Re: [boost] [Review] GGL review starts today, November 5th

Subject: Re: [boost] [Review] GGL review starts today, November 5th
From: Patrick Horgan (phorgan1_at_[hidden])
Date: 2009-11-23 18:07:14

Steven Watanabe wrote:
> AMDG
>
> Patrick Horgan wrote:
>> Steven Watanabe wrote:
>>> FWIW, non-virtual destructors are the norm in Boost.
>>> I strongly dislike the idea of having a policy enabling
>>> virtual destructors.
>> Steven, could you tell me the reason for this?
>
> There aren't a lot of virtual function in Boost either.
A quick grep in trunk shows about 1200 virtual methods including 136 
virtual destructors under boost.
> In ggl, the only virtual functions I see are in exception classes.
>
>> I have always thought that if you had virtual methods, you should 
>> have a virtual destructor since the derived class has something 
>> different about it and if being destructed via a reference or pointer 
>> to base, the wrong thing could happen without a virtual destructor!  
>> Educate me please.  I've never heard of any reason to avoid a virtual 
>> destructor if you already had virtual methods.  Obviously I'm missing 
>> something about something I thought I understood.
>
> I don't think so.  If you're using virtual functions in the
> normal OO way, a virtual destructor almost always
> makes sense.  (The use of virtual in Boost.Exception
> is somewhat different.  virtual is being used as an
> implementation detail to separate a fragment of the
> interface of a concrete type from the rest of the type
> in order to minimize dependencies.  It isn't being used
> for polymorphism at all)
But there are virtual destructors in exception of course (isn't 
exception pretty, elegant code btw?)   I'm still a bit confused.  Are 
you saying that destroying from a base class reference can't be a 
problem?  Of course that's true if the derived class doesn't ever add 
any extra dynamic value or system state, but even in that case, surely 
you'd want to have the virtual destructor anyway if there is a need for 
virtual functions, so that if someone less thoughtful was 
extending/maintaining it later, the design would keep those sorts of 
mistakes from being made.  It's defensive programming.

But what I was really asking about, was when you said:

> Steven Watanabe wrote:
>> FWIW, non-virtual destructors are the norm in Boost.
>> I strongly dislike the idea of having a policy enabling
>> virtual destructors.
It seems there's lots of virtual destructors in Boost, and you haven't 
told me why you are against them.  In what way are they bad?  In this 
statement you seem to come out against them quite strongly.  You seem to 
be saying that you want it to be a rule that they are not to be used.  
Am I mis-reading this somehow?

Thanks,

Patrick