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

From: Iain K. Hanson (iain.hanson_at_[hidden])
Date: 2003-09-05 13:37:31

On Fri, 2003-09-05 at 01:12, David Abrahams wrote:
> "Iain K. Hanson" <iain.hanson_at_[hidden]> writes:
> 
> > 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.
> 
> This rings of FUD.  I don't believe it neccessarily has to incur a
> penalty over a dynamic_(down)cast through non-virtual bases and I
> don't believe there are *any* compilers which will require RTTI to do
> EH with virtual bases but will work with EH, no RTTI and non-virtual
> bases.

I think that there is a mis-understanding here and I'm not sure if it is
yours or mine.

dynamic_cast needs rtti afaik, at least on some compilers e.g. Sun.

My mention of incurring cost was to MI over SI. a quick test shows that
throwing a exception using MI and virtual bases with gcc 3.2.2.5 with
-O4 produces an assemblers file more than twice the size of of the same
program using SI exceptions only. With a line count difference of nearly
exactly double.

That is a significant overhead for every exception.

> 
> >> 
> >> > 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.
> 
> That's their choice.  I don't think Boost should cater to incorrect
> assumptions of inefficiency.

>From the above, it is not an incorrect assumption.

/ikh