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Subject: Re: [Boost-users] Exception: adding diagnostic information from a dtor?
From: Michael Schuerig (michael.lists_at_[hidden])
Date: 2011-06-15 04:08:51

On Wednesday 15 June 2011, Emil Dotchevski wrote:
> On Tue, Jun 14, 2011 at 4:00 PM, Michael Schuerig
> <michael.lists_at_[hidden]> wrote:
> > On Wednesday 15 June 2011, Emil Dotchevski wrote:
> >> Michael,
> >>
> >> You're right, the boost::current_exception function is only valid
> >> within a try block.
> >>
> >> The issue you're raising has to do with the postconditions of
> >> operator<<. Its current semantics guarantee that upon return the
> >> error info is successfully stored in the exception object. This
> >> means that when you catch an exception, you can rely on certain
> >> error info to be present:
> >>
> >> catch( file_read_error & e )
> >> {
> >> assert(get_error_info<errinfo_file_name>(e)!=0);
> >> std::cerr << "Error reading " <<
> >> *get_error_info<errinfo_file_name>(e); }
> >>
> >> Without this guarantee, the catch site would be required to deal
> >> with any exception even if it has no error info in it.
> >
> > Emil,
> >
> > thanks for your explanation. Unfortunately, I don't understand it.
> > I do understand that e << errinfo_file_name(fn), if successful at
> > all (i.e., it doesn't throw itself) guarantees that the respective
> > info is present in e. However, this is a dynamic property, I don't
> > see any static reason why it must be true for every
> > file_read_error. So, in principle, a catch site for that exception
> > would have to take into account that this info might be missing.
> > Am I missing something?
> The postcondition in op<< allows the programmer to make a design
> decision that any and all file_read_error exceptions reaching a
> particular catch site in the program will be guaranteed to have a
> file name in them, and to treat the absence of a file name as a
> logic error (that is, a bug, hence the assert in my example.)
> This is similar to passing something to an exception type's
> constructor: it'll throw (something else) if it failed to construct,
> right? The semantics of op<< simply extend this behavior to the case
> when you stuff something in the exception object at a later time.

Is this a correct paraphrase: op<< either adds additional info to an
exception or it raises another exception indicating its failure.

> >> If you are to add error info to exception objects in destructors,
> >> this guarantee can not hold.
> >
> > I'm not sure I understand this either. If file_read_error can be
> > thrown without initially having an errinfo_file_name and this info
> > is only added during unwinding higher up the call stack, then why
> > does it make a difference if it is in a catch block or in a
> > destructor (ignoring for a moment that calling current_exception
> > isn't legal there)?
> You're right that it doesn't make a difference, provided that you
> don't choose to treat the absence of certain error info as a bug. The
> point is that if you always add error info in destructors, that
> choice is not available and you must be able to deal with any
> exception only knowing its type.

Let's see. When I catch one exception and while I add information to it,
a new exception is thrown, it is this new exception that will propagate
up the call stack. By contrast, when a destructor is called due to an
exception and the code in the destructor does something that throws
another exception, this causes program termination. -- Is that it?


Michael Schuerig

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