Re: [boost] [review] Shmem

From: Ion Gaztañaga (igaztanaga_at_[hidden])
Date: 2006-02-17 14:48:09

Hi Andy,

> Ok ...thanks for the example. Though it raise a lot of questions which perhaps
> cant be anwered. As I understand it the no-exceptions requirement in this
> example is a policy based on the fact that exceptions for error recovery have
> been found to be too slow to respond? 

As Dave mentioned exception handling can be slow when there is an error. 
As a general recommendation, many embedded system enterprises don't use 
exceptions (yes, including EC++ group). Technical Report on C++ (N1668) 
Performance confirms that a "code" exception implementation has run-time 
overhead in normal code but a "table" exception has no runtime overhead 
in normal code. However there is a space overhead. N1668 reports 15% 
overhead in size from one vendor. But the problem is predictability.

There also international norms that regulate what languages can be used 
depending on security/reliability levels in some areas. Those norms 
state that only a subset of C++ can be used to program some 
security/reliability equipment. That subset does not include exceptions, 
so many times you are forced to avoid exceptions. Many enterprises take 
the decision to program without exceptions for all embedded devices 
because they don't know if in the future a developed code can end in one 
of those devices. If you need more information about exceptions N1668 
has some good explanations.

> I am now wondering though if the change over to RAII is going to have other
> unwelcome side-effects on a design that has not been designed around it?  This
> is pure speculation. I havent looked deeply into the matter.

I don't think so.

> Also are you reasonably  happy with that decision (RAII only for Shmem)? I
> guess it is not too helpful for the application above for example.  

Well, not *very* happy, but the point is that I should implement what 
boosters want. If in the future there are requests for a non-throwing 
alternative interface, that can be discussed and developed in the 
future. But this will be more needed in threads, since there are 
hard-realtime embedded systems without exceptions but using threads. 
You always have the option to use the C API.

This RAII issue is closed to me. Now I have a lot for work to do for the 
final Shmem version.

Regards,

Ion