Re: [boost] Re: Virtual inheritance in exception hierarchies

From: Iain K. Hanson (iain.hanson_at_[hidden])
Date: 2003-09-04 13:48:42

On Tue, 2003-09-02 at 19:32, David Abrahams wrote:
> "Iain K. Hanson" <iain.hanson_at_[hidden]> writes:
> 
> >> But is this a good design?  It certainly isn't the only possible one.
> >> (Making all the code depend upon the definitions of both Network_err and
> >> File_system_err - which no doubt drags other stuff into the translation unit
> >> - isn't a design choice I'd make lightly.)
> >
> > In certain places it may be the most natural design choice. Whether it
> > is a good idea to pay for virtual inheritance in every exception derived
> > from class exception is a separate issue. Exception handling incurs a
> > significant cost once an exception is thrown. Adding to that cost could
> > drive more users away from exception handling.
> 
> Do you think dynamic downcasting through a layer of virtual
> inheritance is significantly more expensive than downcasting through a
> layer of regular inheritance?

I don't know as I have not benchmarked it. However, it will incur a
penalty and also has a size penalty. On some compilers it may mean
turning on RTTI which often reduces optimisation and increases general
code bloat.

> 
> > Also, I'm not sure that this use case is sufficiently common to burden
> > all exceptions with.
> 
> What's the burden?
> 
> I'm just askin', is all.

Space and time overhead. There are still people out there who will not
use exceptions for performance reasons ( perceived or real ). Anything
we do that increases the cost will give them further excuse.

/ikh