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List:       boost
Subject:    Re: [boost] [1.34.0beta] many, many warnings... :(
From:       Jeff Garland <jeff () crystalclearsoftware ! com>
Date:       2007-05-05 18:10:12
Message-ID: 463CC884.6030209 () crystalclearsoftware ! com
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JOAQUIN LOPEZ MU?Z wrote:

>> I find inheriting without virtual functions quite useful.  The 
>> main reason for the virtual destructor rule goes back to Scott
>> Meyers Effective C++.  He points out that if you have
>> Base <-- Derived and you call delete Base* the destructor of
>> Derived is not called. In general, it's hard to ensure your 
>> derived classes won't need the destructor called and hence the 
>> rule. 
>> However, if Derived happens to have a trivial/empty destructor 
>> then, really there's no reason to call it.
> 
> I agree and disagree with your previous exposition :)

That's usual ;-)

> I think deriving without virtual constructors is fine in those
> contexts when the user is not expected to delete passed
> objects, which are by far the most usual. Almost every
> function of the form
> 
>   void foo([const] T& t)
> 
> won't certainly try to delete t, so it's fine to pass an
> use here an object of some class derived from T (like in
> your super_string case.) 

Sure, but in the general case you could easily create an interface like this:

   void foo(T* t)

hence Scott's design rule.
 >...snip...
 >
> What I don't agree with you in is your claim that
> "if Derived happens to have a trivial/empty destructor 
> then, really there's no reason to call it." If my
> memory serves me well, deleting a Derived object through
> its Base non-virtual dtor is illegal according to the
> standard, regardless of whether D's dtor is trivial or
> not. You're merely being lucky with the actual behavior
> exhibited by compilers.

By the standard I believe it is technically undefined behavior.  But as a 
practical matter it's defined exactly the same on all compilers -- call the 
base class destructor.

Jeff

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