Re: [boost] Is there any interest in a library that provides cont ainers with virtual destructors?

From: Victor A. Wagner Jr. (vawjr_at_[hidden])
Date: 2005-10-26 09:16:25

At 02:51 2005-10-26, Martin Bonner wrote:
>----Original Message----
>From: Victor A. Wagner Jr. [mailto:vawjr_at_[hidden]]
>Sent: 26 October 2005 05:36
>To: boost_at_[hidden]; boost_at_[hidden]
>Subject: Re: [boost] Is there any interest in a library that provides
>containers with virtual destructors?
>
> > At 10:04 2005-10-25, Andy Tompkins wrote:
> >> I have read a few times that people sometimes wish that std::vector
> >> (or other containers) had a virtual destructor.
>[snip]
> >> This allows one to write something like:
> >>
> >> struct person {
> >>   string first_name;
> >>   string last_name;
> >> };
> >> class people : public virtual_vector<person>
> >> {
> >> public:
> >>   //extra methods
> >> };
> >>
> >> Is this useful?
> >
> > unless I mis-read (or mis-understood...far more likely) a lot of
> > books on how C++ works, I don't believe there is any reason for what
> > _you've_ shown to need a virtual destructor.
>
>You have mis-read or mis-understood (or you need better books).  Consider:
>     virtual_vector<person>* p = new people;

I guees I wasn't clear..  I was suggesting that virtual_vector has no 
need for existence.

>     delete p;
>That invokes undefined behaviour unless virtual_vector<person> has a virtual
>destructor.

are you suggesting that if it were   class people: public 
std::vector<person> {.....};  as above the same would be true?
not "in practice", "in theory".

>In *practise*, you will (probably) get away with it.  The most *likely*
>behaviour is that virtual_vector<>::~virtual_vector() will be called,
>without calling people::~people(). ... but that doesn't alter the fact that
>it *is* undefined behaviour, which is not good.
>
>
>
>--
>Martin Bonner
>Martin.Bonner_at_[hidden]
>Pi Technology, Milton Hall, Ely Road, Milton, Cambridge, CB4 6WZ,
>ENGLAND Tel: [phone]
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Victor A. Wagner Jr.      http://rudbek.com
The five most dangerous words in the English language:
               "There oughta be a law"