Re: Sanity check: public/private

From:

"Alf P. Steinbach" <alfps@start.no>

Newsgroups:

comp.lang.c++.moderated

Date:

18 Sep 2006 05:10:42 -0400

Message-ID:

<4n4rnfF8n3k0U1@individual.net>

* James Kanze:

Alf P. Steinbach wrote:

* Carlos Moreno:

I'm not saying that #defining private as public is the way
to run test protocols; I simply think that it is an
interesting alternative, that can be quite useful in certain
situations -- I wanted to know if I should expect the trick
to work in the general case.

From a practical point of view I think so.

Provided he's not using private inheritance. (Even then, I'd be
surprised if he had a problem unless private inheritance was
used in the exception hierarchy. And only in exceptional cases
then.)

Truth be told I didn't think of that case, thanks! :-) It's subtle.
Illustration code, checking for conversion to inaccessible base at run-time:

     #include <iostream>
     #include <ostream>

     #if 0
     # define private public
     #endif

     void out( char const s[] ) { std::cout << s << std::endl; }

     class IsSerializable {};

     class Base {};
     class Derived: public Base, private IsSerializable {};

     template< class A, class B >
     class IsConvertibleFromTo
     {
         static char (&convert( ... ))[1];
         static char (&convert( B const* ))[2];
         static A const* a();
     public:
         // Fails to compile if B is an inaccessible base class of A.
         enum{ yes = (sizeof( convert(a()) ) == 2) };
     };

     template< class A, class B >
     bool isConvertibleFromTo()
     {
         try { throw static_cast<A const*>( 0 ); }
         catch( B const* ) { return true; }
         catch( ... ) { return false; }
     }

     int main()
     {
         out( IsConvertibleFromTo<Derived, Base>::yes? "B" : "No B" );
         out( isConvertibleFromTo<Derived, IsSerializable>()? "S" : "No
S" );
     }

Change the "#if 0" to "#if 1" and the output of the program is changed.

Now the question for us language definition nit-pickers (I currently
doubt that my question has practical value),

   * How can we can we check for conversion to an inaccessible base class
     at compile time rather than run time?

Additional question: can this have practical value?

--
A: Because it messes up the order in which people normally read text.
Q: Why is it such a bad thing?
A: Top-posting.
Q: What is the most annoying thing on usenet and in e-mail?

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