RE: [boost] Re: Standard C Library and C++ / BOOST

From: Reece Dunn (msclrhd_at_[hidden])
Date: 2004-03-28 14:24:06

Gennaro Prota wrote:
>On Sun, 28 Mar 2004 12:55:10 +0200, "Pavel Vozenilek"
><pavel_vozenilek_at_[hidden]> wrote:

> >"Reece Dunn" <msclrhd_at_[hidden]> wrote
> >> So is it possible to prevent derivation? If not, then at least make it
> >> harder to do so (and put a warning in the docs)?

> >Yes.

> >Legal way:
> >http://www.research.att.com/~bs/bs_faq2.html#no-derivation

>And where is derivation prevented? ;) That only gives an error when
>you try instantiating the derived class. (Not only you may still
>derive, but of course you can access and use any static members).

There aren't any static members to errorcheck, so this is fine. If you try 
to do something like:

   class foobar: public boost::errorcheck<>
   {
      // ...
   };

you will not get any errors until you do something like:

   foobar foo;

which generates an error message like:

VC7.1 --

test.cpp(11) : error C2248: 
'boost::detail::cannot_derive_errorcheck<Storage,MsgStorage>::__ctor' : 
cannot access private member declared in class 
'boost::detail::cannot_derive_errorcheck<Storage,MsgStorage>'
        with
        [
            Storage=boost::policy::error_storage<>,
            MsgStorage=boost::policy::no_errormsg_storage
        ]
        and
        [
            Storage=boost::policy::error_storage<>,
            MsgStorage=boost::policy::no_errormsg_storage
        ]
        and
        [
            Storage=boost::policy::error_storage<>,
            MsgStorage=boost::policy::no_errormsg_storage
        ]
        d:\devel\ADL++\include\boost\errorcheck.hpp(102) : see declaration 
of 'boost::detail::cannot_derive_errorcheck<Storage,MsgStorage>::__ctor'
        with
        [
            Storage=boost::policy::error_storage<>,
            MsgStorage=boost::policy::no_errormsg_storage
        ]
        and
        [
            Storage=boost::policy::error_storage<>,
            MsgStorage=boost::policy::no_errormsg_storage
        ]

VC6 --

test.cpp(70) : error C2248: 'cannot_derive_errorcheck<class 
boost::policy::error_storage<int>,class 
boost::policy::no_errormsg_storage>::cannot_derive_errorcheck<class 
boost::policy::error_storage<int>,class boost::policy::no_errormsg_storage>' 
: cannot access private member declared in class 
'boost::detail::cannot_derive_errorcheck<class 
boost::policy::error_storage<int>,class boost::policy::no_errormsg_storage>'

Borland C++5.5 --

Error E2247 test.cpp 11: 
'boost::detail::cannot_derive_errorcheck<boost::policy::error_storage<int>,boost::policy::no_errormsg_storage>::cannot_derive_errorcheck()' 
is not accessible in function foobar::foobar()

Therefore, the error message is intuitive: cannot_derive_errorcheck!

Regards,
Reece

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