Re: [boost] pure virtual singleton

From: Jody Hagins (jody-boost-011304_at_[hidden])
Date: 2005-03-24 14:14:50

On Thu, 24 Mar 2005 08:47:58 -0800
Dan Rosen <dan.rosen_at_[hidden]> wrote:

> Hi Jason,
> 
> I haven't looked at singleton in any detail, so I can't answer your
> real question, but I wanted to offer a small suggestion, for what it's
> worth:
> 
> Rather than having a special function spelled
> "define_to_enable_creation," I wonder if it would be reasonable to
> declare singleton_base's destructor as pure virtual, forcing client
> code such as test_singleton to implement a destructor. If you wanted
> to enable the virtual behavior only for extra checking in debug mode,
> that'd be fine too:
> 
>   class singleton_base {
>   protected:
>   #ifdef DEBUG
>     virtual ~singleton_base() = 0;
>   #endif
>   // ...
>   };
> 
>   class test_singleton : public singleton_base {
>   protected:
>     ~test_singleton() { }
>     // ...
>   };
> 
> In debug mode, test_singleton's destructor is virtual as defined by
> its base class, and is necessary to implement. In release mode,
> test_singleton's destructor is not virtual unless you specify it to
> be. And implementing a trivial destructor like this is never harmful
> anyway.
> 
> Just a thought, but I might be totally missing something :) Cheers,
> dr

I do not understand what Jason is after either, but I thought a virtual
destructor may offer some assistance, and it may, depending on what he
is really trying to achieve.  However, I thought I'd comment on your
suggestion...

A pure virtual destructor in singleton_base does not require the derived
class to implement a destructor.  Well, it does, but the compiler will
automatically generate a destructor in the derived class.  So you do not
get the benefit or "forcing" the derived class to implement the
destructor.

Second, you still must provide an implementation of
singletom_base::~singleton_base() because derived classes will call it
in their destruction process.  Remember pure virtual functions
(including destructors) can have an implementation, and specifically for
dtors, you must provide such implementation.