Re: [boost] Request for rationale

From: Mike Tegtmeyer (tegtmeye_at_[hidden])
Date: 2007-11-04 08:59:02

Sure. I wasn't really arguing changing shared_ptr, nor did I think  
that there was a number of different ways to work around the problem  
(it was good to see the number of different ways folks would  
though).  I was more making a point about semantic design and the  
fact that if one looked at all of the things that follow this  
paradigm, obtain some resource with a free function returning a  
pointer and release the resource with another free function that  
takes a pointer, I'd argue that more things do not allow you to  
release or 'delete' a null pointer than do (obviously new are free  
are ones that do). That's all.

Mike

On Nov 4, 2007, at 3:00 AM, Daniel Frey wrote:

> On Sat, 2007-11-03 at 19:45 -0400, Mike Tegtmeyer wrote:
>> I guess if shared_ptr was to be done all over again, I think that I
>> would argue that it should act like a pointer, ie you shouldn't be
>> able to 'delete' a 'null' pointer - whatever delete and null mean for
>
> Let's keep shared_ptr like it is. What you (and others) probably  
> need is
> the right helper, as a quick hack it looks like this:
>
> #include <iostream>
> #include <boost/shared_ptr.hpp>
>
> struct X {};
>
> void freeX( X* x )
> {
>   // Should not be called with a null pointer
>   std::cout << "free " << x << std::endl;
> }
>
> template< typename T >
> struct on_nonzero_t
> {
>   typedef void (&F)( T* );
>   F f_;
>
>   on_nonzero_t( F f ) : f_( f ) {}
>   void operator()( T* t ) const { if( t ) f_( t ); }
> };
>
> template< typename T >
> on_nonzero_t< T > on_nonzero( void (&f)( T* ) )
> {
>   return on_nonzero_t< T >( f );
> }
>
> void f( X* x )
> {
>   std::cout << "x = " << x << std::endl;
>   boost::shared_ptr< X > tmp( x, on_nonzero( freeX ) );
> }
>
> int main()
> {
>   X x;
>   f( &x );
>
>   f( 0 );
> }
>
> The naming of the helper might be crap, but anyway, hope you can make
> some use of it...
>
> Regards, Daniel
>
>
>
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