception
> is more intrusive that it has to be.
> 

Boost.Exception helps keep the exception
hierarchy clean.

a) The type of the exception indicates the
   actual error that occurred and contains
   any information specific to the error.
b) boost::exception contains various information
   about the context of the exception.

Your latest solution mixes (a) and (b) in
a way that makes (a) difficult to get at.

In Christ,
Steven Watanabe