Re: [boost] Sealed C++ class.

From: Martin Bonner (martin.bonner_at_[hidden])
Date: 2006-04-07 11:35:27

----Original Message----
From: boost-bounces_at_[hidden]
[mailto:boost-bounces_at_[hidden]] On Behalf Of Sebastian Redl
Sent: 07 April 2006 16:14 To: boost_at_[hidden]
Subject: Re: [boost] Sealed C++ class.

> Vladislav Lazarenko wrote:
> 
>>   template <typename T>
>>   class sealed : virtual private sealed_::sealed_impl<sealed, T>
>> 
>> 
> Interesting idea. Any particular reason why this inheritance is
> virtual? 
> sealed_impl being a template should be rare to duplicate as a base,
> and 
> having no data members or virtual functions, it should be irrelevant
> even if it was duplicated. On the other hand, you introduce the
> troubles 
> that come with virtual inheritance.
> 
> Sebastian Redl

If I write 
        class Broken : public UniquePerson {};
Then the constructor of Broken has to invoke the constructor of
sealed_impl.  However the constructor is private, so it can't.  The
result is a compilation error.

The problem is the code:
     template <typename T1, typename T2>
     class sealed_impl
     {
       friend typename T1;
       friend typename T2;
        ...

7.1.5.3 p2 says "[Note: this implies that, within a class template with
a template type-parameter T, the declaration friend class T; is
ill-formed.]"

In other words, you can't do that :-(

-- 
Martin Bonner
Martin.Bonner_at_[hidden]
Pi Technology, Milton Hall, Ely Road, Milton, Cambridge, CB4 6WZ,
ENGLAND Tel: [phone]