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

From: Martin Bonner (martin.bonner_at_[hidden])
Date: 2005-10-26 03:51:59

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

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]