Re: [boost] Boost and exceptions

Subject: Re: [boost] Boost and exceptions
From: Peter Dimov (pdimov_at_[hidden])
Date: 2012-06-25 13:02:14

Robert Ramey wrote:

> hmmm looks to me that my program is not losing the type.

Looks like you've rediscovered std::nested_exception?

http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2008/n2559.htm

This works but it supports a different (Java-like) style of exception 
handling. This same mindset has led to exception specifications - libraries 
have to only throw whatever is enumerated in their interface, so they are 
forced to wrap. With boost.exception, the high level catch still receives 
the original exception type, without needing to unwrap. It can then probe 
for further information if it likes, or it could ignore it (doesn't 
necessarily have to be rewritten as additional layers are inserted.)

In other words:

With wrapping:

throw X
catch X, throw Y(X)
catch Y

A new layer is inserted:

throw X
catch X, throw Y(X)
catch Y, throw Z(Y)
catch Z // this had to change

With boost.exception

throw X
catch X, add info, rethrow X
catch X

Note how the final catch X doesn't need to change if another layer is 
inserted:

throw X
catch X, add info, rethrow X
catch X, add info, rethrow X
catch X