Re: [boost] Boost and exceptions

Subject: Re: [boost] Boost and exceptions
From: Emil Dotchevski (emildotchevski_at_[hidden])
Date: 2012-06-25 17:12:49

On Mon, Jun 25, 2012 at 11:50 AM, Robert Ramey <ramey_at_[hidden]> wrote:

> permit catch X rather than catch boost::exception which
> syntatically looks clean.

This is not syntactic sugar. I am surprised that I have to defend the
position that if a library's documentation says "the library throws X" then
the application can catch(X &).

> But now you've got an interface - catch X
> which actually is an X++.  To me this is very bad practice.  It hides
> too much. It's not at all obvious whether or not one can extract
> extra data from this - it depends on how the exception is thrown.
>

Look at the postcondition of op<< (
http://www.boost.org/doc/libs/release/libs/exception/doc/exception_operator_shl.html).
As a matter of fact, users can rely on library exceptions to always arrive
with various application-specific data at the catch site.

I would like to know that if I catch a "my_exception" that's what
> I'm getting no more no less.  My interface makes this much clearer.
>

Like Peter said, others have argued your position in the past and the
result is that C++ has exception specifications. I'm warning you, there are
dragons down that route, you don't want to go there.

> That is, I think
> > catch Z // this had to change
> is much better than
> catch X // where X is really an X + some other data
>
> One more thing to argue about.
>
> Looking at my make_exception_wrapper, it might be possible to
> use  the boost exception technique so on could replicate the
> catch X behavior if he wanted to.

How?

> Really, it's just not practical that for a library to be useful one has to
> require
> all the users to change their exception throwing code.  How does
> boost exception deal with third party libraries for which we don't even
> have the source code?
>

There are limits to what can be done. Can Boost Serialization serialize any
object from a third party library for which we don't even have the source
code -- or even if we have it?

> FWIW - The assertion was that it was not possible to implement
> functionality
> of boost exception without wrapping exceptions at the throw site. I believe
> I
> have shown a counter example to this assertion.

You've shown that something different from what Boost Exception can be
implemented with std::exception_ptr, which very few compilers support
anyway.

OTOH, you could use boost::exception_ptr to implement your wrapping code on
pretty much any compiler, though -- bummer -- that won't work for
exceptions emitted by Boost Serialization. :)

Emil Dotchevski
Reverge Studios, Inc.
http://www.revergestudios.com/reblog/index.php?n=ReCode