Re: [boost] [statechart] Problems if I make dtorS virtual?

From: BRIDGES Dick (Dick.Bridges_at_[hidden])
Date: 2005-08-24 17:47:35

Hi Andreas

> -----Original Message-----
> From: boost-bounces_at_[hidden]
[mailto:boost-bounces_at_[hidden]]
> On Behalf Of Andreas Huber
> Sent: Wednesday, August 24, 2005 2:19 PM
> To: boost_at_[hidden]
> Subject: Re: [boost] [statechart] Problems if I make dtorS virtual?
> 
> Hi Dick
> 
> BRIDGES Dick wrote:
> > When compiling the examples (statechart library downloaded from sf
18
> > Aug), I get the following warnings:
>
> [snip warnings]
> 
> > I changed ~simple_State() and ~state_base() to virtual and the
> > warnings disappeared.

[snip]

> > As a quick, not-definitive check, I built PingPong* with both the
> > virtual and non-virtual dtorS using gcc 3.4.2 on an old 
> > i386-redhat-linux machine.
> 
> I would expect the PingPong examples to deliver the least-dependable
> information regarding virtual vs. non-virtual dtors. This is due to
the
> fact that a lot of stuff is going on besides the destruction of
states.
> I think BitMachine would make for a better testing platform...
> 
> > The results appear to be the reverse of what I would expect, but I
> > checked
> > to verify that I have the correct result/config correspendence.
> >
> > ..................non-virtual.....virtual
> > PingPongSingle....  4.08     .....  3.71
> > PingPongMulti1....  6.01997  .....  5.85999
> > PingPongMulti2.... 19.53     ..... 19.55
> 
> Hmmm, here's what I get on a 3.2GHz Intel Pentium 4 Windows XP machine
> (all numbers in microseconds non-virtual / virtual):
> 
> *** MSVC7.1 ***
> 
> PingPongSingle: 1.06 / 1.07
> PingPongMulti1: 5.41 / 5.43
> PingPongMulti2: 12.34 / 12.26
> 
> BitMachineCustom3: 0.18 / 0.19

[snip_similar_results]

Looks more like what I would expect.
BTW: Could I talk you into trading computers?  ;)

> I.e. no significant difference non-virtual vs. virtual. Two things
come
> to mind:
> 1. At the point where states are destructed (see simple_state.hpp,
line
> 983) the compiler might know that the type of the pointer matches the
> runtime type of the referenced object and therefore call the virtual
> destructor non-virtually. However, I'm not sure whether such compiler
> magic is implemented in any of the tested platforms nor do I have a
clue
> whether that's possible at all (it does sound pretty impossible)
> 2. There could be little difference on modern processors anyway (as
you
> have mentioned)
> 
> > Reminds me of the old joke:  "I see fine.  I just don't understand
> > what
> > I'm looking at."  Two questions:  1) will making the dtorS virtual
> > lead
> > to adverse consequences I simply haven't encountered yet;
> 
> Given the numbers above I don't see any problems whatsoever. Not on
the
> tested processors, that is. I can't say whether the picture is the
same
> for processors used in the embedded world.

I may know the answer for some ARM machines soon - I hope. ;)  I'll try
the BitMachine* tests if I can get the cross compilers to build them.

> > and 2) can
> > someone help me understand what I did wrong in setting up this test?
> 
> Difficult to say. Could you build with bjam and repeat the test?

I followed the instructions to build the examples.  Changed to the
example directory and ran <<bjam "sTOOLS=gcc">>.  I got lazy and only
ran the PingPong* progs.  Guess I should have mentioned that.  %>]

Any chance that, in the absence of some future evidence to the contrary,
those dtorS can be made virtual in the library?

[snip]

> Regards,
> 
> --
> Andreas Huber

Regards,
Dick Bridges

"Multithreading is just one damn thing after, before, or simultaneous
with another." Scott Meyers and Andrei Alexandrescu